Multiple interface implementation in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Multiple interface implementation

Define Interface A

↓

Define Interface B

↓

Create Class implementing A and B

↓

Implement methods from A and B

↓

Create object of Class

↓

Call methods from both interfaces

↓

Use combined behavior

This flow shows how a class can implement multiple interfaces by defining all required methods, then using an object to access all interface behaviors.

Execution Sample

C Sharp (C#)

using System;
interface IA { void MethodA(); }
interface IB { void MethodB(); }
class MyClass : IA, IB {
  public void MethodA() { Console.WriteLine("A"); }
  public void MethodB() { Console.WriteLine("B"); }
}
class Program {
  static void Main() {
    var obj = new MyClass();
    obj.MethodA(); obj.MethodB();
  }
}

This code defines two interfaces and a class that implements both, then calls their methods.

Execution Table

Step	Action	Evaluation	Result
1	Define interface IA	Interface IA with MethodA declared	IA ready
2	Define interface IB	Interface IB with MethodB declared	IB ready
3	Define class MyClass implementing IA and IB	Class must implement MethodA and MethodB	MyClass ready
4	Implement MethodA in MyClass	MethodA prints 'A'	MethodA implemented
5	Implement MethodB in MyClass	MethodB prints 'B'	MethodB implemented
6	Create object obj of MyClass	obj is instance of MyClass	obj created
7	Call obj.MethodA()	Calls MethodA	Output: A
8	Call obj.MethodB()	Calls MethodB	Output: B
9	End of execution	All interface methods called	Program ends

💡 All interface methods implemented and called, program ends normally

Variable Tracker

Variable	Start	After 6	After 7	After 8	Final
obj	null	MyClass instance	MyClass instance	MyClass instance	MyClass instance

Key Moments - 2 Insights

Why must MyClass implement all methods from both interfaces?

Because in row 3 of execution_table, the class declares it implements IA and IB, so it must provide all methods declared in both interfaces to compile.

Can we call MethodA and MethodB on obj directly?

Yes, as shown in rows 7 and 8, obj is a MyClass instance that implements both interfaces, so both methods are accessible.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what does MethodA do?

A Prints 'A' to the console

B Prints 'B' to the console

C Throws an error

D Does nothing

💡 Hint

Check the 'Evaluation' and 'Result' columns at step 4 in execution_table

At which step is the object 'obj' created?

A Step 3

B Step 7

C Step 6

D Step 1

💡 Hint

Look for the action mentioning 'Create object obj' in execution_table

If MyClass did not implement MethodB, what would happen?

A Program runs normally

B Compile-time error occurs

C MethodB calls print 'A'

D Runtime error occurs

💡 Hint

Refer to key_moments about implementing all interface methods

Concept Snapshot

Multiple interface implementation in C#:
- Use class : Interface1, Interface2
- Implement all interface methods
- Create object of class
- Call methods from all interfaces
- Enables combining behaviors from multiple sources

Full Transcript

This example shows how a class in C# can implement multiple interfaces by listing them separated by commas after the colon. The class must provide all methods declared in each interface. We define two interfaces IA and IB, each with one method. Then we create MyClass implementing both. It provides MethodA and MethodB. We create an object of MyClass and call both methods, printing 'A' and 'B'. This demonstrates how multiple interface implementation allows a class to combine behaviors from different contracts.

Practice

1.

What does it mean when a C# class implements multiple interfaces?

easy

A. The class inherits code from multiple classes.

B. The class agrees to provide code for all methods defined in those interfaces.

C. The class can only use one interface at a time.

D. The class automatically gets all properties from the interfaces without coding.

Solution

1. Step 1: Understand interface implementation

   Interfaces define method signatures but no code. A class implementing them must provide the code.

2. Step 2: Multiple interfaces require all methods

   When a class implements several interfaces, it must write code for every method in all interfaces.

3. Final Answer:

   The class agrees to provide code for all methods defined in those interfaces. -> Option B

4. Quick Check:

   Multiple interface implementation = implement all methods [OK]

Hint: Interfaces are contracts; class must fulfill all method contracts [OK]

Common Mistakes:

• Thinking interfaces provide code to inherit
• Believing class can skip some interface methods
• Confusing interfaces with classes

2.

Which of the following is the correct syntax to declare a class Car implementing interfaces IMovable and IEngine?

?

easy

A. public class Car : IMovable, IEngine { }

B. public class Car implements IMovable, IEngine { }

C. public class Car inherits IMovable, IEngine { }

D. public class Car : IMovable & IEngine { }

Solution

1. Step 1: Recall C# interface syntax

   In C#, a class uses a colon ':' followed by interface names separated by commas.

2. Step 2: Check each option

   public class Car : IMovable, IEngine { } uses ':' and commas correctly. Options B and C use wrong keywords. public class Car : IMovable & IEngine { } uses '&' which is invalid.

3. Final Answer:

   public class Car : IMovable, IEngine { } -> Option A

4. Quick Check:

   Interfaces listed after ':' separated by commas [OK]

Hint: Use ':' and commas to list interfaces after class name [OK]

Common Mistakes:

• Using 'implements' keyword (Java style)
• Using '&' instead of commas
• Using 'inherits' keyword incorrectly

3.

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

interface IA { void Show(); }
interface IB { void Show(); }
class Demo : IA, IB {
    public void Show() { Console.WriteLine("Hello"); }
}
class Program {
    static void Main() {
        Demo d = new Demo();
        d.Show();
    }
}

medium

A. Hello

B. Compilation error due to ambiguous Show method

C. Runtime error

D. No output

Solution

1. Step 1: Understand method implementation for multiple interfaces

   Both interfaces have Show method. The class Demo implements one Show method that satisfies both.

2. Step 2: Check program output

   Main creates Demo and calls Show, which prints "Hello".

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Single method implements both interfaces' Show [OK]

Hint: One method can implement same method from multiple interfaces [OK]

Common Mistakes:

• Expecting compile error for same method name
• Thinking separate methods needed for each interface
• Confusing interface method calls

4.

Identify the error in this code snippet:

interface IA { void Run(); }
interface IB { void Jump(); }
class Player : IA, IB {
    public void Run() { Console.WriteLine("Running"); }
}

medium

A. Run method should be private.

B. Class Player cannot implement two interfaces.

C. Interfaces cannot have methods.

D. Class Player must implement Jump method from IB interface.

Solution

1. Step 1: Check interface methods

   IA requires Run(), IB requires Jump().

2. Step 2: Verify class implementation

   Player implements Run() but misses Jump(), so it is incomplete.

3. Final Answer:

   Class Player must implement Jump method from IB interface. -> Option D

4. Quick Check:

   All interface methods must be implemented [OK]

Hint: Implement all interface methods to avoid errors [OK]

Common Mistakes:

• Forgetting to implement all interface methods
• Thinking interfaces can't have methods
• Assuming methods can be private

5.

Given these interfaces and class:

interface IAlpha { void Action(); }
interface IBeta { void Action(); }
class Combined : IAlpha, IBeta {
    void IAlpha.Action() { Console.WriteLine("Alpha Action"); }
    void IBeta.Action() { Console.WriteLine("Beta Action"); }
}
class Program {
    static void Main() {
        Combined c = new Combined();
        // Which calls are valid?
    }
}

Which of the following calls will compile and print output?

hard

A. ((IBeta)c).Action(); // prints 'Beta Action'

B. c.Action(); // prints 'Alpha Action'

C. ((IAlpha)c).Action(); // prints 'Alpha Action'

D. c.Action(); // prints 'Beta Action'

Solution

1. Step 1: Understand explicit interface implementation

   Combined class implements IAlpha.Action and IBeta.Action explicitly, so these methods are not accessible via class instance directly.

2. Step 2: Check method calls

   Only calls through interface references like (IAlpha)c or (IBeta)c are valid. c.Action() is invalid and causes compile error.

3. Final Answer:

   ((IAlpha)c).Action(); // prints 'Alpha Action' -> Option C

4. Quick Check:

   Explicit interface methods need interface cast to call [OK]

Hint: Explicit interface methods require casting to interface type [OK]

Common Mistakes:

• Calling explicit interface methods directly on class instance
• Confusing explicit and implicit implementation
• Assuming c.Action() works without cast