Breaking change: ArgumentNullException thrown when null is passed as input type to some JsonSerializer serialize methods

[layomia]

In .NET Core 3.1 and versions 4.6.0-4.7.2 of the System.Text.Json library, null could be passed as the type argument to various serialization methods on JsonSerializer, if the value to serialize was also null. In .NET 5/version 5.0.0, an ArgumentNullException is thrown if null is passed as the type argument.

Version introduced

.NET 5

Old behavior

In .NET Core 3.1 and versions 4.6.0-4.7.2 of the System.Text.Json library, null could be passed as the type argument to various serialization methods on JsonSerializer, if the value to serialize was also null.

New behavior

In .NET 5/version 5.0.0, an ArgumentNullException is thrown if null is passed as the type argument.

Reason for change

null is not expected as a valid input for the type arguments to the various serialization methods. The change was made to ensure correctness and prevent unexpected bugs.

Recommended action

Ensure that a non-null type argument is passed to applicable serialization methods on JsonSerializer. Strategies include handwriting null for values which are null and their type cannot be determined.

For example,

string json;

object? obj = GetObject();
Type? type = obj?.GetType();

if (obj == null)
{
   json = "null";
}
else
{
    json = JsonSerializer.Serialize(obj, type!);
}

Category

• ASP.NET Core
• C#
• Code analysis
• Core .NET libraries
• Cryptography
• Data
• Debugger
• Deployment
• Globalization
• Interop
• JIT
• LINQ
• Managed Extensibility Framework (MEF)
• MSBuild
• Networking
• Printing
• Security
• Serialization
• Visual Basic
• Windows Forms
• Windows Presentation Foundation (WPF)
• XML, XSLT

Affected APIs

• JsonSerializer.Serialize(Object, Type, JsonSerializerOptions)
• JsonSerializer.SerializeToUtf8Bytes(Object, Type, JsonSerializerOptions)
• JsonSerializer.Serialize(Utf8JsonWriter, Object, Type, JsonSerializerOptions)
• JsonSerializer.SerializeAsync(Stream, Object, Type, JsonSerializerOptions, CancellationToken)

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Issue metadata

• Issue type: breaking-change

[layomia]

cc @gewarren, @tdykstra

[layomia]

Looks like this was filed in #20823.