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.Net Book

Exception filters

Context: Exception filters allow you to specify a condition using the when keyword in a catch block. The catch block executes only if the condition evaluates to true. This is useful for catching an exception only in specific circumstances without rethrowing or modifying the exception. Exception filters do not unwind the stack, so the original stack trace remains intact.

Usage Example

Section titled “Usage Example”
using System;


class Program
{
    static void Main()
    {
        try
        {
            int x = 0;
            int y = 10 / x;
        }
        catch (DivideByZeroException ex) when (DateTime.Now.DayOfWeek == DayOfWeek.Monday)
        {
            Console.WriteLine("Caught on Monday only.");
        }
        catch (DivideByZeroException ex)
        {
            Console.WriteLine("Caught on any other day.");
        }
    }
}

Output console (depending on day)

Section titled “Output console (depending on day)”
Terminal window
dotnet run
Caught on any other day.

Important notes

Section titled “Important notes”
  • The condition can use the exception variable or any other context.
  • Filters do not capture the exception unless the condition is true.
  • Useful for logging only certain error codes.

Real-world usage example

Section titled “Real-world usage example”

HTTP client – Catch HttpRequestException only when the status code is 404 (Not Found), otherwise let it propagate.
See .NET docs on exception filters.

Section titled “Related sections”