title: Check string for only digits and one optional decimal point.
body: <p>I need to check if a string contains only digits. How could I achieve this in C#?</p>
  
  <pre><code>string s = "123"    → valid 
  string s = "123.67" → valid 
  string s = "123F"   → invalid 
  </code></pre>
  
  <p>Is there any function like IsNumeric?</p>