Why Does My C# P/Invoke Call With `CallingConvention.Cdecl` Fail When Calling a C `__stdcall` Function?

Troubleshooting C# P/Invoke Calling Convention Mismatches

This article explains a common problem when using Platform Invoke (P/Invoke) to interact between C# and C code. We'll focus on the issue of mismatched calling conventions: CallingConvention.Cdecl in C# versus __stdcall in C .

Calling Conventions Explained

In 32-bit environments, calling conventions dictate parameter passing, stack storage, and cleanup. Key conventions include:

• __stdcall: The caller pushes arguments; the callee cleans the stack. Used by Windows APIs and COM.
• __cdecl: The caller pushes arguments and cleans the stack. Standard for C.
• __fastcall: Uses registers for some arguments, potentially improving speed but reducing compatibility.
• __thiscall: Similar to __cdecl, but handles the "this" pointer in C member functions.
• __clrcall: A managed calling convention balancing elements from others, preventing stack issues.

Code Example Analysis

Consider a C function:

<code class="language-c++">extern "C" int __stdcall InvokedFunction(int);</code>

And its C# P/Invoke counterpart:

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

The Convention Clash

The problem stems from the CallingConvention.Cdecl in C# conflicting with the __stdcall in C . This leads to stack imbalances. While __stdcall is often the P/Invoke default (due to its Windows API prevalence), interfacing with __cdecl C code requires explicit convention specification.

Resolution

The solution is simple: align the C# calling convention with the C one:

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

Key Takeaway

Understanding calling conventions is vital for successful cross-language interoperability. Matching conventions prevents stack errors and ensures smooth C# and C integration.

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