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.