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

Recall & Review

beginner

What is a type pattern in C#?

A type pattern checks if an object is of a specific type and, if so, allows you to use it as that type in a safe way.

beginner

How do you use a type pattern with the is keyword?

You write if (obj is TypeName variableName). This checks if obj is of TypeName and, if true, assigns it to variableName for use inside the block.

beginner

What happens if the type pattern check fails?

If the object is not of the specified type, the pattern check returns false and the variable is not assigned.

intermediate

Can you use type patterns in a switch statement?

Yes! You can match the type of a variable in switch cases using type patterns, making code clearer and safer.

beginner

Example: What does this code do?

if (obj is string s) { Console.WriteLine(s.Length); }

It checks if obj is a string. If yes, it assigns it to s and prints the length of the string.

What does the type pattern if (obj is int number) do?

A Checks if obj is an int and assigns it to number if true

B Converts obj to int without checking

C Always assigns obj to number

D Checks if obj is null

The pattern checks the type and assigns only if obj is an int.

Which keyword is used with type patterns to check an object's type?

A is

B typeof

C switch

D as

The 'is' keyword is used to check type patterns.

What happens if the type pattern check fails?

A The variable is assigned default value

B The code throws an exception

C The variable is assigned null

D The condition is false and variable is not assigned

If the object is not of the specified type, the check returns false and the variable is not assigned.

Can type patterns be used inside a switch statement?

A No, only if statements

B Yes, to match types in cases

C Only with enums

D Only with strings

Type patterns can be used in switch cases to match object types.

What is the benefit of using type patterns?

A They automatically convert types

B They replace all casts

C They allow safe type checking and variable assignment

D They improve performance by skipping checks

Type patterns let you check types safely and use the variable if the check passes.

Explain how type patterns work with the 'is' keyword in C#.

Think about how you check and use an object's type in one step.

Describe how type patterns can be used in a switch statement and why that is useful.

Consider how switch can handle different types safely.

Practice

1. What does a type pattern in C# do when used in an if statement like if (obj is string s)?

easy

A. Always returns true regardless of obj type

B. Converts obj to a string without checking

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

D. Throws an exception if obj is not a string

Solution

1. Step 1: Understand the is keyword with type pattern

   The is keyword checks if the object is of the specified type.

2. Step 2: Recognize the pattern variable assignment

   If the check is true, the object is assigned to the variable s of that type.

3. Final Answer:

   Checks if obj is a string and assigns it to s if true -> Option C

4. Quick Check:

   Type pattern = Check and assign [OK]

Hint: Type pattern checks type and assigns in one step [OK]

Common Mistakes:

• Thinking it converts without checking
• Assuming it always returns true
• Expecting an exception on type mismatch

2. Which of the following is the correct syntax for using a type pattern in an if statement in C#?

easy

A. if (obj as int number) { }

B. if (int obj is number) { }

C. if (obj int is number) { }

D. if (obj is int number) { }

Solution

1. Step 1: Recall correct type pattern syntax

   The correct syntax is if (variable is Type name).

2. Step 2: Check each option

   if (obj is int number) { } matches the correct syntax. Options B, C, and D have incorrect order or keywords.

3. Final Answer:

   if (obj is int number) { } -> Option D

4. Quick Check:

   Correct syntax = if (obj is Type name) [OK]

Hint: Remember: if (obj is Type var) [OK]

Common Mistakes:

• Swapping type and variable order
• Using 'as' instead of 'is' for pattern
• Placing keywords incorrectly

3. What will be the output of this C# code?

object obj = 42;
if (obj is int number)
{
    Console.WriteLine(number + 10);
}
else
{
    Console.WriteLine("Not an int");
}

medium

A. 52

B. Not an int

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Check the type of obj

   obj holds the integer 42, so obj is int is true.

2. Step 2: Calculate the output inside the if block

   The variable number is assigned 42, so number + 10 equals 52, which is printed.

3. Final Answer:

   52 -> Option A

4. Quick Check:

   Type check true, output = 42 + 10 = 52 [OK]

Hint: If type matches, variable holds value for use [OK]

Common Mistakes:

• Expecting 'Not an int' output
• Confusing type check with cast
• Thinking code throws error

4. Identify the error in this code snippet using type patterns:

object obj = "hello";
if (obj is int number)
{
    Console.WriteLine(number);
}

medium

A. No output because obj is not an int

B. Compilation error due to missing else block

C. No error, code runs and prints 'hello'

D. Runtime exception when casting string to int

Solution

1. Step 1: Analyze the type check condition

   obj is a string, so obj is int number is false.

2. Step 2: Understand the effect on output

   Since the if condition is false and no else block exists, nothing is printed.

3. Final Answer:

   No output because obj is not an int -> Option A

4. Quick Check:

   False type check = no output [OK]

Hint: False type check skips block, no output [OK]

Common Mistakes:

• Expecting runtime exception
• Thinking else block is mandatory
• Assuming it prints the string

5. Given this method using type patterns, what will it return?
<pre>string Describe(object input)
{
return input switch
{
int i => $"Integer: {i}",
string s => $"String: {s}",
null => "Null value",
_ => "Unknown type"
};
}

What is the result of Describe(3.14)?

hard

A. "Integer: 3"

B. "Unknown type"

C. "String: 3.14"

D. Compilation error

Solution

1. Step 1: Check the switch cases for input 3.14

   3.14 is a double, which does not match int, string, or null.

2. Step 2: Identify the default case

   The underscore _ matches any other type, so it returns "Unknown type".

3. Final Answer:

   "Unknown type" -> Option B

4. Quick Check:

   Switch default case = "Unknown type" [OK]

Hint: Switch default (_) handles unmatched types [OK]

Common Mistakes:

• Assuming 3.14 is int or string
• Expecting compilation error
• Ignoring default case