A Json.NET serialization binder to allow providing a custom type name using an attribute. License: MIT

JsonTypeNameSerializationBinder.cs

// ==========
// Usage:
[JsonTypeName("CustomNameHere")]
public class Usage
{
  public string Example { get; set; } = "Hello World";
}
// Serialize:
JsonConvert.SerializeObject(new Usage(),
                            new JsonSerializerSettings()
                            {
                                SerializationBinder = new JsonTypeNameSerializationBinder(),
                                TypeNameHandling = TypeNameHandling.All,
                                Formatting = Formatting.Indented
                            });
// Generates:
{
  "$type": "CustomNameHere",
  "Example": "Hello World"
}
// ==========

/// <summary>
/// Set a custom type name for use during serialization and deserialization with Json.NET.
/// </summary>
[AttributeUsage(AttributeTargets.Class)]
public class JsonTypeNameAttribute : Attribute
{
  public string TypeName { get; }

  public JsonTypeNameAttribute(string typeName)
  {
    TypeName = typeName;
  }
}

/// <summary>
/// A serialization binder for use with the <see cref="JsonTypeNameAttribute"/> on classes in the declared or specified assembly.
/// </summary>
public class JsonTypeNameSerializationBinder : DefaultSerializationBinder
{
  private readonly Dictionary<string, Type> _nameToType;
  private readonly Dictionary<Type, string> _typeToName;

  public JsonTypeNameSerializationBinder(Assembly assembly = null)
  {
    (Type type, string name)[] customNames = GetCustomTypeNames().ToArray();
    _nameToType = customNames.ToDictionary(t => t.name, t => t.type);
    _typeToName = customNames.ToDictionary(t => t.type, t => t.name);

    IEnumerable<(Type type, string name)> GetCustomTypeNames()
    {
      foreach (Type type in assembly?.GetTypes() ?? this.GetType().Assembly.GetTypes())
      {
        foreach (object customAttribute in type.GetCustomAttributes(false))
        {
          if (customAttribute is JsonTypeNameAttribute jsonTypeNameAttribute)
          {
            yield return (type, jsonTypeNameAttribute.TypeName);
            break;
          }
        }
      }
    }
  }

  public override void BindToName(Type serializedType, out string assemblyName, out string typeName)
  {
    if (_typeToName.TryGetValue(serializedType, out string name))
    {
      assemblyName = null;
      typeName = name;
    }
    else
    {
      base.BindToName(serializedType, out assemblyName, out typeName);
    }
  }

  public override Type BindToType(string assemblyName, string typeName)
  {
    if (_nameToType.TryGetValue(typeName, out Type type))
    {
      return type;
    }
    else
    {
      return base.BindToType(assemblyName, typeName);
    }
  }
}