Type patterns in C Sharp (C#) - Time & Space Complexity

Time Complexity: Type patterns

O(n)

Understanding Time Complexity

When using type patterns in C#, we often check an object's type to decide what to do next.

We want to see how the time it takes grows as we check more objects or more complex types.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

object[] items = new object[] { 1, "hello", 3.14, true, new List<int>() };

foreach (var item in items)
{
    if (item is int i)
        Console.WriteLine($"Integer: {i}");
    else if (item is string s)
        Console.WriteLine($"String: {s}");
    else
        Console.WriteLine("Other type");
}

This code checks each item in an array to see if it matches certain types and prints a message accordingly.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each item in the array and checking its type.
• How many times: Once for each item in the array (n times, where n is the number of items).

How Execution Grows With Input

As the number of items grows, the program checks each one once.

Input Size (n)	Approx. Operations
10	About 10 type checks
100	About 100 type checks
1000	About 1000 type checks

Pattern observation: The number of operations grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the number of items checked.

Common Mistake

[X] Wrong: "Checking types inside the loop makes it slower in a way that depends on the number of type options."

[OK] Correct: The number of type checks per item is fixed and small, so it does not grow with input size. The main cost grows only with how many items there are.

Interview Connect

Understanding how type checks inside loops affect performance helps you write clear and efficient code, a skill valued in many programming tasks.

Self-Check

"What if we added nested loops that check types inside each item? How would the time complexity change?"

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