How to sum only one textbox value and show result in label in c# : <p>The question could be duplicate but my problem is totally different.</p>

<p>I have a one textbox and one label.</p>

<p>Example: label text is already exist with number like 450 Now I want to add number in textbox and the textbox value should be sum in 450 + 30 = 480 where 30 is textbox value.</p>

<p>what I have tried:</p>

<pre><code>lbl_TotalAmount.Text = lbl_TotalAmount.Text + txt_DeliveryCharges.Text;
</code></pre>

<p>The above Result is: <code>45030</code></p>

<p>Another Code I Have tried:</p>

<pre><code>double total = Convert.ToDouble(lbl_TotalAmount.Text);
double charges = Convert.ToDouble(txt_DeliveryCharges.Text);
double sum = total + charges;
lbl_TotalAmount.Text = String.Format(sum.ToString("0.00"));
</code></pre>

<p>The above code is going to sum every value that I put in the textbox and also when I remove one word the number is sum every time.</p>

<p>Please give me best solution I tried many solution to solve this but unable to do this.</p>

<p>Sorry for bad English.</p>
