Positional patterns in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Positional patterns

Input value

↓

Match with positional pattern

↓

Extract parts

↓

Use extracted parts

↓

Execute matched case block

↓

Output or result

Positional patterns check the shape of data and extract parts to use in code, then run matching case blocks.

Execution Sample

C Sharp (C#)

record Point(int X, int Y);

Point p = new(3, 4);

string result = p switch
{
    (0, 0) => "Origin",
    (var x, 0) => $"X={x} on X-axis",
    (0, var y) => $"Y={y} on Y-axis",
    (var x, var y) => $"Point at ({x},{y})"
};

This code matches a Point record with positional patterns and returns a string describing its position.

Execution Table

Step	Input	Pattern Matched	Extracted Variables	Action	Output
1	Point(3,4)	(0,0)	none	No match, try next
2	Point(3,4)	(var x, 0)	x=3	Y is not 0, no match
3	Point(3,4)	(0, var y)	y=4	X is not 0, no match
4	Point(3,4)	(var x, var y)	x=3, y=4	Match, create string	Point at (3,4)

💡 All patterns checked; last pattern matches and execution stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
p	Point(3,4)	Point(3,4)	Point(3,4)	Point(3,4)	Point(3,4)	Point(3,4)
x	undefined	undefined	3	undefined	3	3
y	undefined	undefined	undefined	4	4	4
result	undefined	undefined	undefined	undefined	Point at (3,4)	Point at (3,4)

Key Moments - 2 Insights

Why does the pattern (var x, 0) not match when Y is 4?

Because the pattern requires the second value to be exactly 0, but Y is 4, so it fails as shown in step 2 of the execution_table.

What happens if no positional pattern matches?

The switch expression would not compile (non-exhaustive patterns); here, the last pattern (var x, var y) matches anything, ensuring a result as in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'x' when the pattern (var x, 0) is checked?

A 3

B 0

C undefined

D 4

💡 Hint

Check the 'Extracted Variables' column at step 2 in the execution_table.

At which step does the pattern match successfully?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the 'Pattern Matched' and 'Action' columns to see when the match happens.

If the point was (0,0), which pattern would match first?

A (var x, var y)

B (var x, 0)

C (0, 0)

D (0, var y)

💡 Hint

Refer to the order of patterns in the code and the execution_table logic.

Concept Snapshot

Positional patterns match data by shape and extract parts.
Syntax: (var1, var2, ...)
Used in switch or is expressions.
Matches in order; first match runs.
Extracted variables can be used in code block.

Full Transcript

This example shows how positional patterns in C# match a Point record's X and Y values. The switch expression tries each pattern in order. It first checks if the point is at the origin (0,0). If not, it checks if Y is zero, then if X is zero. Finally, it matches any point and extracts X and Y. Variables x and y get values only when their pattern matches. The output string describes the point's position based on the matched pattern.