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.

Practice

1. What is the main purpose of a generic interface in C#?

easy

A. To allow the interface to work with any data type specified later

B. To restrict the interface to only one specific data type

C. To make the interface private and inaccessible outside its class

D. To automatically implement all methods without code

Solution

1. Step 1: Understand generic interfaces

   Generic interfaces use placeholders (type parameters) to allow flexibility in the data types they work with.

2. Step 2: Identify the purpose

   This flexibility means the same interface can be reused with different types, making code more reusable and type-safe.

3. Final Answer:

   To allow the interface to work with any data type specified later -> Option A

4. Quick Check:

   Generic interface = flexible type [OK]

Hint: Generic interfaces use placeholders for types [OK]

Common Mistakes:

• Thinking generic interfaces fix one type only
• Confusing generic interfaces with access modifiers
• Assuming generic interfaces auto-implement methods

2. Which of the following is the correct syntax to declare a generic interface named IContainer with a type parameter T?

easy

A. interface IContainer<T> () { }

B. interface IContainer { T }

C. interface IContainer<T> { }

D. class IContainer<T> { }

Solution

1. Step 1: Recall generic interface syntax

   Generic interfaces use angle brackets after the interface name to specify type parameters, like IContainer<T>.

2. Step 2: Check each option

   interface IContainer<T> { } correctly declares a generic interface. interface IContainer { T } is invalid syntax. class IContainer<T> { } uses 'class' instead of 'interface'. interface IContainer<T> () { } has incorrect parentheses.

3. Final Answer:

   interface IContainer<T> { } -> Option C

4. Quick Check:

   Generic interface syntax = interface Name<T> [OK]

Hint: Generic interfaces use angle brackets after name [OK]

Common Mistakes:

• Using class instead of interface keyword
• Omitting angle brackets for type parameter
• Adding parentheses after interface name

3. What will be the output of the following C# code?

interface IPrinter<T> { void Print(T item); }
class StringPrinter : IPrinter<string> {
  public void Print(string item) { Console.WriteLine(item); }
}
var printer = new StringPrinter();
printer.Print("Hello World");

medium

A. Hello World

B. Compile-time error due to missing type parameter

C. Runtime error due to invalid cast

D. No output

Solution

1. Step 1: Understand interface implementation

   The class StringPrinter implements IPrinter<string> and defines the Print method to write the string to console.

2. Step 2: Trace method call

   The Print method is called with "Hello World", so it prints this string to the console.

3. Final Answer:

   Hello World -> Option A

4. Quick Check:

   Implemented method prints input string [OK]

Hint: Implemented method prints passed argument [OK]

Common Mistakes:

• Expecting compile error for correct generic usage
• Confusing interface with abstract class
• Thinking Print method returns a value

4. Identify the error in this code snippet:

interface IRepository<T> {
  void Add(T item);
}
class Repository : IRepository {
  public void Add(object item) { /* implementation */ }
}

medium

A. Interface IRepository cannot have methods

B. Class Repository must specify the type parameter when implementing IRepository

C. Method Add should return a value

D. Class Repository cannot implement interfaces

Solution

1. Step 1: Check interface implementation

   The interface IRepository is generic and requires a type parameter <T>. The class Repository implements it without specifying this type.

2. Step 2: Identify the fix

   The class must specify the type parameter, for example IRepository<object>, to correctly implement the interface.

3. Final Answer:

   Class Repository must specify the type parameter when implementing IRepository -> Option B

4. Quick Check:

   Generic interface requires type parameter [OK]

Hint: Generic interfaces need type parameters when implemented [OK]

Common Mistakes:

• Omitting type parameter in implementation
• Assuming interface methods must return values
• Thinking classes can't implement interfaces

5. Given the generic interface and classes below, what will be the output?

interface ITransformer<T> {
  T Transform(T input);
}
class IntDoubler : ITransformer<int> {
  public int Transform(int input) => input * 2;
}
class StringRepeater : ITransformer<string> {
  public string Transform(string input) => input + input;
}
var doubler = new IntDoubler();
var repeater = new StringRepeater();
Console.WriteLine(doubler.Transform(5));
Console.WriteLine(repeater.Transform("Hi"));

hard

A. 5
HiHi

B. 55
HiHi

C. 10
Hi

D. 10
HiHi

Solution

1. Step 1: Analyze IntDoubler.Transform

   This method doubles the integer input. For input 5, output is 5 * 2 = 10.

2. Step 2: Analyze StringRepeater.Transform

   This method repeats the string input twice. For input "Hi", output is "Hi" + "Hi" = "HiHi".

3. Final Answer:

   10 HiHi -> Option D

4. Quick Check:

   Int doubled = 10, string repeated = HiHi [OK]

Hint: Trace each Transform method with given input [OK]

Common Mistakes:

• Mixing outputs of different classes
• Forgetting string concatenation doubles text
• Assuming Transform returns original input