Validate Textbox is Numeric on ButtonClick in C#
<p>I currently have an WinForm consisting of Name, Address, Zip Code, State, and Age.</p>

<p>Once the user inputs the data, they click the "Exit" button which confirms no fields are blank, then saves the data to a file. I want to add a Zip Code validation that confirms the textbox (ZipField) contains only numbers.</p>

<pre><code>    private void Btn_Click(object sender, EventArgs e)
    {

        if (String.IsNullOrEmpty(NField.Text) || String.IsNullOrEmpty(AField.Text) ||
            String.IsNullOrEmpty(ZField.Text) || String.IsNullOrEmpty(SField.Text) ||
            String.IsNullOrEmpty(AField.Text))
        {
            MessageBox.Show("Please complete", "Unable to save", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);

        return;
        }
        saveInfo();
        Form myForm = Start.getForm();
        myForm.Show();
        this.Close();
    }
</code></pre>
