NullRefrenceException未被用户代码处理

Question

Code below is placed in page_Load. How I should handle this to bypass UrlReferrer when you enter page directly first time and there is no referrer? What I am missing here?

    if (HttpContext.Current.Request.UrlReferrer.AbsoluteUri != null)
    {
        urlReferer = HttpContext.Current.Request.UrlReferrer.AbsoluteUri.ToString();
    }
    else
    {
        urlReferer = "";
    }

Answer 1

Just check UrlReferrer for null:

if (HttpContext.Current.Request.UrlReferrer != null 
    && HttpContext.Current.Request.UrlReferrer.AbsoluteUri != null)
{
     urlReferer = HttpContext.Current.Request.UrlReferrer.AbsoluteUri.ToString();
}     
else     
{         
    urlReferer = "";     
} 

Answer 2

Who says the client passed by the referrer in the HTTP request?

Check if UrlReferrer is null first

if (HttpContext.Current.Request.UrlReferrer != null)
    {
        urlReferer = HttpContext.Current.Request.UrlReferrer.AbsoluteUri.ToString();
    }
    else
    {
        urlReferer = "";
    }

Answer 3

我相信你需要检查HttpContext.Current.Request.UrlReferrer != null 。

Answer 4

Why not this way much cleaner than checking nulls

private void Page_Load()
{
    if (!IsPostBack)
    {
        if (HttpContext.Current.Request.UrlReferrer != null)
        {
            urlReferer = HttpContext.Current.Request.UrlReferrer.AbsoluteUri.ToString();
        }
        else
        {
            urlReferer = "";
        }
    }
}

Answer 5

If UrlReferrer is null, then the test to AbsolutUri will fail.

Try initially testing UrlReferrer for null, this will probably correct the issue.

Answer 6

Use your debugger. If you're running this out of visual studio, than you might be brought to a debugger window when the exception is thrown. There are multiple tabs at the bottom of the debugger including "Locals" and "Watch" you can use those to see what variables are being stored.

If the above code is indeed what's causing the problem than

HttpContext.Current.Request.UrlReferrer.AbsoluteUri
or
HttpContext.Current.Request.UrlReferrer
or
HttpContext.Current.Request
or
HttpContext.Current
or
HttpContext

is set to null