Populate initialized properties

Article
10/25/2023

Starting in .NET 8, you can specify a preference to either replace or populate .NET properties when JSON is deserialized. The JsonObjectCreationHandling enum provides the object creation handling choices:

Default (replace) behavior

The System.Text.Json deserializer always creates a new instance of the target type. However, even though a new instance is created, some properties and fields might already be initialized as part of the object's construction. Consider the following type:

class A
{
    public List<int> Numbers1 { get; } = [1, 2, 3];
    public List<int> Numbers2 { get; set; } = [1, 2, 3];
}

When you create an instance of this class, the Numbers1 (and Numbers2) property's value is a list with three elements (1, 2, and 3). If you deserialize JSON to this type, the default behavior is that property values are replaced:

For Numbers1, since it's read-only (no setter), it still has the values 1, 2, and 3 in its list.
For Numbers2, which is read-write, a new list is allocated and the values from the JSON are added.

For example, if you execute the following deserialization code, Numbers1 contains the values 1, 2, and 3 and Numbers2 contains the values 4, 5, and 6.

A? a = JsonSerializer.Deserialize<A>("""{"Numbers1": [4,5,6], "Numbers2": [4,5,6]}""");

Populate behavior

Starting in .NET 8, you can change the deserialization behavior to modify (populate) properties and fields instead of replace them:

For a collection type property, the object is reused without clearing. If the collection is prepopulated with elements, they'll show in the final deserialized result along with the values from the JSON. For an example, see Collection property example.
For a property that's an object with properties, its mutable properties are updated to the JSON values but the object reference itself doesn't change.
For a struct type property, the effective behavior is that for its mutable properties, any existing values are kept and new values from the JSON are added. However, unlike a reference property, the object itself isn't reused since it's a value type. Instead, a copy of the struct is modified and then reassigned to the property. For an example, see Struct property example.

A struct property must have a setter; otherwise, an InvalidOperationException is thrown at run time.

Note

The populate behavior currently doesn't work for types that have a parameterized constructor. For more information, see dotnet/runtime issue 92877.

Read-only properties

For populating reference properties that are mutable, since the instance that the property references isn't replaced, the property doesn't need to have a setter. This behavior means that deserialization can also populate read-only properties.

Note

Struct properties still require setters because the instance is replaced with a modified copy.

Collection property example

Consider the same class A from the replace behavior example, but this time annotated with a preference for populating properties instead of replacing them:

[JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
class A
{
    public List<int> Numbers1 { get; } = [1, 2, 3];
    public List<int> Numbers2 { get; set; } = [1, 2, 3];
}

If you execute the following deserialization code, both Numbers1 and Numbers2 contain the values 1, 2, 3, 4, 5, and 6:

A? a = JsonSerializer.Deserialize<A>("""{"Numbers1": [4,5,6], "Numbers2": [4,5,6]}""");

Struct property example

The following class contains a struct property, S1, whose deserialization behavior is set to Populate. After executing this code, c.S1.Value1 has a value of 10 (from the constructor) and c.S1.Value2 has a value of 5 (from the JSON).

C? c = JsonSerializer.Deserialize<C>("""{"S1": {"Value2": 5}}""");

class C
{
    public C()
    {
        _s1 = new S
        {
            Value1 = 10
        };
    }

    private S _s1;

    [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
    public S S1
    {
        get { return _s1; }
        set { _s1 = value; }
    }
}

struct S
{
    public int Value1 { get; set; }
    public int Value2 { get; set; }
}

If the default Replace behavior was used instead, c.S1.Value1 would have its default value of 0 after deserialization. That's because the constructor C() would be called, setting c.S1.Value1 to 10, but then the value of S1 would be replaced with a new instance. (c.S1.Value2 would still be 5, since the JSON replaces the default value.)

How to specify

There are multiple ways to specify a preference for replace or populate:

Use the JsonObjectCreationHandlingAttribute attribute to annotate at the type or property level. If you set the attribute at the type level and set its Handling property to Populate, the behavior will only apply to those properties where population is possible (for example, value types must have a setter).

If you want the type-wide preference to be Populate, but want to exclude one or more properties from that behavior, you can add the attribute at the type level and again at the property level to override the inherited behavior. That pattern is shown in the following code.
```
// Type-level preference is Populate.
[JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
class B
{
    // For this property only, use Replace behavior.
    [JsonObjectCreationHandling(JsonObjectCreationHandling.Replace)]
    public List<int> Numbers1 { get; } = [1, 2, 3];
    public List<int> Numbers2 { get; set; } = [1, 2, 3];
}
```
Set JsonSerializerOptions.PreferredObjectCreationHandling (or, for source generation, JsonSourceGenerationOptionsAttribute.PreferredObjectCreationHandling) to specify a global preference.
```
var options = new JsonSerializerOptions
{
    PreferredObjectCreationHandling = JsonObjectCreationHandling.Populate
};
```

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