Default values for types in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the default value of an int in C#?

The default value of an int is 0.

beginner

What is the default value of a bool in C#?

The default value of a bool is false.

beginner

What is the default value of a reference type like string in C#?

The default value of a reference type such as string is null.

intermediate

How can you get the default value of any type T in C#?

You can use the expression default(T) to get the default value of type T.

beginner

What is the default value of a double in C#?

The default value of a double is 0.0.

What is the default value of a char in C#?

A null

B ' ' (space)

C 'a'

D '\0' (null character)

The default value of a char is the null character '\0', which is the Unicode character with code zero.

Which of these types has a default value of null in C#?

A string

B bool

C int

D double

Reference types like string have a default value of null. Value types like int, bool, and double have non-null default values.

What does the expression default(bool) return?

A true

B false

C null

D 0

default(bool) returns false, which is the default value for the bool type.

How do you get the default value of a generic type T in C#?

A default(T)

B T.default

C null

D new T()

The correct way to get the default value of a generic type T is to use default(T).

What is the default value of a decimal type in C#?

A 0.0

B null

C 0

D 0.0m

The default value of decimal is 0. The suffix m is used for literals but not for default values.

Explain what default values are in C# and give examples for both value and reference types.

Think about what happens when you declare a variable but don't assign a value.

How can you use the default keyword with generic types in C#? Why is it useful?

Consider how you write code that works with any type.

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