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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of generic interface implementation

What is the output of the following C# code that uses a generic interface?

C Sharp (C#)

using System;

interface IContainer<T>
{
    T GetItem();
}

class Box<T> : IContainer<T>
{
    private T item;
    public Box(T item) { this.item = item; }
    public T GetItem() => item;
}

class Program
{
    static void Main()
    {
        IContainer<string> container = new Box<string>("Hello");
        Console.WriteLine(container.GetItem());
    }
}

A Hello

B System.String

C Box`1

D Compilation error

💡 Hint

Look at what the GetItem method returns and what is passed to the Box constructor.

The Box class implements IContainer and stores the item passed in the constructor. The GetItem method returns that item, which is the string "Hello".

❓ Predict Output

intermediate

Output of generic interface with multiple types

What will be printed when running this C# program using a generic interface with different types?

C Sharp (C#)

using System;

interface IPrinter<T>
{
    void Print(T value);
}

class Printer<T> : IPrinter<T>
{
    public void Print(T value) => Console.WriteLine($"Value: {value}");
}

class Program
{
    static void Main()
    {
        IPrinter<int> intPrinter = new Printer<int>();
        IPrinter<double> doublePrinter = new Printer<double>();
        intPrinter.Print(5);
        doublePrinter.Print(3.14);
    }
}

A Value: 5\nValue: 3.14

B Value: 5.0\nValue: 3.14

C 5\n3.14

D Compilation error due to interface mismatch

💡 Hint

Check how the Print method formats the output string.

The Print method writes "Value: " followed by the value passed. For int 5 and double 3.14, it prints exactly those values with the prefix.

🔧 Debug

advanced

Identify the error in generic interface implementation

What error will this C# code produce when compiled?

C Sharp (C#)

interface IRepository<T>
{
    void Add(T item);
    T Get(int id);
}

class Repository : IRepository<int>
{
    public void Add(int item) { }
    public string Get(int id) { return "item"; }
}

A Runtime error: InvalidCastException

B No error, compiles and runs fine

C Compilation error: 'Repository.Get(int)' does not implement interface member 'IRepository<int>.Get(int)'

D Compilation error: Missing method Add

💡 Hint

Check the return type of the Get method in the class versus the interface.

The interface expects Get to return int, but the class returns string, causing a compilation error.

📝 Syntax

advanced

Which option correctly declares a generic interface with two type parameters?

Select the correct syntax for declaring a generic interface with two type parameters TKey and TValue in C#.

A interface IDictionary<TKey; TValue> { void Add(TKey key, TValue value); }

B interface IDictionary<TKey, TValue> { void Add(TKey key, TValue value); }

C interface IDictionary<TKey TValue) { void Add(TKey key, TValue value); }

D interface IDictionary<T1, T2> { void Add(TKey key, TValue value); }

💡 Hint

Look for correct generic type parameter syntax with angle brackets and commas.

Option B correctly uses angle brackets with comma-separated type parameters and valid method signature.

🚀 Application

expert

Determine the number of items in the generic interface collection

Given the following code, what is the output of the program?

C Sharp (C#)

using System;
using System.Collections.Generic;

interface IStore<T>
{
    void Add(T item);
    int Count { get; }
}

class Store<T> : IStore<T>
{
    private List<T> items = new List<T>();
    public void Add(T item) => items.Add(item);
    public int Count => items.Count;
}

class Program
{
    static void Main()
    {
        IStore<string> store = new Store<string>();
        store.Add("apple");
        store.Add("banana");
        store.Add("cherry");
        Console.WriteLine(store.Count);
    }
}

A 0

B Compilation error due to missing Count implementation

C Runtime error: NullReferenceException

D 3

💡 Hint

Count returns the number of items added to the list.

Three items are added to the Store, so Count returns 3.