[c#] Multiple Inheritance in C#

Since multiple inheritance is bad (it makes the source more complicated) C# does not provide such a pattern directly. But sometimes it would be helpful to have this ability.

For instance I'm able to implement the missing multiple inheritance pattern using interfaces and three classes like that:

public interface IFirst { void FirstMethod(); }
public interface ISecond { void SecondMethod(); }

public class First:IFirst 
{ 
    public void FirstMethod() { Console.WriteLine("First"); } 
}

public class Second:ISecond 
{ 
    public void SecondMethod() { Console.WriteLine("Second"); } 
}

public class FirstAndSecond: IFirst, ISecond
{
    First first = new First();
    Second second = new Second();
    public void FirstMethod() { first.FirstMethod(); }
    public void SecondMethod() { second.SecondMethod(); }
}

Every time I add a method to one of the interfaces I need to change the class FirstAndSecond as well.

Is there a way to inject multiple existing classes into one new class like it is possible in C++?

Maybe there is a solution using some kind of code generation?

Or it may look like this (imaginary c# syntax):

public class FirstAndSecond: IFirst from First, ISecond from Second
{ }

So that there won't be a need to update the class FirstAndSecond when I modify one of the interfaces.

EDIT

Maybe it would be better to consider a practical example:

You have an existing class (e.g. a text based TCP client based on ITextTcpClient) which you do already use at different locations inside your project. Now you feel the need to create a component of your class to be easy accessible for windows forms developers.

As far as I know you currently have two ways to do this:

1. Write a new class that is inherited from components and implements the interface of the TextTcpClient class using an instance of the class itself as shown with FirstAndSecond.

2. Write a new class that inherits from TextTcpClient and somehow implements IComponent (haven't actually tried this yet).

In both cases you need to do work per method and not per class. Since you know that we will need all the methods of TextTcpClient and Component it would be the easiest solution to just combine those two into one class.

To avoid conflicts this may be done by code generation where the result could be altered afterwards but typing this by hand is a pure pain in the ass.

Multiple inheritance is one of those things that generally causes more problems than it solves. In C++ it fits the pattern of giving you enough rope to hang yourself, but Java and C# have chosen to go the safer route of not giving you the option. The biggest problem is what to do if you inherit multiple classes that have a method with the same signature that the inheritee doesn't implement. Which class's method should it choose? Or should that not compile? There is generally another way to implement most things that doesn't rely on multiple inheritance.

I created a C# post-compiler that enables this kind of thing:

using NRoles;

public interface IFirst { void FirstMethod(); }
public interface ISecond { void SecondMethod(); }

public class RFirst : IFirst, Role {
  public void FirstMethod() { Console.WriteLine("First"); }
}

public class RSecond : ISecond, Role {
  public void SecondMethod() { Console.WriteLine("Second"); }
}

public class FirstAndSecond : Does<RFirst>, Does<RSecond> { }

You can run the post-compiler as a Visual Studio post-build-event:

C:\some_path\nroles-v0.1.0-bin\nutate.exe "$(TargetPath)"

In the same assembly you use it like this:

var fas = new FirstAndSecond();
fas.As<RFirst>().FirstMethod();
fas.As<RSecond>().SecondMethod();

In another assembly you use it like this:

var fas = new FirstAndSecond();
fas.FirstMethod();
fas.SecondMethod();

With C# 8 now you practically have multiple inheritance via default implementation of interface members:

interface ILogger
{
    void Log(LogLevel level, string message);
    void Log(Exception ex) => Log(LogLevel.Error, ex.ToString()); // New overload
}

class ConsoleLogger : ILogger
{
    public void Log(LogLevel level, string message) { ... }
    // Log(Exception) gets default implementation
}

We all seem to be heading down the interface path with this, but the obvious other possibility, here, is to do what OOP is supposed to do, and build up your inheritance tree... (isn't this what class design is all about?)

class Program
{
    static void Main(string[] args)
    {
        human me = new human();
        me.legs = 2;
        me.lfType = "Human";
        me.name = "Paul";
        Console.WriteLine(me.name);
    }
}

public abstract class lifeform
{
    public string lfType { get; set; }
}

public abstract class mammal : lifeform 
{
    public int legs { get; set; }
}

public class human : mammal
{
    public string name { get; set; }
}

This structure provides reusable blocks of code and, surely, is how OOP code should be written?

If this particular approach doesn't quite fit the bill the we simply create new classes based on the required objects...

class Program
{
    static void Main(string[] args)
    {
        fish shark = new fish();
        shark.size = "large";
        shark.lfType = "Fish";
        shark.name = "Jaws";
        Console.WriteLine(shark.name);
        human me = new human();
        me.legs = 2;
        me.lfType = "Human";
        me.name = "Paul";
        Console.WriteLine(me.name);
    }
}

public abstract class lifeform
{
    public string lfType { get; set; }
}

public abstract class mammal : lifeform 
{
    public int legs { get; set; }
}

public class human : mammal
{
    public string name { get; set; }
}

public class aquatic : lifeform
{
    public string size { get; set; }
}

public class fish : aquatic
{
    public string name { get; set; }
}

You could have one abstract base class that implements both IFirst and ISecond, and then inherit from just that base.

If you can live with the restriction that the methods of IFirst and ISecond must only interact with the contract of IFirst and ISecond (like in your example)... you can do what you ask with extension methods. In practice, this is rarely the case.

public interface IFirst {}
public interface ISecond {}

public class FirstAndSecond : IFirst, ISecond
{
}

public static MultipleInheritenceExtensions
{
  public static void First(this IFirst theFirst)
  {
    Console.WriteLine("First");
  }

  public static void Second(this ISecond theSecond)
  {
    Console.WriteLine("Second");
  }
}

///

public void Test()
{
  FirstAndSecond fas = new FirstAndSecond();
  fas.First();
  fas.Second();
}

So the basic idea is that you define the required implementation in the interfaces... this required stuff should support the flexible implementation in the extension methods. Anytime you need to "add methods to the interface" instead you add an extension method.

Consider just using composition instead of trying to simulate Multiple Inheritance. You can use Interfaces to define what classes make up the composition, eg: ISteerable implies a property of type SteeringWheel, IBrakable implies a property of type BrakePedal, etc.

Once you've done that, you could use the Extension Methods feature added to C# 3.0 to further simplify calling methods on those implied properties, eg:

public interface ISteerable { SteeringWheel wheel { get; set; } }

public interface IBrakable { BrakePedal brake { get; set; } }

public class Vehicle : ISteerable, IBrakable
{
    public SteeringWheel wheel { get; set; }

    public BrakePedal brake { get; set; }

    public Vehicle() { wheel = new SteeringWheel(); brake = new BrakePedal(); }
}

public static class SteeringExtensions
{
    public static void SteerLeft(this ISteerable vehicle)
    {
        vehicle.wheel.SteerLeft();
    }
}

public static class BrakeExtensions
{
    public static void Stop(this IBrakable vehicle)
    {
        vehicle.brake.ApplyUntilStop();
    }
}


public class Main
{
    Vehicle myCar = new Vehicle();

    public void main()
    {
        myCar.SteerLeft();
        myCar.Stop();
    }
}

I created a C# post-compiler that enables this kind of thing:

using NRoles;

public interface IFirst { void FirstMethod(); }
public interface ISecond { void SecondMethod(); }

public class RFirst : IFirst, Role {
  public void FirstMethod() { Console.WriteLine("First"); }
}

public class RSecond : ISecond, Role {
  public void SecondMethod() { Console.WriteLine("Second"); }
}

public class FirstAndSecond : Does<RFirst>, Does<RSecond> { }

You can run the post-compiler as a Visual Studio post-build-event:

C:\some_path\nroles-v0.1.0-bin\nutate.exe "$(TargetPath)"

In the same assembly you use it like this:

var fas = new FirstAndSecond();
fas.As<RFirst>().FirstMethod();
fas.As<RSecond>().SecondMethod();

In another assembly you use it like this:

var fas = new FirstAndSecond();
fas.FirstMethod();
fas.SecondMethod();

Consider just using composition instead of trying to simulate Multiple Inheritance. You can use Interfaces to define what classes make up the composition, eg: ISteerable implies a property of type SteeringWheel, IBrakable implies a property of type BrakePedal, etc.

Once you've done that, you could use the Extension Methods feature added to C# 3.0 to further simplify calling methods on those implied properties, eg:

public interface ISteerable { SteeringWheel wheel { get; set; } }

public interface IBrakable { BrakePedal brake { get; set; } }

public class Vehicle : ISteerable, IBrakable
{
    public SteeringWheel wheel { get; set; }

    public BrakePedal brake { get; set; }

    public Vehicle() { wheel = new SteeringWheel(); brake = new BrakePedal(); }
}

public static class SteeringExtensions
{
    public static void SteerLeft(this ISteerable vehicle)
    {
        vehicle.wheel.SteerLeft();
    }
}

public static class BrakeExtensions
{
    public static void Stop(this IBrakable vehicle)
    {
        vehicle.brake.ApplyUntilStop();
    }
}


public class Main
{
    Vehicle myCar = new Vehicle();

    public void main()
    {
        myCar.SteerLeft();
        myCar.Stop();
    }
}

With C# 8 now you practically have multiple inheritance via default implementation of interface members:

interface ILogger
{
    void Log(LogLevel level, string message);
    void Log(Exception ex) => Log(LogLevel.Error, ex.ToString()); // New overload
}

class ConsoleLogger : ILogger
{
    public void Log(LogLevel level, string message) { ... }
    // Log(Exception) gets default implementation
}

i know i know even though its not allowed and so on, sometime u actualy need it so for the those:

class a {}
class b : a {}
class c : b {}

like in my case i wanted to do this class b : Form (yep the windows.forms) class c : b {}

cause half of the function were identical and with interface u must rewrite them all

Yes using Interface is a hassle because anytime we add a method in the class we have to add the signature in the interface. Also, what if we already have a class with a bunch of methods but no Interface for it? we have to manually create Interface for all the classes that we want to inherit from. And the worst thing is, we have to implement all methods in the Interfaces in the child class if the child class is to inherit from the multiple interface.

By following Facade design pattern we can simulate inheriting from multiple classes using accessors. Declare the classes as properties with {get;set;} inside the class that need to inherit and all public properties and methods are from that class, and in the constructor of the child class instantiate the parent classes.

For example:

 namespace OOP
 {
     class Program
     {
         static void Main(string[] args)
         {
             Child somechild = new Child();
             somechild.DoHomeWork();
             somechild.CheckingAround();
             Console.ReadLine();
         }
     }

     public class Father 
     {
         public Father() { }
         public void Work()
         {
             Console.WriteLine("working...");
         }
         public void Moonlight()
         {
             Console.WriteLine("moonlighting...");
         }
     }


     public class Mother 
     {
         public Mother() { }
         public void Cook()
         {
             Console.WriteLine("cooking...");
         }
         public void Clean()
         {
             Console.WriteLine("cleaning...");
         }
     }


     public class Child 
     {
         public Father MyFather { get; set; }
         public Mother MyMother { get; set; }

         public Child()
         {
             MyFather = new Father();
             MyMother = new Mother();
         }

         public void GoToSchool()
         {
             Console.WriteLine("go to school...");
         }
         public void DoHomeWork()
         {
             Console.WriteLine("doing homework...");
         }
         public void CheckingAround()
         {
             MyFather.Work();
             MyMother.Cook();
         }
     }


 }

with this structure class Child will have access to all methods and properties of Class Father and Mother, simulating multiple inheritance, inheriting an instance of the parent classes. Not quite the same but it is practical.

If X inherits from Y, that has two somewhat orthogonal effects:

1. Y will provide default functionality for X, so the code for X only has to include stuff which is different from Y.
2. Almost anyplace a Y would be expected, an X may be used instead.

Although inheritance provides for both features, it is not hard to imagine circumstances where either could be of use without the other. No .net language I know of has a direct way of implementing the first without the second, though one could obtain such functionality by defining a base class which is never used directly, and having one or more classes that inherit directly from it without adding anything new (such classes could share all their code, but would not be substitutable for each other). Any CLR-compliant language, however, will allow the use of interfaces which provide the second feature of interfaces (substitutability) without the first (member reuse).

You could have one abstract base class that implements both IFirst and ISecond, and then inherit from just that base.

If you can live with the restriction that the methods of IFirst and ISecond must only interact with the contract of IFirst and ISecond (like in your example)... you can do what you ask with extension methods. In practice, this is rarely the case.

public interface IFirst {}
public interface ISecond {}

public class FirstAndSecond : IFirst, ISecond
{
}

public static MultipleInheritenceExtensions
{
  public static void First(this IFirst theFirst)
  {
    Console.WriteLine("First");
  }

  public static void Second(this ISecond theSecond)
  {
    Console.WriteLine("Second");
  }
}

///

public void Test()
{
  FirstAndSecond fas = new FirstAndSecond();
  fas.First();
  fas.Second();
}

So the basic idea is that you define the required implementation in the interfaces... this required stuff should support the flexible implementation in the extension methods. Anytime you need to "add methods to the interface" instead you add an extension method.

Yes using Interface is a hassle because anytime we add a method in the class we have to add the signature in the interface. Also, what if we already have a class with a bunch of methods but no Interface for it? we have to manually create Interface for all the classes that we want to inherit from. And the worst thing is, we have to implement all methods in the Interfaces in the child class if the child class is to inherit from the multiple interface.

By following Facade design pattern we can simulate inheriting from multiple classes using accessors. Declare the classes as properties with {get;set;} inside the class that need to inherit and all public properties and methods are from that class, and in the constructor of the child class instantiate the parent classes.

For example:

 namespace OOP
 {
     class Program
     {
         static void Main(string[] args)
         {
             Child somechild = new Child();
             somechild.DoHomeWork();
             somechild.CheckingAround();
             Console.ReadLine();
         }
     }

     public class Father 
     {
         public Father() { }
         public void Work()
         {
             Console.WriteLine("working...");
         }
         public void Moonlight()
         {
             Console.WriteLine("moonlighting...");
         }
     }


     public class Mother 
     {
         public Mother() { }
         public void Cook()
         {
             Console.WriteLine("cooking...");
         }
         public void Clean()
         {
             Console.WriteLine("cleaning...");
         }
     }


     public class Child 
     {
         public Father MyFather { get; set; }
         public Mother MyMother { get; set; }

         public Child()
         {
             MyFather = new Father();
             MyMother = new Mother();
         }

         public void GoToSchool()
         {
             Console.WriteLine("go to school...");
         }
         public void DoHomeWork()
         {
             Console.WriteLine("doing homework...");
         }
         public void CheckingAround()
         {
             MyFather.Work();
             MyMother.Cook();
         }
     }


 }

with this structure class Child will have access to all methods and properties of Class Father and Mother, simulating multiple inheritance, inheriting an instance of the parent classes. Not quite the same but it is practical.

If you can live with the restriction that the methods of IFirst and ISecond must only interact with the contract of IFirst and ISecond (like in your example)... you can do what you ask with extension methods. In practice, this is rarely the case.

public interface IFirst {}
public interface ISecond {}

public class FirstAndSecond : IFirst, ISecond
{
}

public static MultipleInheritenceExtensions
{
  public static void First(this IFirst theFirst)
  {
    Console.WriteLine("First");
  }

  public static void Second(this ISecond theSecond)
  {
    Console.WriteLine("Second");
  }
}

///

public void Test()
{
  FirstAndSecond fas = new FirstAndSecond();
  fas.First();
  fas.Second();
}

So the basic idea is that you define the required implementation in the interfaces... this required stuff should support the flexible implementation in the extension methods. Anytime you need to "add methods to the interface" instead you add an extension method.

This is along the lines of Lawrence Wenham's answer, but depending on your use case, it may or may not be an improvement -- you don't need the setters.

public interface IPerson {
  int GetAge();
  string GetName();
}

public interface IGetPerson {
  IPerson GetPerson();
}

public static class IGetPersonAdditions {
  public static int GetAgeViaPerson(this IGetPerson getPerson) { // I prefer to have the "ViaPerson" in the name in case the object has another Age property.
    IPerson person = getPerson.GetPersion();
    return person.GetAge();
  }
  public static string GetNameViaPerson(this IGetPerson getPerson) {
    return getPerson.GetPerson().GetName();
  }
}

public class Person: IPerson, IGetPerson {
  private int Age {get;set;}
  private string Name {get;set;}
  public IPerson GetPerson() {
    return this;
  }
  public int GetAge() {  return Age; }
  public string GetName() { return Name; }
}

Now any object that knows how to get a person can implement IGetPerson, and it will automatically have the GetAgeViaPerson() and GetNameViaPerson() methods. From this point, basically all Person code goes into IGetPerson, not into IPerson, other than new ivars, which have to go into both. And in using such code, you don't have to be concerned about whether or not your IGetPerson object is itself actually an IPerson.

You could have one abstract base class that implements both IFirst and ISecond, and then inherit from just that base.

Since the question of multiple inheritance (MI) pops up from time to time, I'd like to add an approach which addresses some problems with the composition pattern.

I build upon the IFirst, ISecond,First, Second, FirstAndSecond approach, as it was presented in the question. I reduce sample code to IFirst, since the pattern stays the same regardless of the number of interfaces / MI base classes.

Lets assume, that with MI First and Second would both derive from the same base class BaseClass, using only public interface elements from BaseClass

This can be expressed, by adding a container reference to BaseClass in the First and Second implementation:

class First : IFirst {
  private BaseClass ContainerInstance;
  First(BaseClass container) { ContainerInstance = container; }
  public void FirstMethod() { Console.WriteLine("First"); ContainerInstance.DoStuff(); } 
}
...

Things become more complicated, when protected interface elements from BaseClass are referenced or when First and Second would be abstract classes in MI, requiring their subclasses to implement some abstract parts.

class BaseClass {
  protected void DoStuff();
}

abstract class First : IFirst {
  public void FirstMethod() { DoStuff(); DoSubClassStuff(); }
  protected abstract void DoStuff(); // base class reference in MI
  protected abstract void DoSubClassStuff(); // sub class responsibility
}

C# allows nested classes to access protected/private elements of their containing classes, so this can be used to link the abstract bits from the First implementation.

class FirstAndSecond : BaseClass, IFirst, ISecond {
  // link interface
  private class PartFirst : First {
    private FirstAndSecond ContainerInstance;
    public PartFirst(FirstAndSecond container) {
      ContainerInstance = container;
    }
    // forwarded references to emulate access as it would be with MI
    protected override void DoStuff() { ContainerInstance.DoStuff(); }
    protected override void DoSubClassStuff() { ContainerInstance.DoSubClassStuff(); }
  }
  private IFirst partFirstInstance; // composition object
  public FirstMethod() { partFirstInstance.FirstMethod(); } // forwarded implementation
  public FirstAndSecond() {
    partFirstInstance = new PartFirst(this); // composition in constructor
  }
  // same stuff for Second
  //...
  // implementation of DoSubClassStuff
  private void DoSubClassStuff() { Console.WriteLine("Private method accessed"); }
}

There is quite some boilerplate involved, but if the actual implementation of FirstMethod and SecondMethod are sufficiently complex and the amount of accessed private/protected methods is moderate, then this pattern may help to overcome lacking multiple inheritance.

Consider just using composition instead of trying to simulate Multiple Inheritance. You can use Interfaces to define what classes make up the composition, eg: ISteerable implies a property of type SteeringWheel, IBrakable implies a property of type BrakePedal, etc.

Once you've done that, you could use the Extension Methods feature added to C# 3.0 to further simplify calling methods on those implied properties, eg:

public interface ISteerable { SteeringWheel wheel { get; set; } }

public interface IBrakable { BrakePedal brake { get; set; } }

public class Vehicle : ISteerable, IBrakable
{
    public SteeringWheel wheel { get; set; }

    public BrakePedal brake { get; set; }

    public Vehicle() { wheel = new SteeringWheel(); brake = new BrakePedal(); }
}

public static class SteeringExtensions
{
    public static void SteerLeft(this ISteerable vehicle)
    {
        vehicle.wheel.SteerLeft();
    }
}

public static class BrakeExtensions
{
    public static void Stop(this IBrakable vehicle)
    {
        vehicle.brake.ApplyUntilStop();
    }
}


public class Main
{
    Vehicle myCar = new Vehicle();

    public void main()
    {
        myCar.SteerLeft();
        myCar.Stop();
    }
}

Multiple inheritance is one of those things that generally causes more problems than it solves. In C++ it fits the pattern of giving you enough rope to hang yourself, but Java and C# have chosen to go the safer route of not giving you the option. The biggest problem is what to do if you inherit multiple classes that have a method with the same signature that the inheritee doesn't implement. Which class's method should it choose? Or should that not compile? There is generally another way to implement most things that doesn't rely on multiple inheritance.

In my own implementation I found that using classes/interfaces for MI, although "good form", tended to be a massive over complication since you need to set up all that multiple inheritance for only a few necessary function calls, and in my case, needed to be done literally dozens of times redundantly.

Instead it was easier to simply make static "functions that call functions that call functions" in different modular varieties as a sort of OOP replacement. The solution I was working on was the "spell system" for a RPG where effects need to heavily mix-and-match function calling to give an extreme variety of spells without re-writing code, much like the example seems to indicate.

Most of the functions can now be static because I don't necessarily need an instance for spell logic, whereas class inheritance can't even use virtual or abstract keywords while static. Interfaces can't use them at all.

Coding seems way faster and cleaner this way IMO. If you're just doing functions, and don't need inherited properties, use functions.

This is along the lines of Lawrence Wenham's answer, but depending on your use case, it may or may not be an improvement -- you don't need the setters.

public interface IPerson {
  int GetAge();
  string GetName();
}

public interface IGetPerson {
  IPerson GetPerson();
}

public static class IGetPersonAdditions {
  public static int GetAgeViaPerson(this IGetPerson getPerson) { // I prefer to have the "ViaPerson" in the name in case the object has another Age property.
    IPerson person = getPerson.GetPersion();
    return person.GetAge();
  }
  public static string GetNameViaPerson(this IGetPerson getPerson) {
    return getPerson.GetPerson().GetName();
  }
}

public class Person: IPerson, IGetPerson {
  private int Age {get;set;}
  private string Name {get;set;}
  public IPerson GetPerson() {
    return this;
  }
  public int GetAge() {  return Age; }
  public string GetName() { return Name; }
}

Now any object that knows how to get a person can implement IGetPerson, and it will automatically have the GetAgeViaPerson() and GetNameViaPerson() methods. From this point, basically all Person code goes into IGetPerson, not into IPerson, other than new ivars, which have to go into both. And in using such code, you don't have to be concerned about whether or not your IGetPerson object is itself actually an IPerson.

Multiple inheritance is one of those things that generally causes more problems than it solves. In C++ it fits the pattern of giving you enough rope to hang yourself, but Java and C# have chosen to go the safer route of not giving you the option. The biggest problem is what to do if you inherit multiple classes that have a method with the same signature that the inheritee doesn't implement. Which class's method should it choose? Or should that not compile? There is generally another way to implement most things that doesn't rely on multiple inheritance.

In my own implementation I found that using classes/interfaces for MI, although "good form", tended to be a massive over complication since you need to set up all that multiple inheritance for only a few necessary function calls, and in my case, needed to be done literally dozens of times redundantly.

Instead it was easier to simply make static "functions that call functions that call functions" in different modular varieties as a sort of OOP replacement. The solution I was working on was the "spell system" for a RPG where effects need to heavily mix-and-match function calling to give an extreme variety of spells without re-writing code, much like the example seems to indicate.

Most of the functions can now be static because I don't necessarily need an instance for spell logic, whereas class inheritance can't even use virtual or abstract keywords while static. Interfaces can't use them at all.

Coding seems way faster and cleaner this way IMO. If you're just doing functions, and don't need inherited properties, use functions.

If X inherits from Y, that has two somewhat orthogonal effects:

1. Y will provide default functionality for X, so the code for X only has to include stuff which is different from Y.
2. Almost anyplace a Y would be expected, an X may be used instead.

Although inheritance provides for both features, it is not hard to imagine circumstances where either could be of use without the other. No .net language I know of has a direct way of implementing the first without the second, though one could obtain such functionality by defining a base class which is never used directly, and having one or more classes that inherit directly from it without adding anything new (such classes could share all their code, but would not be substitutable for each other). Any CLR-compliant language, however, will allow the use of interfaces which provide the second feature of interfaces (substitutability) without the first (member reuse).

You could have one abstract base class that implements both IFirst and ISecond, and then inherit from just that base.

If you can live with the restriction that the methods of IFirst and ISecond must only interact with the contract of IFirst and ISecond (like in your example)... you can do what you ask with extension methods. In practice, this is rarely the case.

public interface IFirst {}
public interface ISecond {}

public class FirstAndSecond : IFirst, ISecond
{
}

public static MultipleInheritenceExtensions
{
  public static void First(this IFirst theFirst)
  {
    Console.WriteLine("First");
  }

  public static void Second(this ISecond theSecond)
  {
    Console.WriteLine("Second");
  }
}

///

public void Test()
{
  FirstAndSecond fas = new FirstAndSecond();
  fas.First();
  fas.Second();
}

So the basic idea is that you define the required implementation in the interfaces... this required stuff should support the flexible implementation in the extension methods. Anytime you need to "add methods to the interface" instead you add an extension method.

We all seem to be heading down the interface path with this, but the obvious other possibility, here, is to do what OOP is supposed to do, and build up your inheritance tree... (isn't this what class design is all about?)

class Program
{
    static void Main(string[] args)
    {
        human me = new human();
        me.legs = 2;
        me.lfType = "Human";
        me.name = "Paul";
        Console.WriteLine(me.name);
    }
}

public abstract class lifeform
{
    public string lfType { get; set; }
}

public abstract class mammal : lifeform 
{
    public int legs { get; set; }
}

public class human : mammal
{
    public string name { get; set; }
}

This structure provides reusable blocks of code and, surely, is how OOP code should be written?

If this particular approach doesn't quite fit the bill the we simply create new classes based on the required objects...

class Program
{
    static void Main(string[] args)
    {
        fish shark = new fish();
        shark.size = "large";
        shark.lfType = "Fish";
        shark.name = "Jaws";
        Console.WriteLine(shark.name);
        human me = new human();
        me.legs = 2;
        me.lfType = "Human";
        me.name = "Paul";
        Console.WriteLine(me.name);
    }
}

public abstract class lifeform
{
    public string lfType { get; set; }
}

public abstract class mammal : lifeform 
{
    public int legs { get; set; }
}

public class human : mammal
{
    public string name { get; set; }
}

public class aquatic : lifeform
{
    public string size { get; set; }
}

public class fish : aquatic
{
    public string name { get; set; }
}

i know i know even though its not allowed and so on, sometime u actualy need it so for the those:

class a {}
class b : a {}
class c : b {}

like in my case i wanted to do this class b : Form (yep the windows.forms) class c : b {}

cause half of the function were identical and with interface u must rewrite them all

Multiple inheritance is one of those things that generally causes more problems than it solves. In C++ it fits the pattern of giving you enough rope to hang yourself, but Java and C# have chosen to go the safer route of not giving you the option. The biggest problem is what to do if you inherit multiple classes that have a method with the same signature that the inheritee doesn't implement. Which class's method should it choose? Or should that not compile? There is generally another way to implement most things that doesn't rely on multiple inheritance.

Since the question of multiple inheritance (MI) pops up from time to time, I'd like to add an approach which addresses some problems with the composition pattern.

I build upon the IFirst, ISecond,First, Second, FirstAndSecond approach, as it was presented in the question. I reduce sample code to IFirst, since the pattern stays the same regardless of the number of interfaces / MI base classes.

Lets assume, that with MI First and Second would both derive from the same base class BaseClass, using only public interface elements from BaseClass

This can be expressed, by adding a container reference to BaseClass in the First and Second implementation:

class First : IFirst {
  private BaseClass ContainerInstance;
  First(BaseClass container) { ContainerInstance = container; }
  public void FirstMethod() { Console.WriteLine("First"); ContainerInstance.DoStuff(); } 
}
...

Things become more complicated, when protected interface elements from BaseClass are referenced or when First and Second would be abstract classes in MI, requiring their subclasses to implement some abstract parts.

class BaseClass {
  protected void DoStuff();
}

abstract class First : IFirst {
  public void FirstMethod() { DoStuff(); DoSubClassStuff(); }
  protected abstract void DoStuff(); // base class reference in MI
  protected abstract void DoSubClassStuff(); // sub class responsibility
}

C# allows nested classes to access protected/private elements of their containing classes, so this can be used to link the abstract bits from the First implementation.

class FirstAndSecond : BaseClass, IFirst, ISecond {
  // link interface
  private class PartFirst : First {
    private FirstAndSecond ContainerInstance;
    public PartFirst(FirstAndSecond container) {
      ContainerInstance = container;
    }
    // forwarded references to emulate access as it would be with MI
    protected override void DoStuff() { ContainerInstance.DoStuff(); }
    protected override void DoSubClassStuff() { ContainerInstance.DoSubClassStuff(); }
  }
  private IFirst partFirstInstance; // composition object
  public FirstMethod() { partFirstInstance.FirstMethod(); } // forwarded implementation
  public FirstAndSecond() {
    partFirstInstance = new PartFirst(this); // composition in constructor
  }
  // same stuff for Second
  //...
  // implementation of DoSubClassStuff
  private void DoSubClassStuff() { Console.WriteLine("Private method accessed"); }
}

There is quite some boilerplate involved, but if the actual implementation of FirstMethod and SecondMethod are sufficiently complex and the amount of accessed private/protected methods is moderate, then this pattern may help to overcome lacking multiple inheritance.