.NET 8 – Handle missing members during JSON deserialization

By default, if you have additional properties in a JSON payload you are trying to deserialize, they are simply ignored. But what if you wanted the deserialization to fail and throw an exception when there are extra properties in the JSON? That is possible starting with .NET 8.

There are several ways in which you can opt in for this behavior when using the System.Text.Json serializer.

1. Using the JsonUnmappedMemberHandling attribute

You can annotate your type with the [System.Text.Json.Serialization.JsonUnmappedMemberHandlingAttribute], passing your option as a parameter.

[JsonUnmappedMemberHandling(JsonUnmappedMemberHandling.Disallow)]
public class Foo
{
     public int Bar { get; set; }
}

2. Using JsonSerializerOptions

You can set the JsonSerializerOptions.UnmappedMemberHandling property to Disallow and pass it along to the Deserialize method.

new JsonSerializerOptions 
{ 
    UnmappedMemberHandling = JsonUnmappedMemberHandling.Disallow 
};

An exception is thrown

Be ready to catch it. With JsonUnmappedMemberHandling set to Disallow, the following exception will be thrown when deserializing a JSON payload with additional members.

System.Text.Json.JsonException: ‘The JSON property ‘’ could not be mapped to any .NET member contained in type ‘+’.’