How to receive a string from a native dll?

//C++ function syntax ->     int GetWindowText( HWND hWnd, LPTSTR lpString, int nMaxCount );
[System.Runtime.InteropServices.DllImport("user32.dll")]
static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

//Form's button click event.
private void button1_Click(object sender, EventArgs e)
{
    const int nMaxCount = 255;
    StringBuilder str = new StringBuilder(nMaxCount);
    GetWindowText(this.Handle, str, nMaxCount);
}