SharePoint 2010 Object Model: How to find out if a Web Application exists

Ever run into a situation where you allow a user to configure a custom component with site URLs but wish you can verify if those URLs are valid in SharePoint?

This was the case for a couple of custom components I recently completed.  The user simply types the Site URLs on the WebPart Tool Pane configuration Textbox delimited by semicolon.

One thing I wanted to do is to validate if the Web Application existed before actually iterating through it and searching for a given SPSite Object based on the URL. (line 6)

Here is a snippet on that (other code omitted for brevity)

//check if the Web App exists
_spWebApplication = SPWebApplication.Lookup(new Uri(url));
if (_spWebApplication != null)
 {
      //We have a Web App, find Site by URL
      if (_spWebApplication.Sites.Where(s => s.Url.Equals(url)).Count() > 0)
      {
          SPSite _site = new SPSite(url);
          _sites.Add(_site);
      }
  }

NOTE: Code not optimized etc, but this should give you an idea.

Hope this helps!

Oscar