如何检查NavigationContext.QueryString？

Question

In ASP.NET I can check for the existence of a QueryString key/value like so

if(Request.QueryString["someValue"] != null) 

However, I can't do this with NavigationContext.QueryString

if(NavigationContext.QueryString["someValue"] != null) 

throws an error - The given key was not present in the dictionary

if(NavigationContext.QueryString.ContainsKey("someValue"))

Also throws an error. This code is in the OnNavigatedTo method where it should be.

How do I check for the existence of a key / value in Windows Phone 8? My ugly, ugly workaround at the moment is to enclose each of these blocks in a try / catch with no code in the catch block. If the key is present the code completes, if not it throws the error which is silently caught.

Answer 1

By checking if the QueryString contains the key should work, maybe your not in a proper context. Else try to get the value. But check if the error is on the extraction process and not by accessing the NavigationContext or the QueryString (maybe they are null).

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    if(NavigationContext.QueryString.ContainsKey("someValue"))
    {
        // string someValue = NavigationContext.QueryString["someValue"];
    }

    // OR

    string someValue = string.Empty;
    if (NavigationContext.QueryString.TryGetValue("someValue",out someValue))
    {
         // someValue contains the value
    }
}

Answer 2

The if(NavigationContext.QueryString["someValue"] != null) should work just fine. When do you call it? You should do it in OnNavigatedTo . If you use it in the contrustor the NavigationContext is null.