How to let try-catch works in LINQPad

doggy · January 2019

I'm trying to try-catch the folowing code in LINQPad. But LINQPad always stopped on Line 3. Can I just leave try-catch works as expected?


try
{
    headers.GetValues("User-Agent");
}
catch (IndexOutOfRangeException)
{
    headers.Remove("User-Agent");
    headers.Add("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/7.0)");
}

JoeAlbahari · January 2019

What do you mean by "stopped"? Have you enabled "Break when exception is thrown" in the toolbar (the blue bug icon)? If so, uncheck that and it will run without breaking.

doggy · January 2019

Yes, you are right. That's the point. Thank you!

nescafe · January 2019

Instead of using try/catch to check for existing values, which is considered bad practice (exceptions are costly), you better use headers.AllKeys.Contains("User-Agent") to check for an existing value.

But, a shorter path is redefining the header. Assuming Headers is of type WebHeaderCollection, use the this[string] or this[HttpRequestHeader] property (which you can examine using built-in ILSpy).

Using both named header "User-Agent" and enum HttpRequestHeader.UserAgent interchangeably:

using (var client = new WebClient())
{
  client.Headers["User-Agent"] = "Test";
  client.Headers.Dump();
  client.Headers[HttpRequestHeader.UserAgent] = "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/7.0)";
  client.Headers.Dump();
}

More info on using exceptions for flow control: https://softwareengineering.stackexchange.com/questions/189222/are-exceptions-as-control-flow-considered-a-serious-antipattern-if-so-why

doggy · January 2019

This is actually a workaround due to a ASP.NET Core issue: https://github.com/dotnet/corefx/issues/34933

If the User-Agent malformed, any kind of read on that header will always throw exception!

nescafe · January 2019

Oops, my bad, carry on

How to let try-catch works in LINQPad

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