title: Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default after running
body: <p>I changed my .NET version of my project and now I am getting this error :</p>
  
  <p><a href="https://i.stack.imgur.com/v8wrN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/v8wrN.png" alt="enter image description here"></a></p>
  
  <blockquote>
    <p>Compiler Error Message: CS1617: Invalid option '6' for /langversion;
    must be ISO-1, ISO-2, 3, 4, 5 or Default</p>
  </blockquote>
  
  <p>Here is my web.config:</p>
  
  <pre><code>  &lt;compilers&gt;
            &lt;compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:5/nowarn:1659;1699;1701"&gt;
              &lt;providerOption name="CompilerVersion" value="v4.0"/&gt;
            &lt;/compiler&gt;
            &lt;compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.VisualBasic.VBCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&amp;quot;Web\&amp;quot; /optionInfer+"&gt;
              &lt;providerOption name="CompilerVersion" value="v4.0"/&gt;
            &lt;/compiler&gt;
          &lt;/compilers&gt;
  </code></pre>
  
  <p>Does anyone know what this means?</p>