System NotSupportedException The URI prefix is not recognized

by Trent Guidry8. January 2011 03:42
I found a work around for that "The URI prefix is not recognized" System NotSupportedException that can occur when using a WebClient after turning on then off Silverlight 4’s "enable running application out of the browser" setting.
To see this issue, create a Silverlight 4 application.
Paste the following code into the MainPage.cs file:
public MainPage()
{InitializeComponent();Loaded += (s, e) =>{Uri uri = new Uri("test.txt", UriKind.RelativeOrAbsolute);WebClient webClient = new WebClient();webClient.DownloadStringCompleted += (s2, e2) =>{if (e2.Error != null && e2.Error.InnerException != null){MessageBox.Show(e2.Error.InnerException.Message);}};webClient.DownloadStringAsync(uri);};
}
If this project is run, it gives an exception "The remote server returned an error: NotFound.", which is OK, since I didn’t create a file called test.txt.
If I then go to the Silverlight 4’s project settings and turn on “Enable running application out of the browser” and run the application, I get the same exception.  That is also expected and OK.
However, if I go back to the Silverlight 4’s project settings and turn off “Enable running application out of the browser”, I now get an exception of “The URI prefix is not recognized.”  That is unexpected and not OK.
After digging into it a bit, I found the following work around.  Right click the Silverlight 4 project and select Unload.  Right click on the now unavailable project file and click edit.  In that file there should be a section that looks like that shown below.
<ItemGroup><None Include="Properties\OutOfBrowserSettings.xml" /></ItemGroup>
Comment that out or replace it with the code below.
<!--<ItemGroup><None Include="Properties\OutOfBrowserSettings.xml" /></ItemGroup>-->
Save the file.  Right click on the project file and select Reload.  Click Yes to the dialog box. 
Rerun the application and it should now give the original exception again.

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