title: Check every TextBox for letters
body: <p>Lets assume I have 5 TextBoxes like this:</p>
  
  <pre><code>textBox_Box1
  textBox_Box2
  textBox_Box3
  textBox_Box4
  textBox_Box5
  </code></pre>
  
  <p>And I have a function that checks if the TextBox contains only letters, like this:</p>
  
  <pre><code>public static bool OnlyLetters(string s)
  {
      foreach (char c in s)
      {
          if (!Char.IsLetter(c))
              return false;
      }
      return true;
  }
  </code></pre>
  
  <p>Is there an efficient way to check every textBox with this function? I do not want to write it in this style of course:</p>
  
  <pre><code>OnlyLetters(textBox_Box1.Text);
  OnlyLetters(textBox_Box2.Text);
  OnlyLetters(textBox_Box3.Text);
  OnlyLetters(textBox_Box4.Text);
  OnlyLetters(textBox_Box5.Text);
  </code></pre>
  
  <p>I would prefer to check it in a loop, but I do not know how to realize it at that point.</p>