How to Resolve Error "CS0630: Member cannot implement interface member because it has an '__arglist' parameter" in C#

The Compiler Error CS0630 is a specific restriction error involving the undocumented __arglist keyword. The message reads: "Member cannot implement interface member because it has an __arglist parameter".

In C#, the __arglist keyword is a legacy feature used primarily for interoperability with unmanaged C/C++ functions that accept a variable number of arguments (like printf). It is not part of the standard Common Language Specification (CLS) and does not map to standard .NET types. Because Interfaces define strict, type-safe contracts for .NET objects, they cannot support the unmanaged, untyped nature of __arglist.

This guide explains the incompatibility between interfaces and __arglist and how to replace it with standard C# features.

Understanding __arglist vs. Interfaces

• Interfaces: Define a contract for managed code. Every method signature must be verifiable by the .NET Runtime.
• __arglist: Creates a call site compatible with the unmanaged __cdecl calling convention (variable arguments). It effectively bypasses standard .NET type safety.

Because __arglist relies on specific stack manipulation that differs from standard method calls, the runtime cannot map an interface call (which uses virtual dispatch) to a method expecting an __arglist.

Scenario: Attempting to use __arglist in an Implementation

This error occurs when you have an interface method (even one that looks like it accepts arguments) and you try to satisfy that contract using a method declared with __arglist.

Example of error:

public interface ILogger
{
    // The interface expects a standard method signature
    void Log(string format, params object[] args);
}

public class NativeLogger : ILogger
{
    // ⛔️ Error CS0630: 'NativeLogger.Log' cannot implement interface member 
    // 'ILogger.Log' because it has an __arglist parameter.
    public void Log(string format, __arglist)
    {
        // ... implementation wrapping a C++ call ...
    }
}

Even though both concepts represent "variable arguments," they are structurally incompatible.

Solution 1: Use params object[] (Managed Approach)

If your goal is simply to accept a variable number of arguments in C#, do not use __arglist. Use the standard params keyword. This works perfectly with interfaces.

Solution: update the class to use the managed array syntax.

public interface ILogger
{
    void Log(string format, params object[] args);
}

public class ManagedLogger : ILogger
{
    // ✅ Correct: Signature matches the interface and uses managed types.
    public void Log(string format, params object[] args)
    {
        // You can iterate over 'args' safely
        Console.WriteLine(string.Format(format, args));
    }
}

Solution 2: Decouple from Interface (Interop Approach)

If you must use __arglist (because you are calling a specific external C API like printf via P/Invoke) and you cannot change the signature, you simply cannot make this method part of an interface implementation.

You must remove the interface relationship or implement the interface using a "bridge" method.

Solution: The Bridge Pattern

Implement the interface using standard types, and call the __arglist method internally.

public interface ILogger
{
    void Log(string format, params object[] args);
}

public class InteropLogger : ILogger
{
    // 1. The Interface Implementation (Safe)
    public void Log(string format, params object[] args)
    {
        // You cannot pass 'args' directly to __arglist easily, 
        // but typically you don't expose __arglist methods via interfaces anyway.
        // You would perform specific logic here.
        Console.WriteLine("Interface called");
    }

    // 2. The Internal Unmanaged Method (Unsafe/Interop)
    // This is NOT part of the interface.
    public void LogNative(string format, __arglist)
    {
        // Call external DLL here
    }
}

Conclusion

CS0630 ensures that interface contracts remain safe and verifiable.

1. Avoid __arglist: Unless you are doing deep P/Invoke integration with C++ vararg functions, do not use this keyword.
2. Use params: For 99% of "variable argument" scenarios in C#, params T[] is the correct solution.
3. Separate Concerns: If you need __arglist for interop, keep that method separate from your public interfaces.