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TryParse and BindAsync methods are validated

  • Article

ASP.NET Core now validates TryParse and BindAsync methods on parameter types for Map* methods. If no valid method is found, ASP.NET Core looks for invalid methods and throws an exception at startup if one is found. The exception helps to avoid unexpected behavior by alerting you that your method signature may be incorrect.

Version introduced

ASP.NET Core 6.0 RC 2

Previous behavior

In previous versions of ASP.NET Core 6, if a TryParse or BindAsync method has an invalid signature, no exception was thrown, and the framework tried to bind JSON from the body.

// Todo.TryParse is not in a valid format.
// Will try to bind from body as JSON instead.
app.MapPost("/endpoint", (Todo todo) => todo.Item);

public class Todo
{
    public string Item { get; set; }
    public static bool TryParse(string value) => true;
}

New behavior

If ASP.NET Core finds a public TryParse or BindAsync method that doesn't match the expected syntax, an exception is thrown on startup. The previous example produces an error similar to:

TryParse method found on Todo with incorrect format. Must be a static method with format
bool TryParse(string, IFormatProvider, out Todo)
bool TryParse(string, out Todo)
but found
Boolean TryParse(System.String)

Type of breaking change

This change can affect binary compatibility and source compatibility.

Reason for change

This change was made so that developers are made aware of BindAsync and TryParse methods that have an invalid format. Previously, the framework would fall back to assuming the parameter is JSON from the body. This assumption can result in unexpected behavior.

If your type has a BindAsync or TryParse method with different syntax for a reason other than parameter binding, you'll now encounter an exception at startup. To avoid this behavior, there are multiple strategies available:

  • Change your BindAsync or TryParse method to be internal or private.
  • Add a new BindAsync or TryParse method that has the syntax the framework looks for—invalid methods are ignored if a valid one is found.
  • Mark your parameter as [FromBody].

Affected APIs

  • RequestDelegateFactory.Create()
  • All IEndpointRouteBuilder.Map*() methods, for example, app.MapGet() and app.MapPost()