Generic class declaration in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a generic class instance

What is the output of this C# program using a generic class?

C Sharp (C#)

using System;

class Box<T>
{
    public T Content { get; set; }
    public Box(T content) { Content = content; }
    public override string ToString() => $"Box contains: {Content}";
}

class Program
{
    static void Main()
    {
        var box = new Box<int>(123);
        Console.WriteLine(box);
    }
}

A Box contains: System.Int32

B Box contains: 123

C Box contains: T

D Compilation error

💡 Hint

Look at how the generic type T is used and what value is passed to the constructor.

The generic class Box stores a value of type T. Here, T is int, so Content is 123. The ToString method returns 'Box contains: 123'.

❓ Predict Output

intermediate

Generic class with multiple type parameters

What will be the output of this C# program with a generic class having two type parameters?

C Sharp (C#)

using System;

class Pair<T1, T2>
{
    public T1 First { get; set; }
    public T2 Second { get; set; }
    public Pair(T1 first, T2 second) { First = first; Second = second; }
    public override string ToString() => $"({First}, {Second})";
}

class Program
{
    static void Main()
    {
        var pair = new Pair<string, double>("pi", 3.14);
        Console.WriteLine(pair);
    }
}

A (3.14, pi)

B Compilation error

C (System.String, System.Double)

D (pi, 3.14)

💡 Hint

Check the order of type parameters and constructor arguments.

The Pair class stores two values of types T1 and T2. Here, T1 is string with value "pi", and T2 is double with value 3.14. The ToString method formats them as (pi, 3.14).

🔧 Debug

advanced

Identify the error in generic class declaration

What error does this C# code produce?

C Sharp (C#)

class Container<T>
{
    public T Value;
    public Container(T val)
    {
        Value = val;
    }
}

A SyntaxError: Missing semicolon after 'Value = val'

B TypeError: Cannot assign T to Value

C Runtime error: NullReferenceException

D No error, compiles successfully

💡 Hint

Check punctuation carefully inside the constructor.

The constructor is missing a semicolon after 'Value = val', causing a syntax error.

❓ Predict Output

advanced

Output of generic class with method using type constraint

What is the output of this C# program using a generic class with a type constraint?

C Sharp (C#)

using System;

class Calculator<T> where T : struct
{
    public T Add(T a, T b)
    {
        dynamic x = a;
        dynamic y = b;
        return x + y;
    }
}

class Program
{
    static void Main()
    {
        var calc = new Calculator<int>();
        Console.WriteLine(calc.Add(5, 7));
    }
}

A 12

B Compilation error due to dynamic usage

C Runtime error: InvalidCastException

D 0

💡 Hint

The Add method uses dynamic to add two struct values.

The generic class restricts T to value types (struct). Using dynamic allows adding the two integers 5 and 7, resulting in 12.

🧠 Conceptual

expert

Understanding generic class variance

Which statement about generic class variance in C# is correct?

A Variance allows changing the type parameter of a generic class at runtime.

B Generic classes support covariance and contravariance on their type parameters by default.

C Only interfaces and delegates can declare variance with 'in' and 'out' keywords, not classes.

D Variance means a generic class can only accept reference types as type parameters.

💡 Hint

Think about where 'in' and 'out' keywords are allowed in C# generics.

In C#, only interfaces and delegates can declare variance using 'in' (contravariance) and 'out' (covariance) keywords. Generic classes do not support variance on their type parameters.