Check contents of a string and clear contents

I am trying to find a way to check the contents of a string in my WindowsPhone 8.0 Silverlight app.

So far I have the following code:

if(MyURI2.Contains(App.MyPurchaseLink_URI))
{
    // TODO
}

App.MyPurchaseLink_URI is set later in the code and will contain a url which is then passed through to the following page where it is retrieved as MyURI2.

On the users second visit to this page, I would like to check the contents of App.MyPurchaseLink_URI and if it contains any data within it, I would like to clear that value and continue executing the rest of my code.