How to Resolve Error "CS0257: An '__arglist' parameter must be the last parameter in a parameter list" in C#

The Compiler Error CS0257 is a syntax restriction error related to the legacy __arglist keyword. The message reads: "An __arglist parameter must be the last parameter in a formal parameter list".

The __arglist keyword allows methods to accept a variable number of arguments, similar to C-style varargs (e.g., printf). Because this mechanism allows the caller to push an arbitrary number of arguments onto the stack, the compiler must know where the "fixed" (known) arguments stop and the "variable" arguments begin. To ensure this, the variable argument list must always be the last item in the method signature.

This guide explains how to correctly define methods using this rare interoperability feature.

Understanding __arglist Positioning

The __arglist keyword behaves similarly to the params keyword regarding ordering rules. It tells the runtime: "Everything else provided by the caller goes here."

• Correct: void MyMethod(int a, __arglist)
• Incorrect: void MyMethod(__arglist, int a)

If you place a specific parameter (like int a) after __arglist, the compiler cannot determine which arguments belong to the variable list and which one is meant for the final integer.

Scenario: Placing Parameters After __arglist

This error occurs when defining a method (usually for P/Invoke) and incorrectly ordering the arguments.

Example of error:

public class Interop
{
    // ⛔️ Error CS0257: An __arglist parameter must be the last parameter.
    // The compiler doesn't know when the variable arguments end 
    // and 'format' begins.
    public static void Log(__arglist, string format)
    {
    }
}

Solution: Reorder the Parameters

Always move the __arglist keyword to the very end of the parameter list.

Solution:

public class Interop
{
    // ✅ Correct: Fixed arguments first, variable arguments last.
    public static void Log(string format, __arglist)
    {
        // Implementation logic...
    }
}

public class Program
{
    static void Main()
    {
        // Usage
        Interop.Log("Value: %d", __arglist(10));
    }
}

note

Is this recommended? Unless you are specifically calling unmanaged C/C++ functions that require the __cdecl calling convention with variable arguments, you should avoid __arglist. For standard C# variable arguments, use the type-safe params object[] syntax instead.

Conclusion

CS0257 enforces a predictable stack layout for function calls.

1. Check the Signature: Look at your method definition.
2. Move __arglist: Cut the keyword and paste it as the final argument in the parentheses.