7.4 Exception Filters

Let’s go through exception filters in C#. This is a powerful feature for handling exceptions conditionally.

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📌 1. What Are Exception Filters?

• An exception filter lets you conditionally catch an exception only if a specific condition is true.

• Syntax:

catch (Exception ex) when (condition)

• The when clause runs before entering the catch block.

• If the condition is false, the exception bypasses this catch and can be handled by another catch block or propagate up the call stack.

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📌 2. Why Use Exception Filters?

• Avoid nested if statements inside catch blocks.

• Catch only certain exceptions based on properties (like Message or ErrorCode).

• Keep code clean and readable.

• Filters don’t interfere with stack unwinding — the exception still behaves normally.

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📌 3. Syntax Example

class Program
{
    static void Main()
    {
        try
        {
            int[] numbers = { 1, 2, 3 };
            int index = 5;
            Console.WriteLine(numbers[index]);
        }
        catch (IndexOutOfRangeException ex) when (ex.Message.Contains("index"))
        {
            Console.WriteLine("Index was out of range!");
        }
        catch (Exception ex)
        {
            Console.WriteLine("Other exception: " + ex.Message);
        }
    }
}

• Only exceptions with a message containing “index” are handled by the first catch.

• Others are passed to the next catch block.

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📌 4. Example: Conditional Logging

try
{
    // Some code
}
catch (Exception ex) when (ex is IOException || ex is TimeoutException)
{
    Console.WriteLine($"I/O or Timeout Exception: {ex.Message}");
}
catch (Exception ex)
{
    Console.WriteLine($"Other Exception: {ex.Message}");
}

• Handles specific exceptions in one block without multiple catch statements.

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📌 5. Key Points

Feature	Notes
when clause	Evaluates before entering catch
Multiple catches	Can bypass this catch if condition false
Stack unwinding	Still occurs normally
Use case	Conditional exception handling, cleaner code

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📌 6. Analogy

• Think of it as “only catch this exception if it meets certain criteria”, like a security gate that lets through only people with certain badges.

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✅ Summary:

• Exception filters (catch … when) let you conditionally catch exceptions.

• Helps simplify exception handling and avoid unnecessary nesting.

• Works with all exception types.