How to Resolve Error "CS0224: A method with vararg cannot be generic, be in a generic type, or have a params parameter" in C#

The Compiler Error CS0224 is a compatibility error involving the legacy variable argument list keyword (__arglist). The message reads: "A method with vararg cannot be generic, be in a generic type, or have a params parameter".

In C#, there are two ways to handle variable numbers of arguments:

1. params (Standard): A managed array (e.g., params object[] args). This is type-safe and standard .NET.
2. __arglist (Undocumented/Legacy): A keyword that maps to the C-style varargs calling convention. This is primarily used for advanced Interop scenarios (like calling C++ functions such as printf).

CS0224 occurs because the __arglist feature is extremely low-level and does not support the complexity of modern .NET features like Generics or mixing with the managed params array.

Understanding vararg vs params

Most C# developers use params to pass a variable number of arguments. The error message mentions "vararg", which specifically refers to the __arglist keyword.

• Allowed: void MyMethod(params int[] args)
• Allowed: void MyMethod(__arglist) (In specific non-generic contexts)
• Forbidden: Mixing __arglist with generics <T> or params.

Unless you are doing specific P/Invoke calls to unmanaged code, you likely triggered this error by experimenting with undocumented keywords or copying legacy code.

Scenario 1: Mixing Generics with __arglist

The Common Language Runtime (CLR) cannot handle the combination of a generic type definition and the unmanaged variable argument list structure.

Example of error (Generic Method)

public class InteropWrapper
{
    // ⛔️ Error CS0224: A method with vararg cannot be generic.
    // You cannot combine <T> with __arglist.
    public void CallNative<T>(__arglist)
    {
    }
}

Example of error (Method inside Generic Class)

Even if the method itself isn't generic, if it lives inside a generic class, it cannot use __arglist.

public class GenericWrapper<T>
{
    // ⛔️ Error CS0224: A method with vararg cannot be in a generic type.
    public void CallNative(__arglist)
    {
    }
}

Scenario 2: Mixing params with __arglist

You cannot have two different mechanisms for variable arguments in the same method signature.

Example of error:

public class Program
{
    // ⛔️ Error CS0224: A method with vararg cannot have a params parameter.
    public static void HybridMethod(params int[] numbers, __arglist)
    {
    }
}

Solution: Use params object[]

If you are writing C# code (not calling a C++ DLL), simply replace __arglist with the standard params object[]. This supports generics, is type-safe, and works everywhere.

Solution

public class GenericWrapper<T>
{
    // ✅ Correct: Use standard C# params. 
    // This works perfectly inside a generic class.
    public void LogData(params object[] args)
    {
        foreach (var arg in args)
        {
            System.Console.WriteLine(arg);
        }
    }
}

Alternative Solution: Non-Generic Wrapper (Interop)

If you must use __arglist (e.g., because you are calling a specific C function via P/Invoke), you must move that method out of any generic class and remove any generic parameters from it.

// ✅ Correct: A standard non-generic class
public class NativeInterop
{
    // Valid only in a non-generic context
    public static void CallPrintf(string format, __arglist)
    {
        // Interop logic here...
    }
}

Conclusion

CS0224 is a restriction on a niche feature.

1. Identify the Keyword: Look for __arglist in your code.
2. Determine Intent:
   • If you just want variable arguments in C#, delete __arglist and use params object[].
   • If you strictly need __arglist for Interop, ensure the method and its containing class are not generic.