Why Do __stdcall C and Cdecl C# Calls Conflict in P/Invoke, and How Can This Be Resolved?

Understanding Cdecl/__stdcall Conflicts in P/Invoke

P/Invoke, bridging C# and C , relies on matching calling conventions to ensure proper argument passing and stack management. A common problem arises when a C function using the __stdcall convention is called from C# using the default (often cdecl) P/Invoke convention.

The Root of the Problem

Different calling conventions handle stack cleanup differently:

• __stdcall: Used extensively in Windows APIs and COM, the callee (C function) is responsible for cleaning up the stack.
• __cdecl: The default C calling convention, where the caller (C# code) handles stack cleanup.
• __thiscall: Used for C member functions; it's not directly supported in P/Invoke due to its complexity.

Consequences of the Mismatch

Simply adding extern "C" to the C code and specifying CallingConvention.Cdecl in the C# DllImport attribute won't fix the problem. The differing cleanup responsibilities lead to stack corruption, potentially causing crashes or subtle, hard-to-debug errors, even without immediate debugger warnings. The stack might be cleaned up twice, leading to unpredictable behavior.

The Solution: Consistent Conventions

The key is consistency. Modify the C function declaration to explicitly use __cdecl: Replace extern "C" int __stdcall InvokedFunction(int); with extern "C" int __cdecl InvokedFunction(int);. Then, ensure your C# DllImport attribute matches:

[DllImport("CPlusPlus.dll", ExactSpelling = true, SetLastError = true, CallingConvention = CallingConvention.Cdecl)]

Key Takeaway

Careful attention to calling conventions is essential for reliable P/Invoke interoperability. Using consistent conventions between C and C# eliminates stack imbalances and improves the overall robustness of your code.

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