12. C# (Exceptions)

0. The Real Goal of This Lesson

The goal of this lesson is to physically understand one sentence:

“Why code that compiles successfully
can still crash while running.”

If this is not clear, then:

• TryParse becomes memorization
• repeated input handling makes no sense
• writing stable programs becomes impossible

Everything after this point turns into empty syntax.

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1. Why Everything Worked Until Now

Up to this point, we have been writing code like this:

var input = Console.ReadLine();
int number = int.Parse(input);

And the user entered:

5

Result:

• No error
• Program runs normally

This makes it very easy to assume:

“int.Parse works fine.”

This assumption is dangerous.

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2. What If the User Enters This?

ABC

At this moment, something fundamentally different happens.

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3. What Actually Happens at Runtime

When the program runs and the user types ABC:

• Execution suddenly stops
• It looks like a breakpoint was hit
• A red error window appears

This is not a bug.
This is expected behavior.

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4. What That Red Window Really Is

The message usually looks like this:

System.FormatException
Input string was not in a correct format

This is an exception.

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5. What an Exception Really Is

An exception is a signal raised at runtime
indicating that the program cannot continue normally.

Key points:

• Not a compile error
• Not a syntax error
• Happens during execution

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6. Compile-Time Errors vs Runtime Errors

Compile-Time Error

int x = "ABC"; // compile-time error

• The program never runs
• The IDE blocks execution
• Must be fixed before running

This is safe.

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Runtime Error

int.Parse("ABC"); // runtime exception

• Compilation succeeds
• Program starts running
• Program crashes during execution

This is dangerous.

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7. Why Doesn’t C# Just Convert It to 0?

A very common question:

“Why not just treat invalid input as 0?”

C#’s answer is deliberate:

“That decision belongs to the developer.”

Because what does this mean?

• "ABC" → 0 ?
• " " → 0 ?
• "999999999999" → 0 ?

These all mean different things.

Automatically choosing a value would silently corrupt intent.

So C# says:

“I don’t know what you want.
This is an exceptional situation.”

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8. The Exact Reason the Program Crashed

The real execution flow is:

1. int.Parse(input) is executed
2. The string is not a valid number
3. A FormatException is thrown
4. No one handles it
5. The program terminates immediately

This is called an unhandled exception.

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9. Exceptions Are Not Failures

This perspective is critical.

• Exception = bug ❌
• Exception = expected risk ⭕

Common exception scenarios:

• Division by zero
• Accessing an empty collection
• Missing files
• Parsing non-numeric input

These are normal real-world situations.

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10. The Mental Shift You Must Make

Beginner mindset:

• “Why did my program crash?”
• “Something is wrong with my code.”

Developer mindset:

“This is an exceptional case.”
“This is where I must decide what happens next.”

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11. Exception Messages Are Hints

Input string was not in a correct format

This message already tells you:

• The input format is wrong
• The value is not numeric
• Parsing failed

If you can read the message, the solution direction is obvious.

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12. Searching Errors Is a Core Developer Skill

Copying this message into a search engine leads to:

• Thousands of identical cases
• Explanations
• TryParse solutions

Knowing what to search is part of being a developer.

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13. Why the Next Lesson Is Inevitable

Current situation:

• We want numeric input
• User input is unreliable
• Unhandled exceptions crash the program

This leads to one unavoidable question:

“How do we handle this without crashing?”

That question naturally leads to:

• TryParse
• exception handling
• defensive programming

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One-Line Summary

An exception does not mean your program is wrong —
it means C# is asking you to take responsibility for this situation.