Answers for "text box allow only letters and numbers"

Function that replaces character and allow only numbers into the textbox

This  technique we can use on the onkeyup property of HTML element

<input type="text" id="txtMobileNo"  
onkeyup="if (/D/g.test(this.value)) this.value = this.value.replace(/D/g,'')"

windows form textbox only allow numbers

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        e.Handled = !char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar);
    }