When clause in catch in C Sharp (C#) - Step-by-Step Execution

Concept Flow - When clause in catch

Try block starts

↓

Exception thrown?

No→Try block ends normally

Yes↓

Catch block with when clause

↓

Evaluate when condition

↓

Condition true?

No→Skip this catch

↓

Next catch or unhandled

↓

Execute catch block

↓

Continue after try-catch

When an exception occurs, the catch block with a when clause checks the condition; if true, it runs, else it skips to the next catch.

Execution Sample

C Sharp (C#)

try {
  int x = int.Parse("abc");
} catch (FormatException e) when (e.Message.Contains("Input")) {
  Console.WriteLine("Caught FormatException with 'Input' in message");
}

This code tries to parse a string to int and catches FormatException only if the message contains 'Input'.

Execution Table

Step	Action	Exception Thrown	When Condition Evaluated	Condition Result	Catch Block Executed	Output
1	Enter try block	No	N/A	N/A	No
2	int.Parse("abc") throws FormatException	FormatException	N/A	N/A	No
3	Catch block with when clause checks condition	FormatException	e.Message.Contains("Input")	True	Yes	Caught FormatException with 'Input' in message
4	Catch block executed, program continues	No	N/A	N/A	No

💡 Catch block executed because when condition was true; program continues normally.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
e.Message	N/A	"Input string was not in a correct format."	"Input string was not in a correct format."	"Input string was not in a correct format."

Key Moments - 2 Insights

Why does the catch block only run when the when condition is true?

Because the when clause acts like a filter; if the condition is false, the catch block is skipped (see execution_table row 3).

What happens if the when condition is false?

The catch block is skipped and the runtime looks for another catch block or the exception remains unhandled (not shown in this trace).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of the when condition at step 3?

A True

B False

C Not evaluated

D Exception message

💡 Hint

Check the 'Condition Result' column at step 3 in the execution_table.

At which step does the catch block actually execute?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Catch Block Executed' column in the execution_table.

If the when condition was false, what would happen next?

A The catch block runs anyway

B The exception is ignored

C The runtime looks for another catch block

D The program crashes immediately

💡 Hint

Refer to the concept_flow where the false condition leads to skipping the catch block.

Concept Snapshot

try {
  // code that may throw
} catch (ExceptionType e) when (condition) {
  // runs only if condition is true
}

- The when clause filters exceptions
- If false, catch is skipped
- Helps handle exceptions selectively

Full Transcript

This example shows how the when clause in a catch block works in C#. When an exception is thrown inside the try block, the catch block with a when clause checks the condition. If the condition is true, the catch block runs and handles the exception. If false, it skips this catch and looks for another. This lets you handle exceptions more precisely based on extra conditions.