Label throws error if text boxes are blank
<p>I have the code below which works fine, but if I leave the text boxes blank it throws an error.  I would like to first check if any are blank then throw an error message in lblSalary.Text.  How could I re-write this?  C#</p>

<p>Thanks</p>

<pre><code>    protected void Button1_Click(object sender, EventArgs e)
    {
        Double AnnualHours;
        AnnualHours = Convert.ToDouble(txtAnnualHours.Text);
        Double Rate;
        Rate = Convert.ToDouble(txtRate.Text);
        Double Salary;
        Salary = (AnnualHours * Rate);
        lblSalary.Text = "$" + Salary.ToString();
    }
}
</code></pre>
