looping in c#,net in winform not console

Question

 Hello Guyz, could anyone help me to find the solution of my problem? Thanks in advance (the sample is attached here)

i try but doesn't work

private void txtinput_KeyPress(object sender, KeyPressEventArgs e)

{

if (!(Char.IsDigit(e.KeyChar) || (e.KeyChar == (char)Keys.Back)))

{

int a = Convert.ToInt32(txtinput.Text);

// var str = array.join(',');

// int a = Convert.ToInt32(txtinput.Text);

for (int i = a; i >= 1; i--)

{

label1.Text =string.Join.ToString(i);

}

e.Handled = true;

}

}