c# code to check is string contains only numbers
<p>I'm a C# noob and I have this task. 
I have TextBox, and user need to put 13 digits.
I need to check is user correctly populate that TextBox. Only numbers can be acceptable.</p>

<p>After that, I need to check is that last entered (13th) number equal with my int64 variable I got from other function. </p>

<p>Thanks!</p>
