Casting with as and is operators in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Casting with as and is operators

Start with object

↓

Check type with 'is'

Yes→Safe to cast

↓

Use 'as' to cast

↓

Cannot cast

↓

Cast failed

↓

Cast succeeded

First, check if an object is a certain type using 'is'. If yes, cast safely. Alternatively, use 'as' to cast and check for null to see if cast succeeded.

Execution Sample

C Sharp (C#)

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

This code checks if obj is a string using 'is' and then casts with 'as', printing the string length both times.

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	Console.WriteLine(s.Length)	s = "hello"	5	Print length 5
4	str = obj as string	Try cast obj to string	"hello"	Assign to str
5	Console.WriteLine(str?.Length)	str = "hello"	5	Print length 5
6	End	No more code	-	Stop execution

💡 Reached end of code after printing string lengths

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
obj	null	"hello"	"hello"	"hello"	"hello"
s	null	null	"hello"	"hello"	"hello"
str	null	null	null	"hello"	"hello"

Key Moments - 2 Insights

Why do we check 'obj is string s' before using s?

Because 'is' checks the type and safely casts only if obj is a string, preventing errors. See step 2 and 3 in execution_table.

What happens if 'obj as string' fails?

It returns null instead of throwing an error. We check for null before using the result. See step 4 in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 's' after step 2?

A "hello"

B null

C 5

D obj

💡 Hint

Check the 'Result' column for step 2 in execution_table

At which step does the 'as' operator cast happen?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look for 'as' in the 'Expression' column in execution_table

If obj was not a string, what would 'str' be after step 4?

A 0

B null

C "hello"

D Exception thrown

💡 Hint

Recall that 'as' returns null if cast fails, see key_moments explanation

Concept Snapshot

Casting with 'is' checks type and casts safely:
if (obj is Type t) { use t }

Casting with 'as' tries cast and returns null if fails:
Type? t = obj as Type;
Check for null before use.

Use 'is' to avoid exceptions and 'as' for nullable casts.

Full Transcript

This example shows how to use 'is' and 'as' operators in C# for casting. First, an object 'obj' holds a string "hello". Using 'is', we check if obj is a string and assign it to 's' if true. Then we print the length of 's'. Next, we use 'as' to cast obj to string and assign to 'str'. We print the length of 'str' using null-safe access. The execution table traces each step, showing variable values and actions. Key moments explain why 'is' is used to safely cast and how 'as' returns null if casting fails. The visual quiz tests understanding of variable values and casting steps. The snapshot summarizes the syntax and behavior of 'is' and 'as' casting in C#.