Implicit typing with var keyword in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an implicitly typed variable holding the number 10.

C Sharp (C#)

var number = [1];

Drag options to blanks, or click blank then click option'

A"10"

Bnumber

C10

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around 10 makes it a string, not an int.

Trying to assign 'var' as a value is incorrect.

The var keyword lets the compiler infer the type from the assigned value. Here, 10 is an integer, so number becomes an int.

2fill in blank

medium

Complete the code to declare an implicitly typed variable holding the text "hello".

C Sharp (C#)

var greeting = [1];

Drag options to blanks, or click blank then click option'

A"hello"

Bhello

C'hello'

Dgreeting

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes for strings is invalid in C#.

Not using quotes makes the compiler think it's a variable name.

The var keyword infers the type from the assigned value. Text must be in double quotes to be a string in C#.

3fill in blank

hard

Fix the error in the code by completing the declaration of an implicitly typed variable holding a decimal number.

C Sharp (C#)

var price = [1]m;

Drag options to blanks, or click blank then click option'

A19.99

B19,99

C"19.99"

Dprice

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas instead of dots for decimals.

Putting the number in quotes makes it a string, not a decimal.

Decimal literals in C# require the suffix m. The number must be written with a dot, not a comma, and without quotes.

4fill in blank

hard

Complete the code to declare an implicitly typed variable holding a list of integers and initialize it with values 1, 2, and 3.

C Sharp (C#)

var numbers = new [1]<int> {1, 2, 3};

Drag options to blanks, or click blank then click option'

A()

BList

C[]

DArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of curly braces for initialization.

Using 'Array' without proper syntax for initialization.

To create a list of integers, use List<int> and initialize it with curly braces { } containing the values.

5fill in blank

hard

Complete the code to declare an implicitly typed variable holding a dictionary with string keys and int values, initialized with two entries.

C Sharp (C#)

var ages = new Dictionary<[1], [2]> { {"Alice", 30}, {"Bob", 25} };

Drag options to blanks, or click blank then click option'

Astring

Bint

C{}

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets instead of curly braces for initialization.

Mixing up key and value types.

The dictionary keys are strings and values are integers. Use curly braces { } to initialize the dictionary with key-value pairs.

Practice

1. What does the var keyword do in C#?

easy

A. It lets the compiler decide the variable's type based on the assigned value.

B. It declares a variable without assigning any value.

C. It forces the variable to be of type var.

D. It creates a constant variable that cannot be changed.

Solution

1. Step 1: Understand the purpose of var

   The var keyword tells the compiler to infer the variable type from the value assigned.

2. Step 2: Compare options with this understanding

   Only It lets the compiler decide the variable's type based on the assigned value. correctly describes this behavior. Other options misunderstand var usage.

3. Final Answer:

   It lets the compiler decide the variable's type based on the assigned value. -> Option A

4. Quick Check:

   Implicit typing = compiler infers type [OK]

Hint: Remember: var means 'let compiler guess type' [OK]

Common Mistakes:

• Thinking var means no type is assigned
• Assuming var creates constants
• Believing var is a type itself

2. Which of the following is a correct way to declare a variable using var in C#?

easy

A. var number;

B. var int number = 10;

C. var = 10;

D. var number = 10;

Solution

1. Step 1: Check syntax rules for var

   Variables declared with var must be initialized immediately with a value.

2. Step 2: Evaluate each option

   var number = 10; declares and initializes correctly. var number; misses initialization. var = 10; has wrong syntax. var int number = 10; mixes explicit type with var incorrectly.

3. Final Answer:

   var number = 10; -> Option D

4. Quick Check:

   var requires immediate assignment [OK]

Hint: Always assign a value when using var [OK]

Common Mistakes:

• Declaring var without assigning a value
• Using var as a type name
• Combining var with explicit type

3. What will be the output of the following code?

var message = "Hello";
var count = 5;
Console.WriteLine(message + " " + count);

medium

A. Hello + 5

B. Hello5

C. Hello 5

D. Compilation error

Solution

1. Step 1: Understand variable types inferred

   message is a string "Hello", count is an int 5.

2. Step 2: Analyze the concatenation in WriteLine

   Using + with string and int converts int to string and concatenates with a space in between, so output is "Hello 5".

3. Final Answer:

   Hello 5 -> Option C

4. Quick Check:

   String + int concatenates as string [OK]

Hint: String + int converts int to string automatically [OK]

Common Mistakes:

• Expecting no space between words
• Thinking + prints literal plus sign
• Assuming compilation error due to var

4. Identify the error in the following code snippet:

var x;
x = 10;

medium

A. var requires initialization at declaration.

B. Missing semicolon after var declaration.

C. No error, code runs fine.

D. Cannot assign int to var variable.

Solution

1. Step 1: Recall var declaration rules

   Variables declared with var must be initialized immediately.

2. Step 2: Check the code snippet

   Here, var x; declares without initialization, which is invalid.

3. Final Answer:

   var requires initialization at declaration. -> Option A

4. Quick Check:

   var must be assigned when declared [OK]

Hint: var must have a value when declared [OK]

Common Mistakes:

• Thinking var can be declared without value
• Confusing var with dynamic
• Assuming assignment later is allowed

5. Given the code below, what is the type of result and what will be its value?

var numbers = new[] {1, 2, 3, 4};
var result = numbers.Select(n => n * 2).ToList();
Console.WriteLine(result.Count);

hard

A. Compilation error due to var usage

B. result is a List<int> with 4 elements; output: 4

C. result is a List<string>; output: 4

D. result is an array of int; output: 8

Solution

1. Step 1: Understand the type inferred for numbers

   numbers is an int array: int[].

2. Step 2: Analyze the LINQ Select and ToList

   Select doubles each number, producing IEnumerable<int>. ToList converts it to List<int> with 4 elements.

3. Step 3: Check Console.WriteLine output

   result.Count returns 4, the number of elements in the list.

4. Final Answer:

   result is a List<int> with 4 elements; output: 4 -> Option B

5. Quick Check:

   var infers List<int>, Count = 4 [OK]

Hint: var follows method return types exactly [OK]

Common Mistakes:

• Assuming result is an array, not a List
• Expecting output to be sum or doubled value
• Thinking var causes compilation error here