title: string.Format() giving "Input string is not in correct format"
body: <p>What do I do wrong here?</p>
  
  <pre><code>string tmp = @"
      if (UseImageFiles) {
          vCalHeader += ""&lt;td&gt;&lt;img onmousedown='' src= '{0}cal_fastreverse.gif' width='13px' height='9' onmouseover='changeBorder(this, 0)' onmouseout='changeBorder(this, 1)' style='border:1px solid white'&gt;&lt;/td&gt;\n""; //Year scroller (decrease 1 year)
          calHeight += 22;
      }";
  
  string x = "xter";
  tmp = string.Format(tmp, x);
  </code></pre>
  
  <p>I get</p>
  
  <blockquote>
    <p>Input string was not in correct format</p>
  </blockquote>
  
  <p>when trying to change <code>{0}</code>. I am doing this in C# and WinForms.</p>
  
  <blockquote>
    <p>Format Exception was unhandled<br>
      Input string was not in correct format   </p>
  </blockquote>
  
  <p>Troubleshoot tips I get:</p>
  
  <blockquote>
    <p>Make sure your method arguments are in right format. When converting a string to datetime, parse the string to take out  the date before putting each variable into the DateTime object.</p>
  </blockquote>