如何使用DLLImport将双精度值传递给C ++参数？

Question

I have a C++ method and it takes a parameter of type double eg

extern "C" {
    __declspec(dllexport) void __cdecl GetResult (double resultLine);
}

On the C# side, I can call the method but it always converts the resultLine value to 0. I am calling the C++ DLL method using DLLImport's extern feature:

internal static class UnsafeNativeMethods
{
const string _dllLocation = "CoreDLL.dll";
[DllImport(_dllLocation)]
public static extern void GetResult(double resultLine);
}

I can call the other methods that take the string value and everything works but for some reason, it doesn't work when I pass the double value.

Is that the correct way to pass the double value? or do I need to use ref/out etc?

Answer 1

I'm not 100% sure this is related, but the fundamental mistake I see here is that you got the calling convention wrong. If you don't specify anything, the DllImport attribute will use the Winapi calling convention, which is actually __stdcall , while you need __cdecl .

To fix this problem set the CallingConvention field to CallingConvention.Cdecl :

internal static class UnsafeNativeMethods
{
const string _dllLocation = "CoreDLL.dll";
[DllImport(_dllLocation), CallingConvention=CallingConvention.Cdecl]
public static extern void GetResult(double resultLine);
}

or set the calling convention to __stdcall on the C++ side (if you don't need vararg __stdcall is also a bit more efficient than __cdecl ).

Answer 2

You have a mismatch of calling conventions, cdecl on one side and stdcall on the other. Make them match and you should be back in business.