Marshalling structs from WM_COPYDATA messages

I am trying to get a C# WPF application to communicate with another application written in C using WM_COPYDATA. The C app is trying to send a struct as follows:

typedef struct
{
    int x;
    int y;
    char str[40];
    double d;
    char c;
} DATASTRUCT;

In my C# app I have defined a struct as follows:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct DATASTRUCT
{
    public int x;
    public int y;
    [MarshalAs(UnmanagedType.LPStr, SizeConst=40)]
    public string s;
    public double d;
    public char c;
};

And the code to receive the WM_COPYDATA message is as follows:

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    hwndSource = HwndSource.FromHwnd(new WindowInteropHelper(this).Handle);
    hwndSource.AddHook(new HwndSourceHook(WndProc));
}

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    if (msg == 0x4A)
    {
        DATASTRUCT data = (DATASTRUCT)Marshal.PtrToStructure(lParam, typeof(DATASTRUCT));
        this.updateText(data);
        handled = true;
    }

    return (IntPtr)0;
}

I am receiving messages from the C application, but all the data in the struct is gibberish. Previous to this I was able to manually extract a开发者_开发百科n array of bytes from the lParam pointer and then use System.BitConverter and System.Text.Encoding.ACII to interpret the byte array, and that worked pretty well. But now I am trying to do it in a cleaner way and it's just not working.

After a long time searching for the answer to this question, I realized that I was missing a VERY important step. I feel like an idiot, but here's the answer to my own question.

The C# struct should look like this:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public unsafe struct DataStruct
{
    public int x;
    public int y;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst=40)]
    public string s;
    public double d;
    public char c;
};

And another struct must be defined to receive the WM_COPYDATA info. It looks like this:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public unsafe struct CopyDataStruct
{
    public IntPtr dwData;
    public int cbData;
    public IntPtr lpData;
}

And the WndProc method should be changed to look like this:

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    if (msg == 0x4A)
    {
        CopyDataStruct cps = (CopyDataStruct)Marshal.PtrToStructure(lParam, typeof(CopyDataStruct));
        DataStruct data = (DataStruct)Marshal.PtrToStructure(cps.lpData, typeof(DataStruct));
        updateText(data);
        handled = true;
    }

    return (IntPtr)0;
}

I was using the CopyDataStruct in my previous working solution, and I just forgot to use it in the newer version.

Part of the problem is that the str member needs to be a ByValTStr not a LPSTR since it's an inlined string array. Try the following definition

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct DATASTRUCT
{
    public int x;
    public int y;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst=40)]
    public string s;
    public double d;
    public char c;
};