Maybe you can try this now - long changed to Int32
[DllImport("ScanDll.dll")]
public static extern Int32 IO_HasScanner([MarshalAs(UnmanagedType.VBByRefStr)] ref string IVS_600DS, Int32 hHandle);
private void Form1_Load(object sender, EventArgs e)
{
Int32 result;
result = IO_HasScanner("IDCapture", this.Handle.ToInt32());
MessageBox.Show(result.ToString());
}