Type checking patterns in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Type checking patterns

Start with object 'obj'

↓

Check type with 'is' pattern

↓

Cast and use

↓

End

The program checks if an object matches a type pattern using 'is'. If yes, it casts and uses it; if no, it skips or handles differently.

Execution Sample

C Sharp (C#)

object obj = "hello";
if (obj is string s)
{
    Console.WriteLine(s.Length);
}

Checks if obj is a string, then prints its length.

Execution Table

Step	Expression	Evaluation	Result	Action
1	obj = "hello"	Assign string to obj	obj = "hello"	Store string in obj
2	obj is string s	Check if obj is string	True	Cast obj to s
3	s.Length	Get length of s	5	Print 5
4	End	No more code	-	Program ends

💡 After printing length, program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
obj	null	"hello"	"hello"	"hello"	"hello"
s	undefined	undefined	"hello"	"hello"	"hello"

Key Moments - 2 Insights

Why can we use 's' inside the if block without explicit casting?

Because the 'is' pattern both checks type and casts obj to s if true, as shown in step 2 of the execution_table.

What happens if obj is not a string?

The condition in step 2 would be false, so the code inside the if block would not run, skipping the cast and print.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the result of 'obj is string s'?

A False

B True

C Throws error

D Undefined

💡 Hint

Check the 'Result' column in step 2 of execution_table.

At which step is the variable 's' assigned a value?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at variable_tracker and execution_table step 2 where casting happens.

If obj was an int instead of a string, what would happen at step 2?

A Result would be True and s assigned

B Program would crash

C Result would be False and s undefined

D s would be null

💡 Hint

Refer to key_moments about what happens if obj is not a string.

Concept Snapshot

Type checking patterns use 'is' to check and cast in one step.
Syntax: if (obj is Type varName) { use varName }
If true, varName is assigned and usable inside block.
If false, block is skipped.
This avoids separate casting and null checks.

Full Transcript

This example shows how C# uses type checking patterns with the 'is' keyword. We start with an object 'obj' assigned a string "hello". Then we check if 'obj' is a string and simultaneously cast it to variable 's'. Since obj is a string, the condition is true, and 's' holds the string value. We then print the length of 's', which is 5. If obj was not a string, the if block would be skipped. This pattern helps safely check and use types without extra casting code.