When clause in catch in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the when clause in a catch block in C#?

The when clause allows you to specify a condition that must be true for the catch block to handle the exception. It helps filter exceptions based on custom logic.

beginner

How does the when clause improve exception handling?

It lets you catch exceptions only if certain conditions are met, making your error handling more precise and avoiding catching exceptions you don't want to handle there.

beginner

Show a simple example of a catch block using a when clause.

Example:<br>

try {
  // code that may throw
} catch (Exception ex) when (ex.Message.Contains("timeout")) {
  // handle timeout exceptions only
}

intermediate

Can multiple catch blocks use when clauses in the same try statement?

Yes, you can have multiple catch blocks with different when conditions to handle different cases of exceptions separately.

intermediate

What happens if the when condition evaluates to false in a catch block?

If the when condition is false, the catch block is skipped, and the runtime looks for another matching catch block or rethrows the exception if none match.

What does the when clause in a catch block do?

A Throws a new exception

B Catches all exceptions unconditionally

C Filters exceptions based on a condition

D Terminates the program immediately

The when clause filters exceptions so the catch block only runs if the condition is true.

Which of these is a valid use of when in a catch block?

A catch (Exception ex) where (ex.Message.Contains("error"))

B catch when (ex.Message.Contains("error"))

C catch (Exception ex) if (ex.Message.Contains("error"))

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

The correct syntax uses when after the exception variable declaration.

If a when condition is false, what happens?

A The program crashes

B The <code>catch</code> block is skipped

C The exception is ignored

D The <code>catch</code> block runs anyway

If the when condition is false, the catch block is skipped and the runtime looks for another handler.

Can you have multiple catch blocks with different when clauses?

A Yes, to handle different exception cases

B No, only one <code>catch</code> block can have a <code>when</code> clause

C Only if they catch the same exception type

D Only if they are nested

Multiple catch blocks can use different when clauses to handle different conditions.

Which C# version introduced the when clause in catch blocks?

A C# 6.0

B C# 5.0

C C# 7.0

D C# 8.0

The when clause in catch blocks was introduced in C# 6.0.

Explain how the when clause works in a catch block and why it is useful.

Think about how you might want to catch only specific cases of an error.

Write a simple C# code snippet using a catch block with a when clause to handle exceptions with a specific message.

Use <code>ex.Message.Contains("text")</code> inside the <code>when</code> clause.