Call .NET methods from JavaScript functions in ASP.NET Core Blazor

This article explains how to invoke .NET methods from JavaScript (JS).

For information on how to call JS functions from .NET, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

Invoke a static .NET method

To invoke a static .NET method from JavaScript (JS), use the JS functions:

• DotNet.invokeMethodAsync (recommended): Asynchronous for both server-side and client-side components.
• DotNet.invokeMethod: Synchronous for client-side components only.

Pass in the name of the assembly containing the method, the identifier of the static .NET method, and any arguments.

In the following example:

• The {ASSEMBLY NAME} placeholder is the app's assembly name.
• The {.NET METHOD ID} placeholder is the .NET method identifier.
• The {ARGUMENTS} placeholder are optional, comma-separated arguments to pass to the method, each of which must be JSON-serializable.

DotNet.invokeMethodAsync('{ASSEMBLY NAME}', '{.NET METHOD ID}', {ARGUMENTS});

DotNet.invokeMethodAsync returns a JS Promise representing the result of the operation. DotNet.invokeMethod (client-side components) returns the result of the operation.

Important

For server-side components, we recommend the asynchronous function (invokeMethodAsync) over the synchronous version (invokeMethod).

The .NET method must be public, static, and have the [JSInvokable] attribute.

In the following example:

• The {<T>} placeholder indicates the return type, which is only required for methods that return a value.
• The {.NET METHOD ID} placeholder is the method identifier.

@code {
    [JSInvokable]
    public static Task{<T>} {.NET METHOD ID}()
    {
        ...
    }
}

Note

Calling open generic methods isn't supported with static .NET methods but is supported with instance methods. For more information, see the Call .NET generic class methods section.

In the following component, the ReturnArrayAsync C# method returns an int array. The [JSInvokable] attribute is applied to the method, which makes the method invokable by JS.

CallDotnet1.razor:

@page "/call-dotnet-1"
@implements IAsyncDisposable
@inject IJSRuntime JS

<PageTitle>Call .NET 1</PageTitle>

<h1>Call .NET Example 1</h1>

<p>
    <button id="btn">Trigger .NET static method</button>
</p>

<p>
    See the result in the developer tools console.
</p>

@code {
    private IJSObjectReference? module;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            module = await JS.InvokeAsync<IJSObjectReference>("import",
                "./Components/Pages/CallDotnet1.razor.js");

            await module.InvokeVoidAsync("addHandlers");
        }
    }

    [JSInvokable]
    public static Task<int[]> ReturnArrayAsync() =>
        Task.FromResult(new int[] { 11, 12, 13 });

    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        if (module is not null)
        {
            try
            {
                await module.DisposeAsync();
            }
            catch (JSDisconnectedException)
            {
            }
        }
    }
}

CallDotnet1.razor.js:

export function returnArrayAsync() {
  DotNet.invokeMethodAsync('BlazorSample', 'ReturnArrayAsync')
    .then(data => {
      console.log(data);
    });
}

export function addHandlers() {
  const btn = document.getElementById("btn");
  btn.addEventListener("click", returnArrayAsync);
}

The addHandlers JS function adds a click event to the button. The returnArrayAsync JS function is assigned as the handler.

The returnArrayAsync JS function calls the ReturnArrayAsync .NET method of the component, which logs the result to the browser's web developer tools console. BlazorSample is the app's assembly name.

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

When the Trigger .NET static method button is selected, the browser's developer tools console output displays the array data. The format of the output differs slightly among browsers. The following output shows the format used by Microsoft Edge:

Array(3) [ 11, 12, 13 ]

Pass data to a .NET method when calling the invokeMethodAsync function by passing the data as arguments.

To demonstrate passing data to .NET, pass a starting position to the ReturnArrayAsync method where the method is invoked in JS:

export function returnArrayAsync() {
  DotNet.invokeMethodAsync('BlazorSample', 'ReturnArrayAsync', 14)
    .then(data => {
      console.log(data);
    });
}

The component's invokable ReturnArrayAsync method receives the starting position and constructs the array from it. The array is returned for logging to the console:

[JSInvokable]
public static Task<int[]> ReturnArrayAsync(int startPosition) => 
    Task.FromResult(Enumerable.Range(startPosition, 3).ToArray());

After the app is recompiled and the browser is refreshed, the following output appears in the browser's console when the button is selected:

Array(3) [ 14, 15, 16 ]

The .NET method identifier for the JS call is the .NET method name, but you can specify a different identifier using the [JSInvokable] attribute constructor. In the following example, DifferentMethodName is the assigned method identifier for the ReturnArrayAsync method:

[JSInvokable("DifferentMethodName")]

In the call to DotNet.invokeMethodAsync (server-side or client-side components) or DotNet.invokeMethod (client-side components only), call DifferentMethodName to execute the ReturnArrayAsync .NET method:

• DotNet.invokeMethodAsync('BlazorSample', 'DifferentMethodName');
• DotNet.invokeMethod('BlazorSample', 'DifferentMethodName'); (client-side components only)

Note

The ReturnArrayAsync method example in this section returns the result of a Task without the use of explicit C# async and await keywords. Coding methods with async and await is typical of methods that use the await keyword to return the value of asynchronous operations.

ReturnArrayAsync method composed with async and await keywords:

[JSInvokable]
public static async Task<int[]> ReturnArrayAsync() => 
    await Task.FromResult(new int[] { 11, 12, 13 });

For more information, see Asynchronous programming with async and await in the C# guide.

Create JavaScript object and data references to pass to .NET

Call DotNet.createJSObjectReference(jsObject) to construct a JS object reference so that it can be passed to .NET, where jsObject is the JS Object used to create the JS object reference. The following example passes a reference to the non-serializable window object to .NET, which receives it in the ReceiveWindowObject C# method as an IJSObjectReference:

DotNet.invokeMethodAsync('{ASSEMBLY NAME}', 'ReceiveWindowObject', 
  DotNet.createJSObjectReference(window));

[JSInvokable]
public static void ReceiveWindowObject(IJSObjectReference objRef)
{
    ...
}

In the preceding example, the {ASSEMBLY NAME} placeholder is the app's namespace.

Note

The preceding example doesn't require disposal of the JSObjectReference, as a reference to the window object isn't held in JS.

Maintaining a reference to a JSObjectReference requires disposing of it to avoid leaking JS memory on the client. The following example refactors the preceding code to capture a reference to the JSObjectReference, followed by a call to DotNet.disposeJSObjectReference() to dispose of the reference:

var jsObjectReference = DotNet.createJSObjectReference(window);

DotNet.invokeMethodAsync('{ASSEMBLY NAME}', 'ReceiveWindowObject', jsObjectReference);

DotNet.disposeJSObjectReference(jsObjectReference);

In the preceding example, the {ASSEMBLY NAME} placeholder is the app's namespace.

Call DotNet.createJSStreamReference(streamReference) to construct a JS stream reference so that it can be passed to .NET, where streamReference is an ArrayBuffer, Blob, or any typed array, such as Uint8Array or Float32Array, used to create the JS stream reference.

Invoke an instance .NET method

To invoke an instance .NET method from JavaScript (JS):

• Pass the .NET instance by reference to JS by wrapping the instance in a DotNetObjectReference and calling Create on it.

• Invoke a .NET instance method from JS using invokeMethodAsync (recommended) or invokeMethod (client-side components only) from the passed DotNetObjectReference. Pass the identifier of the instance .NET method and any arguments. The .NET instance can also be passed as an argument when invoking other .NET methods from JS.

  In the following example:

  • dotNetHelper is a DotNetObjectReference.
  • The {.NET METHOD ID} placeholder is the .NET method identifier.
  • The {ARGUMENTS} placeholder are optional, comma-separated arguments to pass to the method, each of which must be JSON-serializable.

  dotNetHelper.invokeMethodAsync('{.NET METHOD ID}', {ARGUMENTS});
  

  Note

  invokeMethodAsync and invokeMethod don't accept an assembly name parameter when invoking an instance method.

  invokeMethodAsync returns a JS Promise representing the result of the operation. invokeMethod (client-side components only) returns the result of the operation.

  Important

  For server-side components, we recommend the asynchronous function (invokeMethodAsync) over the synchronous version (invokeMethod).

• Dispose of the DotNetObjectReference.

The following sections of this article demonstrate various approaches for invoking an instance .NET method:

• Pass a DotNetObjectReference to an individual JavaScript function
• Pass a DotNetObjectReference to a class with multiple JavaScript functions
• Call .NET generic class methods
• Class instance examples
• Component instance .NET method helper class

Avoid trimming JavaScript-invokable .NET methods

This section applies to client-side apps with ahead-of-time (AOT) compilation and runtime relinking enabled.

Several of the examples in the following sections are based on a class instance approach, where the JavaScript-invokable .NET method marked with the [JSInvokable] attribute is a member of a class that isn't a Razor component. When such .NET methods are located in a Razor component, they're protected from runtime relinking/trimming. In order to protect the .NET methods from trimming outside of Razor components, implement the methods with the DynamicDependency attribute on the class's constructor, as the following example demonstrates:

using System.Diagnostics.CodeAnalysis;
using Microsoft.JSInterop;

public class ExampleClass {

    [DynamicDependency(nameof(ExampleJSInvokableMethod))]
    public ExampleClass()
    {
    }

    [JSInvokable]
    public string ExampleJSInvokableMethod()
    {
        ...
    }
}

For more information, see Prepare .NET libraries for trimming: DynamicDependency.

Pass a DotNetObjectReference to an individual JavaScript function

The example in this section demonstrates how to pass a DotNetObjectReference to an individual JavaScript (JS) function.

The following sayHello1 JS function receives a DotNetObjectReference and calls invokeMethodAsync to call the GetHelloMessage .NET method of a component:

<script>
  window.sayHello1 = (dotNetHelper) => {
    return dotNetHelper.invokeMethodAsync('GetHelloMessage');
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

For the following component:

• The component has a JS-invokable .NET method named GetHelloMessage.
• When the Trigger .NET instance method button is selected, the JS function sayHello1 is called with the DotNetObjectReference.
• sayHello1:
  • Calls GetHelloMessage and receives the message result.
  • Returns the message result to the calling TriggerDotNetInstanceMethod method.
• The returned message from sayHello1 in result is displayed to the user.
• To avoid a memory leak and allow garbage collection, the .NET object reference created by DotNetObjectReference is disposed in the Dispose method.

CallDotnet2.razor:

@page "/call-dotnet-2"
@implements IDisposable
@inject IJSRuntime JS

<PageTitle>Call .NET 2</PageTitle>

<h1>Call .NET Example 2</h1>

<p>
    <label>
        Name: <input @bind="name" />
    </label>
</p>

<p>
    <button @onclick="TriggerDotNetInstanceMethod">
        Trigger .NET instance method
    </button>
</p>

<p>
    @result
</p>

@code {
    private string? name;
    private string? result;
    private DotNetObjectReference<CallDotnet2>? objRef;

    protected override void OnInitialized() =>
        objRef = DotNetObjectReference.Create(this);

    public async Task TriggerDotNetInstanceMethod() =>
        result = await JS.InvokeAsync<string>("sayHello1", objRef);

    [JSInvokable]
    public string GetHelloMessage() => $"Hello, {name}!";

    public void Dispose() => objRef?.Dispose();
}

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

Use the following guidance to pass arguments to an instance method:

Add parameters to the .NET method invocation. In the following example, a name is passed to the method. Add additional parameters to the list as needed.

<script>
  window.sayHello2 = (dotNetHelper, name) => {
    return dotNetHelper.invokeMethodAsync('GetHelloMessage', name);
  };
</script>

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

Provide the parameter list to the .NET method.

CallDotnet3.razor:

@page "/call-dotnet-3"
@implements IDisposable
@inject IJSRuntime JS

<PageTitle>Call .NET 3</PageTitle>

<h1>Call .NET Example 3</h1>

<p>
    <label>
        Name: <input @bind="name" />
    </label>
</p>

<p>
    <button @onclick="TriggerDotNetInstanceMethod">
        Trigger .NET instance method
    </button>
</p>

<p>
    @result
</p>

@code {
    private string? name;
    private string? result;
    private DotNetObjectReference<CallDotnet3>? objRef;

    protected override void OnInitialized() => 
        objRef = DotNetObjectReference.Create(this);

    public async Task TriggerDotNetInstanceMethod() =>
        result = await JS.InvokeAsync<string>("sayHello2", objRef, name);

    [JSInvokable]
    public string GetHelloMessage(string passedName) => $"Hello, {passedName}!";

    public void Dispose() => objRef?.Dispose();
}

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

Pass a DotNetObjectReference to a class with multiple JavaScript functions

The example in this section demonstrates how to pass a DotNetObjectReference to a JavaScript (JS) class with multiple functions.

Create and pass a DotNetObjectReference from the OnAfterRenderAsync lifecycle method to a JS class for multiple functions to use. Make sure that the .NET code disposes of the DotNetObjectReference, as the following example shows.

In the following component, the Trigger JS function buttons call JS functions by setting the JS onclick property, not Blazor's @onclick directive attribute.

CallDotNetExampleOneHelper.razor:

@page "/call-dotnet-example-one-helper"
@implements IAsyncDisposable
@inject IJSRuntime JS

<PageTitle>Call .NET Example</PageTitle>

<h1>Pass <code>DotNetObjectReference</code> to a JavaScript class</h1>

<p>
    <label>
        Message: <input @bind="name" />
    </label>
</p>

<p>
    <button id="sayHelloBtn">
        Trigger JS function <code>sayHello</code>
    </button>
</p>

<p>
    <button id="welcomeVisitorBtn">
        Trigger JS function <code>welcomeVisitor</code>
    </button>
</p>

@code {
    private IJSObjectReference? module;
    private string? name;
    private DotNetObjectReference<CallDotNetExampleOneHelper>? dotNetHelper;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            module = await JS.InvokeAsync<IJSObjectReference>("import",
                "./Components/Pages/CallDotNetExampleOneHelper.razor.js");

            dotNetHelper = DotNetObjectReference.Create(this);
            await module.InvokeVoidAsync("GreetingHelpers.setDotNetHelper", 
                dotNetHelper);

            await module.InvokeVoidAsync("addHandlers");
        }
    }

    [JSInvokable]
    public string GetHelloMessage() => $"Hello, {name}!";

    [JSInvokable]
    public string GetWelcomeMessage() => $"Welcome, {name}!";

    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        if (module is not null)
        {
            try
            {
                await module.DisposeAsync();
            }
            catch (JSDisconnectedException)
            {
            }
        }

        dotNetHelper?.Dispose();
    }
}

In the preceding example:

• JS is an injected IJSRuntime instance. IJSRuntime is registered by the Blazor framework.
• The variable name dotNetHelper is arbitrary and can be changed to any preferred name.
• The component must explicitly dispose of the DotNetObjectReference to permit garbage collection and prevent a memory leak.
• JSDisconnectedException is trapped during module disposal in case Blazor's SignalR circuit is lost. If the preceding code is used in a Blazor WebAssembly app, there's no SignalR connection to lose, so you can remove the try-catch block and leave the line that disposes the module (await module.DisposeAsync();). For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

CallDotNetExampleOneHelper.razor.js:

export class GreetingHelpers {
  static dotNetHelper;

  static setDotNetHelper(value) {
    GreetingHelpers.dotNetHelper = value;
  }

  static async sayHello() {
    const msg =
      await GreetingHelpers.dotNetHelper.invokeMethodAsync('GetHelloMessage');
    alert(`Message from .NET: "${msg}"`);
  }

  static async welcomeVisitor() {
    const msg =
      await GreetingHelpers.dotNetHelper.invokeMethodAsync('GetWelcomeMessage');
    alert(`Message from .NET: "${msg}"`);
  }
}

export function addHandlers() {
  const sayHelloBtn = document.getElementById("sayHelloBtn");
  sayHelloBtn.addEventListener("click", GreetingHelpers.sayHello);

  const welcomeVisitorBtn = document.getElementById("welcomeVisitorBtn");
  welcomeVisitorBtn.addEventListener("click", GreetingHelpers.welcomeVisitor);
}

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

Call .NET generic class methods

JavaScript (JS) functions can call .NET generic class methods, where a JS function calls a .NET method of a generic class.

In the following generic type class (GenericType<TValue>):

• The class has a single type parameter (TValue) with a single generic Value property.
• The class has two non-generic methods marked with the [JSInvokable] attribute, each with a generic type parameter named newValue:
  • Update synchronously updates the value of Value from newValue.
  • UpdateAsync asynchronously updates the value of Value from newValue after creating an awaitable task with Task.Yield that asynchronously yields back to the current context when awaited.
• Each of the class methods write the type of TValue and the value of Value to the console. Writing to the console is only for demonstration purposes. Production apps usually avoid writing to the console in favor of app logging. For more information, see ASP.NET Core Blazor logging and Logging in .NET Core and ASP.NET Core.

Note

Open generic types and methods don't specify types for type placeholders. Conversely, closed generics supply types for all type placeholders. The examples in this section demonstrate closed generics, but invoking JS interop instance methods with open generics is supported. Use of open generics isn't supported for static .NET method invocations, which were described earlier in this article.

For more information, see the following articles:

• Generic classes and methods (C# documentation)
• Generic Classes (C# Programming Guide)
• Generics in .NET (.NET documentation)

GenericType.cs:

using Microsoft.JSInterop;

public class GenericType<TValue>
{
    public TValue? Value { get; set; }

    [JSInvokable]
    public void Update(TValue newValue)
    {
        Value = newValue;

        Console.WriteLine($"Update: GenericType<{typeof(TValue)}>: {Value}");
    }

    [JSInvokable]
    public async Task UpdateAsync(TValue newValue)
    {
        await Task.Yield();
        Value = newValue;

        Console.WriteLine($"UpdateAsync: GenericType<{typeof(TValue)}>: {Value}");
    }
}

In the following invokeMethodsAsync function:

• The generic type class's Update and UpdateAsync methods are called with arguments representing strings and numbers.
• Client-side components support calling .NET methods synchronously with invokeMethod. syncInterop receives a boolean value indicating if the JS interop is occurring on the client. When syncInterop is true, invokeMethod is safely called. If the value of syncInterop is false, only the asynchronous function invokeMethodAsync is called because the JS interop is executing in a server-side component.
• For demonstration purposes, the DotNetObjectReference function call (invokeMethod or invokeMethodAsync), the .NET method called (Update or UpdateAsync), and the argument are written to the console. The arguments use a random number to permit matching the JS function call to the .NET method invocation (also written to the console on the .NET side). Production code usually doesn't write to the console, either on the client or the server. Production apps usually rely upon app logging. For more information, see ASP.NET Core Blazor logging and Logging in .NET Core and ASP.NET Core.

<script>
  const randomInt = () => Math.floor(Math.random() * 99999);

  window.invokeMethodsAsync = async (syncInterop, dotNetHelper1, dotNetHelper2) => {
    var n = randomInt();
    console.log(`JS: invokeMethodAsync:Update('string ${n}')`);
    await dotNetHelper1.invokeMethodAsync('Update', `string ${n}`);

    n = randomInt();
    console.log(`JS: invokeMethodAsync:UpdateAsync('string ${n}')`);
    await dotNetHelper1.invokeMethodAsync('UpdateAsync', `string ${n}`);

    if (syncInterop) {
      n = randomInt();
      console.log(`JS: invokeMethod:Update('string ${n}')`);
      dotNetHelper1.invokeMethod('Update', `string ${n}`);
    }

    n = randomInt();
    console.log(`JS: invokeMethodAsync:Update(${n})`);
    await dotNetHelper2.invokeMethodAsync('Update', n);

    n = randomInt();
    console.log(`JS: invokeMethodAsync:UpdateAsync(${n})`);
    await dotNetHelper2.invokeMethodAsync('UpdateAsync', n);

    if (syncInterop) {
      n = randomInt();
      console.log(`JS: invokeMethod:Update(${n})`);
      dotNetHelper2.invokeMethod('Update', n);
    }
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

In the following GenericsExample component:

• The JS function invokeMethodsAsync is called when the Invoke Interop button is selected.
• A pair of DotNetObjectReference types are created and passed to the JS function for instances of the GenericType as a string and an int.

GenericsExample.razor:

@page "/generics-example"
@implements IDisposable
@inject IJSRuntime JS

<p>
    <button @onclick="InvokeInterop">Invoke Interop</button>
</p>

<ul>
    <li>genericType1: @genericType1?.Value</li>
    <li>genericType2: @genericType2?.Value</li>
</ul>

@code {
    private GenericType<string> genericType1 = new() { Value = "string 0" };
    private GenericType<int> genericType2 = new() { Value = 0 };
    private DotNetObjectReference<GenericType<string>>? objRef1;
    private DotNetObjectReference<GenericType<int>>? objRef2;

    protected override void OnInitialized()
    {
        objRef1 = DotNetObjectReference.Create(genericType1);
        objRef2 = DotNetObjectReference.Create(genericType2);
    }

    public async Task InvokeInterop()
    {
        var syncInterop = OperatingSystem.IsBrowser();

        await JS.InvokeVoidAsync(
            "invokeMethodsAsync", syncInterop, objRef1, objRef2);
    }

    public void Dispose()
    {
        objRef1?.Dispose();
        objRef2?.Dispose();
    }
}

In the preceding example, JS is an injected IJSRuntime instance. IJSRuntime is registered by the Blazor framework.

The following demonstrates typical output of the preceding example when the Invoke Interop button is selected in a client-side component:

JS: invokeMethodAsync:Update('string 37802')
.NET: Update: GenericType<System.String>: string 37802
JS: invokeMethodAsync:UpdateAsync('string 53051')
JS: invokeMethod:Update('string 26784')
.NET: Update: GenericType<System.String>: string 26784
JS: invokeMethodAsync:Update(14107)
.NET: Update: GenericType<System.Int32>: 14107
JS: invokeMethodAsync:UpdateAsync(48995)
JS: invokeMethod:Update(12872)
.NET: Update: GenericType<System.Int32>: 12872
.NET: UpdateAsync: GenericType<System.String>: string 53051
.NET: UpdateAsync: GenericType<System.Int32>: 48995

If the preceding example is implemented in a server-side component, the synchronous calls with invokeMethod are avoided. For server-side components, we recommend the asynchronous function (invokeMethodAsync) over the synchronous version (invokeMethod).

Typical output of a server-side component:

JS: invokeMethodAsync:Update('string 34809')
.NET: Update: GenericType<System.String>: string 34809
JS: invokeMethodAsync:UpdateAsync('string 93059')
JS: invokeMethodAsync:Update(41997)
.NET: Update: GenericType<System.Int32>: 41997
JS: invokeMethodAsync:UpdateAsync(24652)
.NET: UpdateAsync: GenericType<System.String>: string 93059
.NET: UpdateAsync: GenericType<System.Int32>: 24652

The preceding output examples demonstrate that asynchronous methods execute and complete in an arbitrary order depending on several factors, including thread scheduling and the speed of method execution. It isn't possible to reliably predict the order of completion for asynchronous method calls.

Class instance examples

The following sayHello1 JS function:

• Calls the GetHelloMessage .NET method on the passed DotNetObjectReference.
• Returns the message from GetHelloMessage to the sayHello1 caller.

<script>
  window.sayHello1 = (dotNetHelper) => {
    return dotNetHelper.invokeMethodAsync('GetHelloMessage');
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

The following HelloHelper class has a JS-invokable .NET method named GetHelloMessage. When HelloHelper is created, the name in the Name property is used to return a message from GetHelloMessage.

HelloHelper.cs:

using Microsoft.JSInterop;

namespace BlazorSample;

public class HelloHelper(string? name)
{
    public string? Name { get; set; } = name ?? "No Name";

    [JSInvokable]
    public string GetHelloMessage() => $"Hello, {Name}!";
}

The CallHelloHelperGetHelloMessage method in the following JsInteropClasses3 class invokes the JS function sayHello1 with a new instance of HelloHelper.

JsInteropClasses3.cs:

using Microsoft.JSInterop;

namespace BlazorSample;

public class JsInteropClasses3(IJSRuntime js)
{
    private readonly IJSRuntime js = js;

    public async ValueTask<string> CallHelloHelperGetHelloMessage(string? name)
    {
        using var objRef = DotNetObjectReference.Create(new HelloHelper(name));
        return await js.InvokeAsync<string>("sayHello1", objRef);
    }
}

To avoid a memory leak and allow garbage collection, the .NET object reference created by DotNetObjectReference is disposed when the object reference goes out of scope with using var syntax.

When the Trigger .NET instance method button is selected in the following component, JsInteropClasses3.CallHelloHelperGetHelloMessage is called with the value of name.

CallDotnet4.razor:

@page "/call-dotnet-4"
@inject IJSRuntime JS

<PageTitle>Call .NET 4</PageTitle>

<h1>Call .NET Example 4</h1>

<p>
    <label>
        Name: <input @bind="name" />
    </label>
</p>

<p>
    <button @onclick="TriggerDotNetInstanceMethod">
        Trigger .NET instance method
    </button>
</p>

<p>
    @result
</p>

@code {
    private string? name;
    private string? result;
    private JsInteropClasses3? jsInteropClasses;

    protected override void OnInitialized() => 
        jsInteropClasses = new JsInteropClasses3(JS);

    private async Task TriggerDotNetInstanceMethod()
    {
        if (jsInteropClasses is not null)
        {
            result = await jsInteropClasses.CallHelloHelperGetHelloMessage(name);
        }
    }
}

The following image shows the rendered component with the name Amy Pond in the Name field. After the button is selected, Hello, Amy Pond! is displayed in the UI:

The preceding pattern shown in the JsInteropClasses3 class can also be implemented entirely in a component.

CallDotnet5.razor:

@page "/call-dotnet-5"
@inject IJSRuntime JS

<PageTitle>Call .NET 5</PageTitle>

<h1>Call .NET Example 5</h1>

<p>
    <label>
        Name: <input @bind="name" />
    </label>
</p>

<p>
    <button @onclick="TriggerDotNetInstanceMethod">
        Trigger .NET instance method
    </button>
</p>

<p>
    @result
</p>

@code {
    private string? name;
    private string? result;

    public async Task TriggerDotNetInstanceMethod()
    {
        using var objRef = DotNetObjectReference.Create(new HelloHelper(name));
        result = await JS.InvokeAsync<string>("sayHello1", objRef);
    }
}

To avoid a memory leak and allow garbage collection, the .NET object reference created by DotNetObjectReference is disposed when the object reference goes out of scope with using var syntax.

The output displayed by the component is Hello, Amy Pond! when the name Amy Pond is provided in the name field.

In the preceding component, the .NET object reference is disposed. If a class or component doesn't dispose the DotNetObjectReference, dispose it from the client by calling dispose on the passed DotNetObjectReference:

window.{JS FUNCTION NAME} = (dotNetHelper) => {
  dotNetHelper.invokeMethodAsync('{.NET METHOD ID}');
  dotNetHelper.dispose();
}

In the preceding example:

• The {JS FUNCTION NAME} placeholder is the JS function's name.
• The variable name dotNetHelper is arbitrary and can be changed to any preferred name.
• The {.NET METHOD ID} placeholder is the .NET method identifier.

Component instance .NET method helper class

A helper class can invoke a .NET instance method as an Action. Helper classes are useful in scenarios where using static .NET methods aren't applicable:

• When several components of the same type are rendered on the same page.
• In server-side apps with multiple users concurrently using the same component.

In the following example:

• The component contains several ListItem1 components.
• Each ListItem1 component is composed of a message and a button.
• When a ListItem1 component button is selected, that ListItem1's UpdateMessage method changes the list item text and hides the button.

The following MessageUpdateInvokeHelper class maintains a JS-invokable .NET method, UpdateMessageCaller, to invoke the Action specified when the class is instantiated.

MessageUpdateInvokeHelper.cs:

using Microsoft.JSInterop;

namespace BlazorSample;

public class MessageUpdateInvokeHelper(Action action)
{
    private readonly Action action = action;

    [JSInvokable]
    public void UpdateMessageCaller() => action.Invoke();
}

The following updateMessageCaller JS function invokes the UpdateMessageCaller .NET method.

<script>
  window.updateMessageCaller = (dotNetHelper) => {
    dotNetHelper.invokeMethodAsync('UpdateMessageCaller');
    dotNetHelper.dispose();
  }
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

The following ListItem1 component is a shared component that can be used any number of times in a parent component and creates list items (<li>...</li>) for an HTML list (<ul>...</ul> or <ol>...</ol>). Each ListItem1 component instance establishes an instance of MessageUpdateInvokeHelper with an Action set to its UpdateMessage method.

When a ListItem1 component's InteropCall button is selected, updateMessageCaller is invoked with a created DotNetObjectReference for the MessageUpdateInvokeHelper instance. This permits the framework to call UpdateMessageCaller on that ListItem1's MessageUpdateInvokeHelper instance. The passed DotNetObjectReference is disposed in JS (dotNetHelper.dispose()).

ListItem1.razor:

@inject IJSRuntime JS

<li>
    @message
    <button @onclick="InteropCall" style="display:@display">InteropCall</button>
</li>

@code {
    private string message = "Select one of these list item buttons.";
    private string display = "inline-block";
    private MessageUpdateInvokeHelper? messageUpdateInvokeHelper;

    protected override void OnInitialized()
    {
        messageUpdateInvokeHelper = new MessageUpdateInvokeHelper(UpdateMessage);
    }

    protected async Task InteropCall()
    {
        if (messageUpdateInvokeHelper is not null)
        {
            await JS.InvokeVoidAsync("updateMessageCaller",
                DotNetObjectReference.Create(messageUpdateInvokeHelper));
        }
    }

    private void UpdateMessage()
    {
        message = "UpdateMessage Called!";
        display = "none";
        StateHasChanged();
    }
}

StateHasChanged is called to update the UI when message is set in UpdateMessage. If StateHasChanged isn't called, Blazor has no way of knowing that the UI should be updated when the Action is invoked.

The following parent component includes four list items, each an instance of the ListItem1 component.

CallDotnet6.razor:

@page "/call-dotnet-6"

<PageTitle>Call .NET 6</PageTitle>

<h1>Call .NET Example 6</h1>

<ul>
    <ListItem1 />
    <ListItem1 />
    <ListItem1 />
    <ListItem1 />
</ul>

The following image shows the rendered parent component after the second InteropCall button is selected:

• The second ListItem1 component has displayed the UpdateMessage Called! message.
• The InteropCall button for the second ListItem1 component isn't visible because the button's CSS display property is set to none.

Component instance .NET method called from DotNetObjectReference assigned to an element property

The assignment of a DotNetObjectReference to a property of an HTML element permits calling .NET methods on a component instance:

• An element reference is captured (ElementReference).
• In the component's OnAfterRender{Async} method, a JavaScript (JS) function is invoked with the element reference and the component instance as a DotNetObjectReference. The JS function attaches the DotNetObjectReference to the element in a property.
• When an element event is invoked in JS (for example, onclick), the element's attached DotNetObjectReference is used to call a .NET method.

Similar to the approach described in the Component instance .NET method helper class section, this approach is useful in scenarios where using static .NET methods aren't applicable:

• When several components of the same type are rendered on the same page.
• In server-side apps with multiple users concurrently using the same component.
• The .NET method is invoked from a JS event (for example, onclick), not from a Blazor event (for example, @onclick).

In the following example:

• The component contains several ListItem2 components, which is a shared component.
• Each ListItem2 component is composed of a list item message <span> and a second <span> with a display CSS property set to inline-block for display.
• When a ListItem2 component list item is selected, that ListItem2's UpdateMessage method changes the list item text in the first <span> and hides the second <span> by setting its display property to none.

The following assignDotNetHelper JS function assigns the DotNetObjectReference to an element in a property named dotNetHelper. The following interopCall JS function uses the DotNetObjectReference for the passed element to invoke a .NET method named UpdateMessage.

ListItem2.razor.js:

export function assignDotNetHelper(element, dotNetHelper) {
  element.dotNetHelper = dotNetHelper;
}

export async function interopCall(element) {
  await element.dotNetHelper.invokeMethodAsync('UpdateMessage');
}

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

In the preceding example, the variable name dotNetHelper is arbitrary and can be changed to any preferred name.

The following ListItem2 component is a shared component that can be used any number of times in a parent component and creates list items (<li>...</li>) for an HTML list (<ul>...</ul> or <ol>...</ol>).

Each ListItem2 component instance invokes the assignDotNetHelper JS function in OnAfterRenderAsync with an element reference (the first <span> element of the list item) and the component instance as a DotNetObjectReference.

When a ListItem2 component's message <span> is selected, interopCall is invoked passing the <span> element as a parameter (this), which invokes the UpdateMessage .NET method. In UpdateMessage, StateHasChanged is called to update the UI when message is set and the display property of the second <span> is updated. If StateHasChanged isn't called, Blazor has no way of knowing that the UI should be updated when the method is invoked.

The DotNetObjectReference is disposed when the component is disposed.

ListItem2.razor:

@inject IJSRuntime JS
@implements IAsyncDisposable

<li>
    <span style="font-weight:bold;color:@color" @ref="elementRef"
        @onclick="CallJSToInvokeDotnet">
        @message
    </span>
    <span style="display:@display">
        Not Updated Yet!
    </span>
</li>

@code {
    private IJSObjectReference? module;
    private DotNetObjectReference<ListItem2>? objRef;
    private ElementReference elementRef;
    private string display = "inline-block";
    private string message = "Select one of these list items.";
    private string color = "initial";

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            module = await JS.InvokeAsync<IJSObjectReference>("import",
                "./Components/ListItem2.razor.js");

            objRef = DotNetObjectReference.Create(this);
            await module.InvokeVoidAsync("assignDotNetHelper", elementRef, objRef);
        }
    }

    public async Task CallJSToInvokeDotnet()
    {
        if (module is not null)
        {
            await module.InvokeVoidAsync("interopCall", elementRef);
        }
    }

    [JSInvokable]
    public void UpdateMessage()
    {
        message = "UpdateMessage Called!";
        display = "none";
        color = "MediumSeaGreen";
        StateHasChanged();
    }

    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        if (module is not null)
        {
            try
            {
                await module.DisposeAsync();
            }
            catch (JSDisconnectedException)
            {
            }
        }

        objRef?.Dispose();
    }
}

The following parent component includes four list items, each an instance of the ListItem2 component.

CallDotnet7.razor:

@page "/call-dotnet-7"

<PageTitle>Call .NET 7</PageTitle>

<h1>Call .NET Example 7</h1>

<ul>
    <ListItem2 />
    <ListItem2 />
    <ListItem2 />
    <ListItem2 />
</ul>

Synchronous JS interop in client-side components

This section only applies to client-side components.

JS interop calls are asynchronous, regardless of whether the called code is synchronous or asynchronous. Calls are asynchronous to ensure that components are compatible across server-side and client-side render modes. On the server, all JS interop calls must be asynchronous because they're sent over a network connection.

If you know for certain that your component only runs on WebAssembly, you can choose to make synchronous JS interop calls. This has slightly less overhead than making asynchronous calls and can result in fewer render cycles because there's no intermediate state while awaiting results.

To make a synchronous call from JavaScript to .NET in a client-side component, use DotNet.invokeMethod instead of DotNet.invokeMethodAsync.

Synchronous calls work if:

• The component is only rendered for execution on WebAssembly.
• The called function returns a value synchronously. The function isn't an async method and doesn't return a .NET Task or JavaScript Promise.

JavaScript location

Load JavaScript (JS) code using any of approaches described by the article on JavaScript location:

• Load a script in <head> markup (Not generally recommended)
• Load a script in <body> markup
• Load a script from an external JavaScript file (.js) collocated with a component
• Load a script from an external JavaScript file (.js)
• Inject a script before or after Blazor starts

Using JS modules to load JS is described in this article in the JavaScript isolation in JavaScript modules section.

Warning

Only place a <script> tag in a component file (.razor) if the component is guaranteed to adopt static server-side rendering (static SSR) because the <script> tag can't be updated dynamically.

JavaScript isolation in JavaScript modules

Blazor enables JavaScript (JS) isolation in standard JavaScript modules (ECMAScript specification). JavaScript module loading works the same way in Blazor as it does for other types of web apps, and you're free to customize how modules are defined in your app. For a guide on how to use JavaScript modules, see MDN Web Docs: JavaScript modules.

JS isolation provides the following benefits:

• Imported JS no longer pollutes the global namespace.
• Consumers of a library and components aren't required to import the related JS.

For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

Dynamic import with the import() operator is supported with ASP.NET Core and Blazor:

if ({CONDITION}) import("/additionalModule.js");

In the preceding example, the {CONDITION} placeholder represents a conditional check to determine if the module should be loaded.

For browser compatibility, see Can I use: JavaScript modules: dynamic import.

Avoid circular object references

Objects that contain circular references can't be serialized on the client for either:

• .NET method calls.
• JavaScript method calls from C# when the return type has circular references.

Byte array support

Blazor supports optimized byte array JavaScript (JS) interop that avoids encoding/decoding byte arrays into Base64. The following example uses JS interop to pass a byte array to .NET.

Provide a sendByteArray JS function. The function is called statically, which includes the assembly name parameter in the invokeMethodAsync call, by a button in the component and doesn't return a value:

CallDotnet8.razor.js:

export function sendByteArray() {
  const data = new Uint8Array([0x45, 0x76, 0x65, 0x72, 0x79, 0x74, 0x68, 0x69,
    0x6e, 0x67, 0x27, 0x73, 0x20, 0x73, 0x68, 0x69, 0x6e, 0x79, 0x2c,
    0x20, 0x43, 0x61, 0x70, 0x74, 0x61, 0x69, 0x6e, 0x2e, 0x20, 0x4e,
    0x6f, 0x74, 0x20, 0x74, 0x6f, 0x20, 0x66, 0x72, 0x65, 0x74, 0x2e]);
  DotNet.invokeMethodAsync('BlazorSample', 'ReceiveByteArray', data)
    .then(str => {
      alert(str);
    });
}

export function addHandlers() {
  const btn = document.getElementById("btn");
  btn.addEventListener("click", sendByteArray);
}

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

CallDotnet8.razor:

@page "/call-dotnet-8"
@using System.Text
@implements IAsyncDisposable
@inject IJSRuntime JS

<PageTitle>Call .NET 8</PageTitle>

<h1>Call .NET Example 8</h1>

<p>
    <button id="btn">Send Bytes</button>
</p>

<p>
    Quote ©2005 <a href="https://www.uphe.com">Universal Pictures</a>:
    <a href="https://www.uphe.com/movies/serenity-2005">Serenity</a><br>
    <a href="https://www.imdb.com/name/nm0821612/">Jewel Staite on IMDB</a>
</p>

@code {
    private IJSObjectReference? module;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            module = await JS.InvokeAsync<IJSObjectReference>("import", 
                "./Components/Pages/CallDotnet8.razor.js");

            await module.InvokeVoidAsync("addHandlers");
        }
    }

    [JSInvokable]
    public static Task<string> ReceiveByteArray(byte[] receivedBytes) => 
        Task.FromResult(Encoding.UTF8.GetString(receivedBytes, 0, 
            receivedBytes.Length));

    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        if (module is not null)
        {
            try
            {
                await module.DisposeAsync();
            }
            catch (JSDisconnectedException)
            {
            }
        }
    }
}

For information on using a byte array when calling JavaScript from .NET, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

Stream from JavaScript to .NET

Blazor supports streaming data directly from JavaScript to .NET. Streams are requested using the Microsoft.JSInterop.IJSStreamReference interface.

Microsoft.JSInterop.IJSStreamReference.OpenReadStreamAsync returns a Stream and uses the following parameters:

• maxAllowedSize: Maximum number of bytes permitted for the read operation from JavaScript, which defaults to 512,000 bytes if not specified.
• cancellationToken: A CancellationToken for cancelling the read.

In JavaScript:

function streamToDotNet() {
  return new Uint8Array(10000000);
}

In C# code:

var dataReference = 
    await JS.InvokeAsync<IJSStreamReference>("streamToDotNet");
using var dataReferenceStream = 
    await dataReference.OpenReadStreamAsync(maxAllowedSize: 10_000_000);

var outputPath = Path.Combine(Path.GetTempPath(), "file.txt");
using var outputFileStream = File.OpenWrite(outputPath);
await dataReferenceStream.CopyToAsync(outputFileStream);

In the preceding example:

• JS is an injected IJSRuntime instance. IJSRuntime is registered by the Blazor framework.
• The dataReferenceStream is written to disk (file.txt) at the current user's temporary folder path (GetTempPath).

Call JavaScript functions from .NET methods in ASP.NET Core Blazor covers the reverse operation, streaming from .NET to JavaScript using a DotNetStreamReference.

ASP.NET Core Blazor file uploads covers how to upload a file in Blazor. For a forms example that streams <textarea> data in a server-side component, see Troubleshoot ASP.NET Core Blazor forms.

JavaScript [JSImport]/[JSExport] interop

This section applies to client-side components.

As an alternative to interacting with JavaScript (JS) in client-side components using Blazor's JS interop mechanism based on the IJSRuntime interface, a JS [JSImport]/[JSExport] interop API is available to apps targeting .NET 7 or later.

For more information, see JavaScript JSImport/JSExport interop with ASP.NET Core Blazor.

Disposal of JavaScript interop object references

Examples throughout the JavaScript (JS) interop articles demonstrate typical object disposal patterns:

• When calling .NET from JS, as described in this article, dispose of a created DotNetObjectReference either from .NET or from JS to avoid leaking .NET memory.

• When calling JS from .NET, as described in Call JavaScript functions from .NET methods in ASP.NET Core Blazor, dispose any created IJSObjectReference/IJSInProcessObjectReference/JSObjectReference either from .NET or from JS to avoid leaking JS memory.

JS interop object references are implemented as a map keyed by an identifier on the side of the JS interop call that creates the reference. When object disposal is initiated from either the .NET or JS side, Blazor removes the entry from the map, and the object can be garbage collected as long as no other strong reference to the object is present.

At a minimum, always dispose objects created on the .NET side to avoid leaking .NET managed memory.

DOM cleanup tasks during component disposal

For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

JavaScript interop calls without a circuit

For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

Additional resources

• Call JavaScript functions from .NET methods in ASP.NET Core Blazor
• InteropComponent.razor example (dotnet/AspNetCore GitHub repository main branch): The main branch represents the product unit's current development for the next release of ASP.NET Core. To select the branch for a different release (for example, release/{VERSION}, where the {VERSION} placeholder is the release version), use the Switch branches or tags dropdown list to select the branch. For a branch that no longer exists, use the Tags tab to find the API (for example, v7.0.0).
• Interaction with the DOM
• Blazor samples GitHub repository (dotnet/blazor-samples) (how to download)
• Handle errors in ASP.NET Core Blazor apps (JavaScript interop section)
• Threat mitigation: .NET methods invoked from the browser