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

• In a catch clause of atry-catch ortry-catch-finally statement.
• As acase guard in theswitch statement.
• As acase guard in theswitch expression.

when in a catch clause

Thewhen 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)

whereexpr is an expression that evaluates to a Boolean value. If it returnstrue, the exception handler executes; iffalse, it does not.

The following example uses thewhen keyword to conditionally execute handlers for anHttpRequestException 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

• C# keywords