title: C# dll hooks: passing null as parameter
body: <p>I have successfully hooked a dll function in c#</p>
  
  <pre><code>[DllImport ("ftusbsrv.dll", EntryPoint="FtEnumDevices")]
  public unsafe static extern bool FtEnumDevices(ref FT_SERVER_USB_DEVICE lpUsbDevices, ref ulong pulBufferSize, ref FT_ERROR_STATE lpES);
  </code></pre>
  
  <p>I need to pass null as as first parameter as below</p>
  
  <pre><code>FtEnumDevices(null, pulBufferSize,lpES);
  </code></pre>
  
  <p>I am getting the following compile time error:</p>
  
  <blockquote>
    <p>Argument '1': cannot convert from
    '' to 'ref
    FebulaTechWrapper.USBOverNetWrapper.FT_SERVER_USB_DEVICE'</p>
  </blockquote>
  
  <p>I need to know: How can I pass null  as a first parameter?</p>