将字符串从.NET传递到本机（64位）

Question

I am having an issue passing a simple string from my .NET app, compiled as 64-bit, to my native DLL, also compiled as 64-bit.

C++ signature:

DllExport void SetFoo(LPWSTR foo);

C# signature:

[DllImport(Library, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
internal static extern void SetFoo(
     [In][MarshalAs(UnmanagedType.LPWStr)] string foo);

Then I run the following code in C#:

X.SetFoo("some string");

When it reaches the debugger in the DLL, the value is swearing at me in Chinese: Ⴐ虘翺

When I change both the .NET and native code to 32-bit, the value I get in the debugger is the correct string. Where am I being stupid?

Minimal reproduction as a Visual Studio 2015 Solution: download here

To reproduce:

• Create a new Visual Studio solution with a WinForms project.
• Add a Win32 Project, of type DLL to the solution
• Add the following files:

Foo.h:

extern "C" {
    __declspec( dllexport ) void SetFoo(LPWSTR);
}

Foo.cpp:

#include "stdafx.h"
#include "Foo.h"

DllExport void SetFoo(LPWSTR foo) 
{

}

• Set a breakpoint on the opening brace in SetFoo
• Add a button to the winforms form
• Double click it, and call SetFoo("abc123") .
• Implement SetFoo :

Form1.cs:

[DllImport("Win32Project1.dll", CallingConvention = CallingConvention.Cdecl,  CharSet = CharSet.Unicode)]
public static extern void SetFoo(string text);

• Change the apps to build in 64-bit mode by opening Configuration Manager.
• Set Win32Project1 to build as x64
• Set WindowsFormApplication1 to build as x64, by picking platform new, pick x64, OK.
• Change the output directory of WindowsFormsApplication1 to match the output directory of the other app.
• Start without debugging.
• Attach debugger (Ctrl+Alt+P) by setting Attach to to Managed (v4.5, v4.0) code, Native code and finding the process.
• Observe value at breakpoint.

Answer 1

When you interpret ANSI encoded latin text as UTF-16 you see Chinese characters. That's clearly what is happening. So your C# code is sending ANSI encoded text somehow.

The p/invoke would be better written like this:

[DllImport(Library, CallingConvention = CallingConvention.Cdecl, 
    CharSet = CharSet.Unicode)]
internal static extern void SetFoo(string foo);

The [in] is not needed, it is the default. And the MarshalAs is pointless since you specified CharSet.Unicode . However, neither change affects the semantics.

The only sound explanations for what you describe in the question are:

• The actual code is not as you have described it, or
• There is a defect in the debugger.

I suggest that you change the unmanaged function to

DllExport void SetFoo(LPWSTR foo) 
{
    MessageBoxW(0, L"", foo, MB_OK);
}

If the message box displays the correct text then the conclusion would appear to be that the debugger is defective.