How to pass strings from C# to C++ (and from C++ to C#) using DLLImport?

Question

I've been trying to send a string to/from C# to/from C++ for a long time but didn't manage to get it working yet ...

So my question is simple :
Does anyone know some way to send a string from C# to C++ and from C++ to C# ?
(Some sample code would be helpful)

Answer 1

Passing string from C# to C++ should be straightforward - PInvoke will manage the conversion for you.

Getting a string from C++ to C# can be done using a StringBuilder. You need to get the length of the string in order to create a buffer of the correct size.

Here are two examples of a well-known Win32 API:

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

public static string GetText(IntPtr hWnd)
{
    // Allocate correct string length first
    int length = GetWindowTextLength(hWnd);
    StringBuilder sb = new StringBuilder(length + 1);
    GetWindowText(hWnd, sb, sb.Capacity);
    return sb.ToString();
}

[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
public static extern bool SetWindowText(IntPtr hwnd, String lpString);
SetWindowText(Process.GetCurrentProcess().MainWindowHandle, "Amazing!");