Type checking patterns in C Sharp (C#)

Introduction

Type checking patterns help you check what kind of data you have and use it safely in your code.

When you want to do something only if a variable is a certain type.

When you want to get a value from an object if it matches a type.

When you want to avoid errors by checking types before using data.

When you want to write clear and simple code that handles different data types.

Syntax

C Sharp (C#)

if (variable is TypeName name) {
    // use 'name' here
}

The is keyword checks the type and assigns the variable if it matches.

This pattern helps avoid extra casting and makes code safer and cleaner.

Examples

This checks if obj is a string, then uses it as s.

C Sharp (C#)

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

This checks if obj is an int, then adds 10 to it.

C Sharp (C#)

object obj = 123;
if (obj is int number) {
    Console.WriteLine(number + 10);
}

You can also check if an object is null using is null.

C Sharp (C#)

object obj = null;
if (obj is null) {
    Console.WriteLine("Object is null");
}

Sample Program

This program checks each item in an array. It prints the uppercase string if it's a string, adds 1 if it's an integer, notes if it's null, and says "Other type detected" for anything else.

C Sharp (C#)

using System;

class Program {
    static void Main() {
        object[] items = { "apple", 42, 3.14, null };

        foreach (var item in items) {
            if (item is string text) {
                Console.WriteLine($"String: {text.ToUpper()}");
            } else if (item is int number) {
                Console.WriteLine($"Integer: {number + 1}");
            } else if (item is null) {
                Console.WriteLine("Null value found");
            } else {
                Console.WriteLine("Other type detected");
            }
        }
    }
}

Important Notes

Type checking patterns reduce the need for manual casting and make code safer.

They work well with switch statements for cleaner multi-type checks.

Summary

Type checking patterns let you test and use variables by their type easily.

They help avoid errors by ensuring you only use data in the right way.

Using is TypeName name is a simple and clear way to check types and get values.

Practice

1. What does the C# pattern if (obj is string s) do?

easy

A. Checks if obj is null

B. Converts obj to 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 pattern

   The is keyword checks if an object is of a certain type.

2. Step 2: Assign variable if type matches

   If obj is a string, it assigns the value to s for use inside the block.

3. Final Answer:

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

4. Quick Check:

   is Type var checks type and assigns [OK]

Hint: Remember: is Type var checks and assigns together [OK]

Common Mistakes:

• Thinking it converts without checking
• Assuming it checks for null only
• Believing it throws exceptions automatically

2. Which of the following is the correct syntax to check if obj is an int and assign it to number?

easy

A. if (obj is int number) { }

B. if (obj as int number) { }

C. if (obj == int number) { }

D. if (obj instanceof int number) { }

Solution

1. Step 1: Identify correct C# type pattern syntax

   The correct syntax uses is Type variable to check and assign.

2. Step 2: Compare options

   if (obj is int number) { } uses is int number, which is valid. Others use invalid keywords or syntax.

3. Final Answer:

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

4. Quick Check:

   is Type var syntax is correct [OK]

Hint: Use is Type var for type check and assignment [OK]

Common Mistakes:

• Using 'as' instead of 'is' for type checking
• Using '==' to compare types
• Using JavaScript or Java keywords like 'instanceof'

3. What is the output of this code?

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

medium

A. Not an int

B. 52

C. 42

D. Compilation error

Solution

1. Step 1: Check the type of obj

   obj holds the integer 42, so obj is int n is true and assigns 42 to n.

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

   It prints n + 10 which is 42 + 10 = 52.

3. Final Answer:

   52 -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Printing the original obj instead of n + 10
• Choosing else output wrongly
• Thinking it causes a compile error

4. Identify the error in this code snippet:

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

medium

A. No error, code runs and prints number

B. Runtime exception thrown

C. Syntax error in the if statement

D. The variable 'number' is not assigned because obj is not int

Solution

1. Step 1: Analyze the type check

   obj is a string, so obj is int number is false and number is not assigned.

2. Step 2: Understand the effect on code execution

   The if block is skipped, so nothing prints. No error or exception occurs.

3. Final Answer:

   The variable 'number' is not assigned because obj is not int -> Option D

4. Quick Check:

   Type check false means variable not assigned [OK]

Hint: If type check fails, variable is not assigned [OK]

Common Mistakes:

• Thinking it causes syntax error
• Expecting runtime exception
• Assuming variable is assigned anyway

5. Given a list of objects List<object> items = new() { 1, "two", 3, null, 4.5 };, which code snippet correctly sums only the integer values using type checking patterns?

hard

A. int sum = 0;
foreach (var item in items)
{
if (item is int n) sum += n;
}
Console.WriteLine(sum);

B. int sum = 0;
foreach (int n in items)
{
sum += n;
}
Console.WriteLine(sum);

C. int sum = 0;
foreach (var item in items)
{
if (item is double d) sum += (int)d;
}
Console.WriteLine(sum);

D. int sum = 0;
foreach (var item in items)
{
sum += (int)item;
}
Console.WriteLine(sum);

Solution

1. Step 1: Understand the list contents

   The list has integers, a string, null, and a double. We want to sum only integers.

2. Step 2: Check each option for correct type filtering

   int sum = 0; foreach (var item in items) { if (item is int n) sum += n; } Console.WriteLine(sum); uses if (item is int n) to add only integers. int sum = 0; foreach (int n in items) { sum += n; } Console.WriteLine(sum); tries to cast all items to int in foreach, causing error. int sum = 0; foreach (var item in items) { if (item is double d) sum += (int)d; } Console.WriteLine(sum); sums doubles cast to int, which is incorrect. int sum = 0; foreach (var item in items) { sum += (int)item; } Console.WriteLine(sum); casts all items to int without checking, causing runtime errors.

3. Final Answer:

   int sum = 0; foreach (var item in items) { if (item is int n) sum += n; } Console.WriteLine(sum); -> Option A

4. Quick Check:

   Use is int var to filter integers safely [OK]

Hint: Use if (item is int n) to sum integers safely [OK]

Common Mistakes:

• Casting without checking causing exceptions
• Summing wrong types like double or string
• Assuming foreach int works on object list