Only Double Must be Accepted in the textBox.

try this

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
        if ((e.KeyChar < 48 || e.KeyChar > 57) && e.KeyChar != 46
                                 && e.KeyChar != 8)
                e.Handled = true;
}

48-57 will be numbers
46- decimal point
8-backspace

This will allow users to type only numbers, decimal and backspace