Exception filtering

C# course · Advanced · Lesson 17/83

Learn catch filtering with the when clause.

Exception filtering (when)

Since C# 6 you can add a when clause to catch blocks: catch (Exception ex) when (ex.Message == "..."). This is called an exception filter. If the condition is true the catch runs; otherwise it is skipped.

Filtering lets you decide behavior based on the error content, not just the type. For example you can catch only errors with certain messages and let others bubble up. You can also use side effects like Log(ex) inside the filter.

Filter examples

You can distinguish multiple catch blocks with when. Example: catch (Exception ex) when (ex.Message.Contains("disk")) { ... } and separately catch (Exception ex) when (ex.Message.Contains("network")) { ... }. This lets you take different actions based on the message.

When the filter returns false, the catch is skipped and another catch is searched; otherwise the exception propagates. Avoid side effects in filters; they make debugging harder.

Exercise Filter with when

Define static async Task FailAsync(string msg). In its body write await Task.CompletedTask; and throw new Exception(msg);. Make Main a static async Task Main. Apply the filter with try { await FailAsync("Expected"); } catch (Exception ex) when (ex.Message == "Expected") { Console.WriteLine("Caught"); } and print.

Show example solution

In FailAsync body write await Task.CompletedTask; and throw new Exception(msg);. Make Main static async Task Main() and write try { await FailAsync("Expected"); } catch (Exception ex) when (ex.Message == "Expected") { Console.WriteLine("Caught"); }
Example solution:
using System;
using System.Threading.Tasks;

class Program
{
    static async Task FailAsync(string msg)
    {
        await Task.CompletedTask;
        throw new Exception(msg);
    }

    static async Task Main()
    {
        try
        {
            await FailAsync("Expected");
        }
        catch (Exception ex) when (ex.Message == "Expected")
        {
            Console.WriteLine("Caught");
        }
    }
}

Test yourself

Complete the when clause

catch (Exception ex) ____ (ex.Message == "Hata") { Console.WriteLine(ex.Message); }

Show answer

catch (Exception ex) when (ex.Message == "Hata") { Console.WriteLine(ex.Message); }

What does the when clause do?
- It adds a finally block
- It repeats the try block
- It silently swallows the exception
- It runs the catch only when the condition is true
Show answer

It runs the catch only when the condition is true — Correct. The when clause runs the catch only when true; otherwise skipped.
Match filtering concepts

when (cond)

Filters the catch
ex.Message.Contains(...)

Condition by message
catch (Exception ex)

General catch
if it returns false

Catch is skipped

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Exception filtering