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when (C# reference)

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You use the when contextual keyword to specify a filter condition in the following contexts:

when in a catch clause

The when keyword can be used in a catch clause to specify a condition that must be true for the handler for a specific exception to execute. Its syntax is:

catch (ExceptionType [e]) when (expr)

where expr is an expression that evaluates to a Boolean value. If it returns true, the exception handler executes; if false, it does not.

The following example uses the when keyword to conditionally execute handlers for an HttpRequestException depending on the text of the exception message.

using System;
using System.Net.Http;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        Console.WriteLine(MakeRequest().Result);
    }

    public static async Task<string> MakeRequest()
    {
        var client = new HttpClient();
        var streamTask = client.GetStringAsync("https://localHost:10000");
        try
        {
            var responseText = await streamTask;
            return responseText;
        }
        catch (HttpRequestException e) when (e.Message.Contains("301"))
        {
            return "Site Moved";
        }
        catch (HttpRequestException e) when (e.Message.Contains("404"))
        {
            return "Page Not Found";
        }
        catch (HttpRequestException e)
        {
            return e.Message;
        }
    }
}

See also