Generic interfaces in C Sharp (C#)

Introduction

Generic interfaces let you create flexible blueprints that work with any data type. This helps you write code that can handle many kinds of data without repeating yourself.

When you want to create a contract for classes that can work with different types of data.

When you want to write reusable code that can handle multiple data types safely.

When you want to enforce that certain methods or properties work with a specific type, but that type can vary.

When building collections or services that should work with any type of object.

When you want to avoid code duplication for similar operations on different data types.

Syntax

C Sharp (C#)

interface IMyInterface<T>
{
    void DoSomething(T item);
    T GetItem();
}

The <T> after the interface name means it is generic and uses a placeholder type T.

Classes that implement this interface must specify the actual type for T and implement the methods using that type.

Examples

This interface defines a repository that can store and find items of any type T.

C Sharp (C#)

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

This class implements IRepository for strings. It stores strings in a list.

C Sharp (C#)

class StringRepository : IRepository<string>
{
    private List<string> items = new();
    public void Add(string item) => items.Add(item);
    public string FindById(int id) => items[id];
}

This interface defines a way to compare two objects of type T.

C Sharp (C#)

interface IComparer<T>
{
    int Compare(T a, T b);
}

Sample Program

This program shows a generic interface IContainer<T> and a class Container<T> that implements it. It stores items of any type. We create containers for int and string and print their items.

C Sharp (C#)

using System;
using System.Collections.Generic;

interface IContainer<T>
{
    void AddItem(T item);
    T GetItem(int index);
}

class Container<T> : IContainer<T>
{
    private List<T> items = new();

    public void AddItem(T item) => items.Add(item);

    public T GetItem(int index) => items[index];
}

class Program
{
    static void Main()
    {
        IContainer<int> intContainer = new Container<int>();
        intContainer.AddItem(10);
        intContainer.AddItem(20);

        Console.WriteLine(intContainer.GetItem(0));
        Console.WriteLine(intContainer.GetItem(1));

        IContainer<string> stringContainer = new Container<string>();
        stringContainer.AddItem("hello");
        stringContainer.AddItem("world");

        Console.WriteLine(stringContainer.GetItem(0));
        Console.WriteLine(stringContainer.GetItem(1));
    }
}

Important Notes

Generic interfaces help avoid repeating code for different types.

When implementing a generic interface, you must specify the type or keep it generic.

You can use multiple generic parameters like interface IPair.

Summary

Generic interfaces use placeholders for types to make code reusable.

They define methods or properties that work with any specified type.

Classes implement these interfaces by specifying the actual type.

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