Generic interfaces in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a generic interface in C#?

A generic interface is an interface that has one or more type parameters. It allows you to define methods and properties that work with any data type specified when the interface is implemented.

beginner

How do you declare a generic interface with one type parameter named T?

You declare it like this:

public interface IExample<T> {
    void DoSomething(T item);
}

intermediate

Why use generic interfaces instead of non-generic ones?

Generic interfaces provide type safety and code reuse. They let you write flexible code that works with any data type without losing the benefits of compile-time type checking.

beginner

How do you implement a generic interface in a class?

You specify the type when implementing the interface. For example:<br><pre>public class MyClass : IExample&lt;int&gt; {<br>    public void DoSomething(int item) {<br>        // implementation<br>    }<br>}</pre>

intermediate

Can a generic interface have multiple type parameters? Give an example.

Yes, it can. Example:

public interface IPair<T1, T2> {
    T1 First { get; }
    T2 Second { get; }
}

What does the <T> mean in a generic interface declaration?

A It is a placeholder for a data type specified later

B It is a method name

C It is a variable

D It is a keyword to create an object

The <T> is a placeholder for a type that will be specified when the interface is implemented.

How do you specify the type when implementing a generic interface?

A By providing the type in angle brackets after the interface name

B By declaring a new interface

C By using the 'new' keyword

D By casting the interface

You specify the type by writing the interface name followed by the type in angle brackets, like IExample.

Which of these is a valid generic interface declaration?

A public interface ITest { void Do(T item); }

B public class ITest<T> { void Do(T item); }

C public interface ITest<T> { void Do(T item); }

D public interface ITest<T> { T Do(); }

Option C correctly declares a generic interface with a method using the generic type T.

Can a generic interface have properties using the generic type?

A Only static properties can use the generic type

B Yes, properties can use the generic type

C No, generic interfaces cannot have properties

D No, only methods can use the generic type

Generic interfaces can have properties that use the generic type parameter.

What is the main advantage of using generic interfaces?

A They allow private methods in interfaces

B They make code run faster

C They reduce the number of classes needed

D They allow code reuse with type safety

Generic interfaces allow writing reusable code that works with any type while keeping type safety.

Explain what a generic interface is and why it is useful.

Think about how you can write one interface that works with many data types.

Describe how to implement a generic interface in a class with an example.

Remember to specify the type in angle brackets after the interface name.

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