Type checking patterns in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a type checking pattern in C#?

A type checking pattern is a way to test if an object is of a certain type and, if so, to extract it into a variable in a single step using the is keyword.

beginner

How do you use the is keyword with a pattern to check type and assign a variable?

You write if (obj is TypeName variableName). This checks if obj is of type TypeName and if true, assigns it to variableName.

intermediate

What is the benefit of using type checking patterns over traditional casting?

Type checking patterns combine the type check and variable assignment in one step, making code safer and cleaner by avoiding explicit casts and potential exceptions.

intermediate

Explain the difference between is TypeName variable and as TypeName casting.

is TypeName variable checks type and assigns if true, while as TypeName tries to cast and returns null if it fails. The is pattern is safer and more concise.

beginner

Show a simple example of type checking pattern in C#.

Example:<br>

object obj = "hello";<br>if (obj is string s)<br>{<br>    Console.WriteLine($"String length: {s.Length}");<br>}

What does the C# expression if (obj is string s) do?

A Casts obj to string without checking

B Checks if obj is a string and assigns it to s if true

C Always assigns obj to s regardless of type

D Throws an exception if obj is not a string

The expression checks the type and assigns the variable only if obj is a string.

Which keyword is used in C# for type checking patterns?

A cast

B as

C is

D typeof

The 'is' keyword is used for type checking patterns in C#.

What happens if the type check fails in if (obj is int i)?

A The if block is skipped and i is not assigned

B An exception is thrown

C i is assigned null

D The program crashes

If the type check fails, the if block does not run and the variable is not assigned.

Which is safer to avoid exceptions when checking type and using the object?

A Type checking pattern with 'is'

B Direct casting with (Type)

C Using 'as' without null check

D Ignoring type and using object directly

'is' pattern safely checks type and assigns variable only if correct.

What is the main advantage of type checking patterns?

A Replace all if statements

B Make code run faster

C Allow casting to any type without error

D Combine type check and assignment in one step

They combine checking and assignment, making code cleaner and safer.

Explain how type checking patterns work in C# and why they are useful.

Think about how you check type and get the object ready to use in one step.

Write a simple C# code snippet using a type checking pattern to check if an object is a string and print its length.

Use 'if (obj is string s)' and then print s.Length.