C#

[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);

private void btnClickBack_Click(object sender, System.EventArgs e)
{
 //Pointer to Calculator window
	 IntPtr calcPtr=IntPtr.Zero;
 //Search For the Calculator Window SciCalc is Windows class
 calcPtr=FindWindow("SciCalc","Calculator");
 //If  Found it
 if(!calcPtr.Equals(IntPtr.Zero))
 {
   IntPtr backSpaceButton=IntPtr.Zero;
//Search For Backspace Button
 backSpaceButton=FindWindowEx(calcPtr,IntPtr.Zero,"Button","Backspace");
 //If Found it
 if(!backSpaceButton.Equals(IntPtr.Zero))
 {
   // 0x00F5 is BM_Click Message
  SendMessage(backSpaceButton,0x00F5,IntPtr.Zero,IntPtr.Zero);
 }
 }
}