I have a simple increment on textbox by pressing down arrow key which are as below.
protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
You'll need to handle other commands that existed before and return when you handle ones you are looking for. Try changing it to this and see if that helps:
protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
if (msg.WParam.ToInt32() == (int)Keys.Down)
{
int c = int.Parse(textBox1.Text);
c++;
textBox1.Text = c.ToString();
return true;
}
return base.ProcessCmdKey(ref msg, keyData);
}