Passing string from .NET to native (64-bit)

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.

David Heffernan · Accepted Answer · 2015-08-18 09:25:58Z

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.

edited Aug 18, 2015 at 9:25

answered Aug 18, 2015 at 8:30

David Heffernan

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8 Comments

Vegard Larsen Over a year ago

David, thanks for your comment, but this is the case. I have reproduced a minimal solution and put it in my Dropbox. Run it without debugging, and then attach the debugger (with native support) after it has started, but before you press the button.

David Heffernan Over a year ago

Please include your minimal repro in the question. Off site links are no good. When I use the code in my answer, it works correctly.

Vegard Larsen Over a year ago

I've added a link to the solution in the question. There are quite a few steps to set it up to fail, so I suggest looking at the solution and seeing it fail.

David Heffernan Over a year ago

Off site links are not very useful. I don't know why you won't make an MCVE here.

David Heffernan Over a year ago

This would appear to be a debugger defect.

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