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How to Resolve Error "CS0735: Invalid type specified as an argument for TypeForwardedTo attribute" in C#

The Compiler Error CS0735 is a configuration error related to Type Forwarding. The message reads: "Invalid type specified as an argument for TypeForwardedTo attribute".

Type Forwarding is a mechanism used to move a type from one assembly to another without breaking existing applications that reference it. You leave a "signpost" attribute ([TypeForwardedTo]) in the old assembly pointing to the new location. However, this attribute strictly requires a valid, fully defined type (a System.Type object). This error occurs when you pass something that is not a valid type definition, such as a generic open type without the proper syntax, or a type that cannot be resolved in the current context.

This guide explains the correct syntax for forwarding types.

Understanding Type Forwarding Arguments​

The [TypeForwardedTo] attribute signature looks like this: public TypeForwardedToAttribute(Type destination)

It expects a System.Type object. In C#, you obtain this using the typeof() operator.

  • Valid: typeof(MyClass), typeof(List<int>), typeof(List<>).
  • Invalid: MyClass (without typeof), List<> (raw syntax), or 0 (null/invalid inputs).

CS0735 usually indicates a syntax error inside the attribute parentheses where the compiler expected a type token but found something else.

Scenario: Forwarding Generic Types Incorrectly​

This error is most common when forwarding Generic Types. Developers often struggle with the syntax for "Open Generics" (a generic type without specific arguments, like List<T>).

Example of error: attempting to forward a generic type using invalid syntax inside typeof.

using System.Runtime.CompilerServices;

// ⛔️ Error CS0735: Invalid type specified.
// You cannot use 'typeof(MyGenericClass<T>)' here because 'T' is not defined.
// You also cannot simply write 'MyGenericClass' if it requires type parameters.
[assembly: TypeForwardedTo(typeof(MyGenericClass<T>))]

namespace OldAssembly
{
    // The class definition has been moved to another assembly.
}

Solution: Use typeof Correctly​

To fix this, you must use the correct C# syntax for obtaining a type token.

Forwarding a Standard Class​

Simply wrap the class name in typeof().

using System.Runtime.CompilerServices;

// ✅ Correct: Standard type forwarding
[assembly: TypeForwardedTo(typeof(MyMovedClass))]

Forwarding an Open Generic Class​

If you are forwarding public class MyGeneric<T>, use the "unbound" syntax with empty angle brackets.

using System.Runtime.CompilerServices;

// ✅ Correct: The empty brackets '<>' represent the open generic definition.
[assembly: TypeForwardedTo(typeof(MyGenericClass<>))]

Forwarding a Multi-Parameter Generic​

If the class is MyMap<TKey, TValue>, use commas to indicate the number of parameters.

using System.Runtime.CompilerServices;

// ✅ Correct: One comma means two parameters.
[assembly: TypeForwardedTo(typeof(MyMap<,>))]
note

Resolution Requirement: For typeof(MyClass) to work, the compiler must be able to find MyClass. Since you deleted the code from the current assembly (to move it), you must reference the destination assembly (where the code now lives) in your project references. If the compiler can't find the type at all, you will get a different error (CS0246).

Conclusion​

CS0735 is a syntax check on the attribute argument.

  1. Check the Operator: Ensure you are using typeof(...).
  2. Check Generics:
    • Do not put T inside the brackets.
    • Use <> for List<T>.
    • Use <,> for Dictionary<K,V>.

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