When clause in catch in C Sharp (C#)

Introduction

The when clause in a catch block lets you handle exceptions only if a specific condition is true. This helps you write cleaner and more precise error handling.

You want to catch an exception only if a certain variable has a specific value.

You want to handle errors differently based on the error message or error code.

You want to ignore some exceptions and let them pass if they don't meet a condition.

You want to add extra checks before deciding how to handle an error.

Syntax

C Sharp (C#)

try
{
    // code that might throw
}
catch (ExceptionType ex) when (condition)
{
    // handle exception only if condition is true
}

The when condition is a boolean expression evaluated when an exception is caught.

If the condition is false, the exception is not handled here and can be caught by another catch block or propagate up.

Examples

This catches the IndexOutOfRangeException only if the error message contains the number 5.

C Sharp (C#)

try
{
    int[] numbers = {1, 2, 3};
    int x = numbers[5];
}
catch (IndexOutOfRangeException ex) when (ex.Message.Contains("5"))
{
    Console.WriteLine("Caught index error with 5 in message.");
}

This catches a FormatException only if the current time is before noon.

C Sharp (C#)

try
{
    int.Parse("abc");
}
catch (FormatException ex) when (DateTime.Now.Hour < 12)
{
    Console.WriteLine("Morning format error caught.");
}

Sample Program

This program throws an exception with a message depending on the number. The first catch block only handles exceptions with "big" in the message. Others are handled by the second catch.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        int number = 10;
        try
        {
            if (number > 5)
                throw new InvalidOperationException("Number is too big");
            else
                throw new InvalidOperationException("Number is small");
        }
        catch (InvalidOperationException ex) when (ex.Message.Contains("big"))
        {
            Console.WriteLine("Caught exception for big number.");
        }
        catch (InvalidOperationException ex)
        {
            Console.WriteLine("Caught other invalid operation exception.");
        }
    }
}

Important Notes

The when clause helps avoid nested if checks inside catch blocks.

Use when to make your error handling clearer and more specific.

Summary

The when clause adds a condition to a catch block.

It runs the catch block only if the condition is true.

This helps write cleaner and more precise error handling code.

Practice

1.

What does the when clause do in a catch block in C#?

easy

A. It adds a condition to run the catch block only if the condition is true.

B. It defines a new exception type to catch.

C. It skips the catch block entirely.

D. It makes the catch block run before the try block.

Solution

1. Step 1: Understand the purpose of the when clause

   The when clause adds a condition to a catch block.

2. Step 2: Effect of the when clause in exception handling

   The catch block runs only if the condition after when is true, otherwise it skips.

3. Final Answer:

   It adds a condition to run the catch block only if the condition is true. -> Option A

4. Quick Check:

   when clause = conditional catch [OK]

Hint: when clause filters catch by condition [OK]

Common Mistakes:

• Thinking when defines a new exception type
• Assuming catch runs always regardless of when
• Confusing when with finally block

2.

Which of the following is the correct syntax to use a when clause in a catch block?

try {
    // code
} catch (Exception ex) _____ {
    // handle
}

easy

A. while (ex.Message.Contains("error"))

B. if (ex.Message.Contains("error"))

C. where (ex.Message.Contains("error"))

D. when (ex.Message.Contains("error"))

Solution

1. Step 1: Recall the correct keyword for condition in catch

   The correct keyword to add a condition in catch is when.

2. Step 2: Match the syntax with the options

   Only when (ex.Message.Contains("error")) uses when correctly with the condition.

3. Final Answer:

   when (ex.Message.Contains("error")) -> Option D

4. Quick Check:

   Use 'when' keyword for catch condition [OK]

Hint: Use 'when' keyword, not if/where/while in catch [OK]

Common Mistakes:

• Using if instead of when in catch
• Confusing when with where or while
• Missing parentheses after when

3.

What will be the output of this code?

try {
    throw new InvalidOperationException("Invalid operation");
} catch (InvalidOperationException ex) when (ex.Message.Contains("Invalid")) {
    Console.WriteLine("Caught invalid operation");
} catch (Exception) {
    Console.WriteLine("Caught general exception");
}

medium

A. Caught general exception

B. Caught invalid operation

C. No output

D. Runtime error

Solution

1. Step 1: Identify the thrown exception and matching catch

   The code throws InvalidOperationException with message containing "Invalid".

2. Step 2: Check the when condition in the first catch

   The first catch has a when clause checking if message contains "Invalid" which is true, so it runs.

3. Final Answer:

   Caught invalid operation -> Option B

4. Quick Check:

   when condition true runs first catch [OK]

Hint: Check exception type and when condition carefully [OK]

Common Mistakes:

• Ignoring the when condition and picking second catch
• Assuming no output if when is used
• Confusing exception types

4.

Find the error in this code snippet:

try {
    // some code
} catch (Exception ex) when ex.Message == "Error" {
    Console.WriteLine("Error caught");
}

medium

A. catch block must not have a condition

B. Cannot use when with Exception type

C. Missing parentheses around the when condition

D. Console.WriteLine is not allowed in catch

Solution

1. Step 1: Check syntax of when clause

   The condition after when must be enclosed in parentheses.

2. Step 2: Identify the missing parentheses in the code

   The code uses when ex.Message == "Error" without parentheses, which is invalid syntax.

3. Final Answer:

   Missing parentheses around the when condition -> Option C

4. Quick Check:

   when condition needs parentheses [OK]

Hint: Always put parentheses after when [OK]

Common Mistakes:

• Omitting parentheses around when condition
• Thinking when can't be used with Exception
• Believing catch can't have code inside

5.

Consider this code:

try {
    throw new ArgumentNullException("param");
} catch (ArgumentNullException ex) when (ex.ParamName == "param") {
    Console.WriteLine("Parameter error");
} catch (ArgumentNullException ex) {
    Console.WriteLine("Other argument null error");
}

What will be printed and why?

hard

A. Parameter error, because the when condition matches the ParamName.

B. Other argument null error, because when clause is ignored.

C. No output, because exception is not caught.

D. Runtime error due to duplicate catch blocks.

Solution

1. Step 1: Identify the thrown exception and its property

   The code throws ArgumentNullException with ParamName set to "param".

2. Step 2: Check the when clause condition in the first catch

   The first catch has a when clause checking if ex.ParamName == "param", which is true, so this catch runs.

3. Step 3: Understand catch block selection

   The second catch is ignored because the first matching catch with true when condition handles the exception.

4. Final Answer:

   Parameter error, because the when condition matches the ParamName. -> Option A

5. Quick Check:

   when true catches first matching block [OK]

Hint: when filters catch; first true condition wins [OK]

Common Mistakes:

• Ignoring when condition and picking second catch
• Thinking duplicate catch causes error
• Assuming exception is uncaught