Runtime: [System.Text.Json] Dictionary is not supported

[mrpmorris]

The following code

string json = System.Text.Json.JsonSerializer.Serialize(new Dictionary<int, int>());
Throws an exception

System.NotSupportedException
  HResult=0x80131515
  Message=The collection type 'System.Collections.Generic.Dictionary`2[System.Int32,System.Int32]' is not supported.
  Source=System.Text.Json
  StackTrace:
   at System.Text.Json.JsonClassInfo.GetElementType(Type propertyType, Type parentType, MemberInfo memberInfo, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo.CreateProperty(Type declaredPropertyType, Type runtimePropertyType, Type implementedPropertyType, PropertyInfo propertyInfo, Type parentClassType, JsonConverter converter, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo.AddProperty(Type propertyType, PropertyInfo propertyInfo, Type classType, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo.AddPolicyProperty(Type propertyType, JsonSerializerOptions options)
   at System.Text.Json.JsonClassInfo..ctor(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type classType)
   at System.Text.Json.WriteStackFrame.Initialize(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.WriteCore(Utf8JsonWriter writer, Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.WriteCore(PooledByteBufferWriter output, Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.WriteCoreString(Object value, Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
   at ConsoleApp18.Program.Main(String[] args) in C:\Users\x\source\repos\ConsoleApp18\ConsoleApp18\Program.cs:line 11

[artelk]

The Serialize method has an optional parameter JsonSerializerOptions.
You can provide a "Converter": https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to?view=netcore-3.1#registration-sample---converters-collection

[alanisaac]

My understanding is that this is currently by design in 3.x.

There's a good discussion of this issue you can follow in #30524. This may be a duplicate of that issue (though the issue title could be updated for clarity).

The current state is that there is a proposal in progress, see https://github.com/dotnet/runtime/pull/32676