Passing String from Native C++ DLL to C# App

Question

I have written a DLL in C++. One of the functions writes to a character array. C++ Function ``` EXPORT int xmain(int argc, char argv[], char argv2[]) { char pTypeName[4096]; ... //Other pTypeName ends up populated with "Portable Network Graphics" //This code verifies that pTypeName is populated with what I think it is: char szBuff[64]; sprintf(szBuff, pTypeName, 0); MessageBoxA(NULL, szBuff, szBuff, MB_OK); //The caption and title are "Portable Network Graphics" ... //Here, I attempt to copy the value in pTypeName to parameter 3. sprintf(argv2, szBuff, 0); return ret; } ``` C# Import ``` //I believe I have to use CharSet.Ansi because by the C++ code uses char[], [DllImport("FirstDll.dll", CharSet=CharSet.Ansi)] public static extern int xmain(int argc, string argv, ref string zzz); ``` C# Function ``` private void button2_Click(object sender, EventArgs e) { string zzz = ""; int xxx = xmain(2, @"C:\hhh.bmp", ref zzz); MessageBox.Show(zzz); //The message box displays //MessageBox.Show di…

Accepted Answer

Use a `StringBuilder` to pass a character array that native code can fill in (see Fixed-Length String Buffers).

Declare the function:

```
[DllImport("FirstDll.dll", CharSet=CharSet.Ansi)]
public static extern int xmain(int argc, string argv, StringBuilder argv2);
```

Use it:

```
// allocate a StringBuilder with enough space; if it is too small,
// the native code will corrupt memory
StringBuilder sb = new StringBuilder(4096);
xmain(2, @"C:\hhh.bmp", sb);
string argv2 = sb.ToString();
```

Passing String from Native C++ DLL to C# App

Solution

Problem