Generic interfaces in C Sharp (C#) - Mini Project: Build & Apply

Using Generic Interfaces in C#

📖 Scenario: You are building a simple system to store and display items of different types. You want to use a generic interface to handle items in a flexible way.

🎯 Goal: Create a generic interface called IItemHolder<T> with a method to get the item. Then implement this interface in a class and display the stored item.

📋 What You'll Learn

Create a generic interface IItemHolder<T> with a method T GetItem().

Create a class ItemHolder<T> that implements IItemHolder<T>.

Add a constructor to ItemHolder<T> to store an item of type T.

Implement the GetItem() method to return the stored item.

Create an instance of ItemHolder<string> with the value "Hello World".

Print the item returned by GetItem().

💡 Why This Matters

🌍 Real World

Generic interfaces let you write flexible code that works with many types without repeating yourself. For example, you can create containers or handlers for different data types easily.

💼 Career

Understanding generic interfaces is important for writing reusable and maintainable code in professional C# development, especially in libraries and frameworks.

1

Create the generic interface

Create a generic interface called IItemHolder<T> with a method T GetItem().

C Sharp (C#)

// Your code here

Hint

Use the interface keyword and add a generic type parameter <T>. Define a method T GetItem() inside.

2

Create the class that implements the interface

Create a class called ItemHolder<T> that implements the interface IItemHolder<T>. Add a private field item of type T and a constructor that takes a parameter item of type T and stores it.

C Sharp (C#)

public interface IItemHolder
{
    T GetItem();
}

// Your code here

Hint

Use class ItemHolder<T> : IItemHolder<T>. Add a private field item and a constructor to set it.

3

Create an instance and call the method

Create an instance of ItemHolder<string> called holder with the value "Hello World". Then call holder.GetItem() and store the result in a variable called result.

C Sharp (C#)

public interface IItemHolder
{
    T GetItem();
}

public class ItemHolder : IItemHolder
{
    private T item;

    public ItemHolder(T item)
    {
        this.item = item;
    }

    public T GetItem()
    {
        return item;
    }
}

// Your code here

Hint

Create holder using new ItemHolder<string>("Hello World"). Call GetItem() and save to result.

4

Print the result

Write a line to print the variable result to the console.

C Sharp (C#)

public interface IItemHolder
{
    T GetItem();
}

public class ItemHolder : IItemHolder
{
    private T item;

    public ItemHolder(T item)
    {
        this.item = item;
    }

    public T GetItem()
    {
        return item;
    }
}

var holder = new ItemHolder("Hello World");
var result = holder.GetItem();

// Your code here

Hint

Use Console.WriteLine(result); inside a Main method to print the value.

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