Default keyword for generic types in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of default keyword with generic type parameter

What is the output of this C# code snippet?

C Sharp (C#)

using System;

class Program {
    static void ShowDefault<T>() {
        T value = default;
        Console.WriteLine(value == null ? "null" : value.ToString());
    }

    static void Main() {
        ShowDefault<int>();
        ShowDefault<string>();
    }
}

A Compilation error

B

null
null

C

0
""

D

0
null

💡 Hint

Consider what default returns for value types and reference types.

The default keyword returns the default value for the generic type T. For int (a value type), it returns 0. For string (a reference type), it returns null.

❓ Predict Output

intermediate

Default keyword with nullable generic type

What will be the output of this C# program?

C Sharp (C#)

using System;

class Program {
    static void ShowDefault<T>() {
        T value = default;
        Console.WriteLine(value == null ? "null" : value.ToString());
    }

    static void Main() {
        ShowDefault<int?>();
        ShowDefault<double>();
    }
}

A

null
0

B

0
0

C

null
null

D Compilation error

💡 Hint

Nullable types are structs that can hold null.

int? is a nullable int, so its default is null. double is a value type, so default is 0.

❓ Predict Output

advanced

Default keyword with struct generic type

What is the output of this C# code?

C Sharp (C#)

using System;

struct Point {
    public int X, Y;
    public override string ToString() => $"({X},{Y})";
}

class Program {
    static void ShowDefault<T>() where T : struct {
        T value = default;
        Console.WriteLine(value.ToString());
    }

    static void Main() {
        ShowDefault<Point>();
    }
}

A null

B (,)

C (0,0)

D Compilation error

💡 Hint

Default of a struct initializes all fields to zero.

The default value of a struct sets all fields to zero. So Point has X=0 and Y=0, printing (0,0).

❓ Predict Output

advanced

Default keyword with generic class and null check

What will this program print?

C Sharp (C#)

using System;

class Container<T> {
    public T Value = default;
    public bool IsNull() => Value == null;
}

class Program {
    static void Main() {
        var c1 = new Container<string>();
        var c2 = new Container<int>();
        Console.WriteLine(c1.IsNull());
        Console.WriteLine(c2.IsNull());
    }
}

A

True
False

B

False
False

C

True
True

D Compilation error

💡 Hint

Value types cannot be null, reference types can.

string is a reference type, so default is null and IsNull() returns true. int is a value type, default is 0, so IsNull() returns false.

🧠 Conceptual

expert

Behavior of default keyword with unconstrained generic types

Consider a generic method void Test() with no constraints on T. Which statement about default(T) is true?

A It returns null for all types, including value types.

B It returns the default value for value types and null for reference types.

C It causes a compile-time error if <code>T</code> is unconstrained.

D It returns zero for numeric types and null for reference types.

💡 Hint

Think about how default behaves for value and reference types.

default(T) returns the default value for value types (like 0 for int) and null for reference types. It works even if T is unconstrained.

Practice

1. What does the default keyword return when used with a generic type parameter T in C#?

easy

A. A new instance of type T created with the default constructor

B. The default value of type T, like 0 for int or null for reference types

C. An exception is thrown if T is a reference type

D. The string representation of type T

Solution

1. Step 1: Understand the purpose of default keyword with generic T

   The default keyword returns the default value for any type. For value types like int, it returns 0. For reference types, it returns null. When used with a generic type parameter T, default(T) returns the default value of that type without creating an instance.

2. Final Answer:

   The default value of type T, like 0 for int or null for reference types -> Option B

3. Quick Check:

   default(T) = default value [OK]

Hint: Remember: default returns zero/null, not a new object [OK]

Common Mistakes:

• Thinking default creates a new instance
• Assuming default throws exceptions
• Confusing default with ToString()

2. Which of the following is the correct syntax to get the default value of a generic type T inside a method?

easy

A. default(T)

B. default<T>

C. default T

D. default::T

Solution

1. Step 1: Recall syntax for default and check options

   The correct syntax to get the default value of a type is default(TypeName). For generic types, it becomes default(T). default<T> uses invalid angle brackets. default T misses parentheses. default::T uses invalid scope operator. Only default(T) is correct.

2. Final Answer:

   default(T) -> Option A

3. Quick Check:

   Use parentheses with default: default(T) [OK]

Hint: Use parentheses with default keyword for types [OK]

Common Mistakes:

• Using angle brackets instead of parentheses
• Omitting parentheses
• Using invalid operators like ::

3. What is the output of this C# code?

static T GetDefault<T>() {
    return default(T);
}

Console.WriteLine(GetDefault<int>());
Console.WriteLine(GetDefault<string>() == null);

medium

A. 0
False

B. null
True

C. 0
True

D. null
False

Solution

1. Step 1: Trace the code execution

   For GetDefault<int>(), default(int) returns 0, so first line prints 0. For GetDefault<string>(), default(string) returns null, so null == null is true, second line prints True.

2. Final Answer:

   0 True -> Option C

3. Quick Check:

   default int = 0, default string = null [OK]

Hint: Value types default to zero; reference types default to null [OK]

Common Mistakes:

• Assuming default string returns empty string
• Confusing default int with null
• Forgetting default returns null for references

4. Identify the error in this generic method using default keyword:

public T GetValue<T>() {
    T value = default;
    return value;
}

medium

A. Syntax error: default must be used as default(T) in this context

B. Missing return type in method signature

C. Runtime error because default cannot be assigned to generic types

D. No error; code compiles and returns default value

Solution

1. Step 1: Validate default literal usage in generic context

   Since C# 7.1, default can be used as a literal without specifying type when the compiler can infer it from context like T value = default;. The method correctly initializes and returns the default value of T. No error.

2. Final Answer:

   No error; code compiles and returns default value -> Option D

3. Quick Check:

   default literal works with inferred generic type [OK]

Hint: Since C# 7.1, default literal can omit type in generics [OK]

Common Mistakes:

• Thinking default must always have parentheses
• Assuming runtime error for default assignment
• Missing that default literal is valid syntax

5. You want to write a generic method that returns the default value of T but only if T is a value type. Otherwise, it should return null. Which code snippet correctly implements this?

hard

A. public object GetDefaultIfValueType<T>() {
return typeof(T).IsValueType ? default(T) : null;
}

B. public T GetDefaultIfValueType<T>() {
return default(T) ?? null;
}

C. public T? GetDefaultIfValueType<T>() where T : struct {
return default(T);
}

D. public T GetDefaultIfValueType<T>() {
if (default(T) == null) return null;
else return default(T);
}

Solution

1. Step 1: Evaluate options against requirement for value/reference types

   Needs runtime check as constraints can't handle both; return object. C: typeof(T).IsValueType ? default(T) : null works. A restricts to struct; B ?? null invalid for values; D default(T) == null compile error unconstrained.

2. Final Answer:

   public object GetDefaultIfValueType<T>() { return typeof(T).IsValueType ? default(T) : null; } -> Option A

3. Quick Check:

   Use typeof(T).IsValueType to decide default or null [OK]

Hint: Use typeof(T).IsValueType to check type at runtime [OK]

Common Mistakes:

• Restricting generic type with where struct and missing reference types
• Using null-coalescing on value types
• Comparing default(T) to null directly