Why Do Cdecl and Stdcall Calling Conventions Mismatch in C# P/Invoke, and How Can This Be Fixed?

Understanding C# P/Invoke Calling Convention Conflicts: Cdecl vs. Stdcall

Platform Invoke (P/Invoke) bridges managed (C#) and unmanaged (e.g., C ) code. However, discrepancies in calling conventions can cause interoperability issues.

The Source of the Problem

The conflict stems from differing calling convention handling between C and the default P/Invoke settings in C#.

• C : C functions often employ the __stdcall convention. The called function (callee) is responsible for stack cleanup.
• C# P/Invoke Default: C#'s P/Invoke defaults to CallingConvention.Winapi, which, on Windows, is equivalent to __stdcall.

Key Calling Convention Differences:

The core differences lie in:

• Convention Keyword: __stdcall in C versus CallingConvention.Winapi (or CallingConvention.Stdcall) in C#.
• Stack Management: __cdecl (often used in C) requires the calling function (caller) to clean the stack; __stdcall assigns this to the callee.
• Argument Order: While less common as a source of error in this specific context, argument order can vary between conventions.

Incorrect P/Invoke Declaration Example:

An incorrect C# declaration might use CallingConvention.Cdecl when the C function uses __stdcall. This leads to stack corruption and runtime errors.

The Correct C# Declaration:

To ensure compatibility, the C# DllImport attribute must match the C function's convention:

<code class="language-csharp">[DllImport("CPlusPlus.dll", ExactSpelling = true, SetLastError = true, CallingConvention = CallingConvention.Winapi)]
private static extern int InvokedFunction(IntPtr intArg);</code>

Preventing Convention Mismatches:

Always confirm the unmanaged function's calling convention and mirror it precisely in your C# DllImport declaration. Consult Microsoft's documentation for detailed information on supported calling conventions.

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