Switch expressions with patterns in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Switch expressions with patterns

Start with input value

↓

Evaluate switch expression

↓

Match pattern 1?

No→Match pattern 2?

↓

Return result 1

↓

End

The switch expression checks the input against patterns in order and returns the matching result or a default.

Execution Sample

C Sharp (C#)

object input = 42;
string result = input switch
{
    int i when i > 0 => "Positive integer",
    int i when i < 0 => "Negative integer",
    string s => $"String: {s}",
    _ => "Unknown"
};

This code uses a switch expression to check the input's type and value, returning a matching string.

Execution Table

Step	Input Value	Pattern Checked	Condition Result	Branch Taken	Output
1	42	int i when i > 0	True	Yes	"Positive integer"
2	42	int i when i < 0	Skipped	No
3	42	string s	Skipped	No
4	42	_ (default)	Skipped	No
Exit	42	N/A	Matched at step 1	End	"Positive integer"

💡 Input 42 matches pattern 'int i when i > 0' at step 1, so switch returns "Positive integer" and stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
input	42	42	42	42	42
result	null	"Positive integer"	"Positive integer"	"Positive integer"	"Positive integer"

Key Moments - 3 Insights

Why does the switch expression stop checking after the first pattern matches?

Because switch expressions return the result immediately when a pattern matches, as shown in execution_table step 1 where the first pattern matches and the rest are skipped.

What does the underscore (_) pattern mean in the switch expression?

The underscore (_) is the default pattern that matches anything not matched before, as seen in execution_table step 4, but it is skipped here because an earlier pattern matched.

How does the 'when' clause affect pattern matching?

The 'when' clause adds an extra condition to the pattern; the pattern only matches if the condition is true, like 'int i when i > 0' in step 1.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 1?

A "Unknown"

B "Positive integer"

C "Negative integer"

D "String: 42"

💡 Hint

Check the Output column at step 1 in the execution_table.

At which step does the switch expression stop checking patterns?

A Step 3

B Step 4

C Step 1

D Step 2

💡 Hint

Look at the Branch Taken column and see where the first 'Yes' appears.

If input was "hello" (a string), which pattern would match?

A string s

B int i when i < 0

C int i when i > 0

D _ (default)

💡 Hint

Refer to the patterns and their types in the execution_sample code.

Concept Snapshot

Switch expressions check an input against patterns in order.
Each pattern can have a condition with 'when'.
The first matching pattern returns its result immediately.
Use '_' as a default catch-all pattern.
Syntax: result = input switch { pattern1 => value1, pattern2 => value2, _ => defaultValue };

Full Transcript

This example shows how a switch expression in C# tests an input value against several patterns. The input 42 is checked first against 'int i when i > 0', which matches because 42 is a positive integer. The switch returns "Positive integer" immediately and stops checking other patterns. The underscore (_) pattern is a default that matches anything else but is skipped here because an earlier pattern matched. The 'when' clause adds extra conditions to patterns. This flow helps write clear, concise code for multiple conditions.