Default values for types in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign the default value of an int variable.

C Sharp (C#)

int number = [1];

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

Adefault(int)

B0

Cnull

Dint()

Attempts:

3 left

💡 Hint

Common Mistakes

Using null for value types like int causes errors.

Using 0 directly is correct but the task asks for default keyword.

In C#, default(int) returns the default value for the int type, which is 0.

2fill in blank

medium

Complete the code to assign the default value of a bool variable.

C Sharp (C#)

bool isActive = [1];

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

Atrue

Bnull

Cdefault(bool)

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using null for bool causes errors because bool is a value type.

Using true is not the default value.

default(bool) returns false, the default value for bool.

3fill in blank

hard

Fix the error in the code to assign the default value of a string variable.

C Sharp (C#)

string name = [1];

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

A""

Bdefault(string)

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes a type error.

Using empty string "" is not the default value.

Using default(string) assigns null which is the default for reference types like string.

4fill in blank

hard

Fill both blanks to create a method that returns the default value of a generic type T.

C Sharp (C#)

public T GetDefaultValue<[1]>() {
    return [2];
}

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

AT

Bdefault(T)

Cvoid

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as a generic type parameter is invalid.

Returning null may cause errors for value types.

The generic type parameter is T, and default(T) returns its default value.

5fill in blank

hard

Fill all three blanks to create a dictionary with keys as strings and values as default values of type int.

C Sharp (C#)

var defaults = new Dictionary<string, int> {
    {"one", [1],
    {"two", [2],
    {"three", [3]
};

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

Adefault(int)

B0

Cdefault

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null causes errors because int is a value type.

Using 0 is correct but the task asks for default keyword.

Using default(int) assigns the default int value 0 to each dictionary entry.

Practice

1. What is the default value of an int type variable in C#?

easy

A. 0

B. null

C. 1

D. "" (empty string)

Solution

1. Step 1: Understand default values for value types

   In C#, value types like int have a default value of zero when not initialized.

2. Step 2: Recall default for int

   The default for int is 0, not null or any other value.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Default int = 0 [OK]

Hint: Remember: numbers default to zero in C# [OK]

Common Mistakes:

• Thinking default int is null
• Confusing default int with 1
• Assuming default is empty string

2. Which of the following is the correct syntax to get the default value of a type T in C#?

easy

A. T.default()

B. default = T

C. default(T)

D. T.default

Solution

1. Step 1: Recall C# syntax for default values

   The correct syntax to get the default value of a type T is default(T).

2. Step 2: Check other options

   Options like T.default(), default = T, and T.default are invalid syntax in C#.

3. Final Answer:

   default(T) -> Option C

4. Quick Check:

   Use default(T) to get default value [OK]

Hint: Use default(Type) syntax to get default value [OK]

Common Mistakes:

• Using invalid syntax like T.default()
• Confusing assignment with default keyword
• Trying to access default as a property

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

bool flag = default;
Console.WriteLine(flag);

medium

A. False

B. null

C. True

D. Compilation error

Solution

1. Step 1: Understand default value of bool

   The default value of bool in C# is false.

2. Step 2: Analyze the code output

   The variable flag is assigned default, so it holds false. Printing it outputs "False".

3. Final Answer:

   False -> Option A

4. Quick Check:

   Default bool = false [OK]

Hint: Default bool is always false, not true [OK]

Common Mistakes:

• Assuming default bool is true
• Expecting null for bool
• Thinking code causes error

4. Identify the error in this C# code snippet:

string text = default;
int number = default;
Console.WriteLine(text.Length);

medium

A. Compile-time error on text.Length

B. No error, code runs fine

C. Syntax error on default keyword

D. NullReferenceException at runtime

Solution

1. Step 1: Understand default for reference types

   For reference types like string, default is null.

2. Step 2: Analyze accessing property on null

   Accessing text.Length when text is null causes a NullReferenceException at runtime.

3. Final Answer:

   NullReferenceException at runtime -> Option D

4. Quick Check:

   Default string = null; accessing Length causes error [OK]

Hint: Default reference types are null; accessing members causes errors [OK]

Common Mistakes:

• Thinking default string is empty string
• Expecting compile-time error instead of runtime
• Ignoring null checks before member access

5. Given this struct and code:

struct Point { public int X; public int Y; }
Point p = default;
Console.WriteLine(p.X + "," + p.Y);

What is the output and why?

hard

A. Random values because struct fields are not set

B. 0,0 because struct fields default to zero

C. Compilation error because fields are uninitialized

D. null,null because struct is a reference type

Solution

1. Step 1: Understand default for structs

   Structs are value types; their fields get default values when using default.

2. Step 2: Default values of int fields in struct

   Both X and Y are int, so default is 0 for each.

3. Step 3: Output concatenation

   Printing p.X + "," + p.Y outputs "0,0".

4. Final Answer:

   0,0 because struct fields default to zero -> Option B

5. Quick Check:

   Struct fields default to zero [OK]

Hint: Struct fields default to zero, not null [OK]

Common Mistakes:

• Thinking structs are reference types
• Expecting null values for struct fields
• Assuming uninitialized fields cause errors