Why do both the abstract class and interface exist in C#?

Question

Why do both the abstract class and interface exist in C# if we can achieve the interface feature by making all the members in the class as abstract.

Is it because:

Answer 1

The idea is simple - if your class(YourClass) is already deriving from a parent class(SomeParentClass) and at the same time you want your class(YourClass) to have a new behavior that is defined in some abstract class(SomeAbstractClass), you can't do that by simply deriving from that abstract class(SomeAbstractClass), C# doesn't allow multiple inheritance. However if your new behavior was instead defined in an interface (IYourInterface), you could easily derive from the interface(IYourInterface) along with parent class(SomeParentClass).

Consider having a class Fruit that is derived by two children(Apple & Banana) as shown below:

class Fruit
{
    public virtual string GetColor()
    {
        return string.Empty;
    }
}

class Apple : Fruit
{
    public override string GetColor()
    {
        return "Red";
    }
}

class Banana : Fruit
{
    public override string GetColor()
    {
        return "Yellow";
    }
}

We have an existing interface ICloneable in C#. This interface has a single method as shown below, a class that implements this interface guarantees that it can be cloned:

public interface ICloneable
{
    object Clone();
}

Now if I want to make my Apple class(not Banana class) clonable, I can simpley implement ICloneable like this:

 class Apple : Fruit , ICloneable
{
    public object Clone()
    {
        // add your code here
    }

    public override string GetColor()
    {
        return "Red";
    }
}

Now considering your argument of pure abstract class, if C# had a pure abstract class say Clonable instead of interface IClonable like this:

abstract class Clonable
{
    public abstract object Clone();
}

Could you now make your Apple class clonable by inheriting the abstract Clonable instead of IClonable? like this:

// Error: Class 'Apple' cannot have multiple base classes: 'Fruit' & 'Clonable'
class Apple : Fruit, Clonable
{
    public object Clone()
    {
        // add your code here
    }

    public override string GetColor()
    {
        return "Red";
    }
}

No, you can't, because a class cannot derive from multiple classes.