“Cannot create an instance of the abstract class or interface” C# error message

Question

I changed my base class to abstract for a project and now I\'m receiving the following error:

Cannot create an instance of the abstract c

Answer 1

I saw your earlier question, in which you asked about the meaning of "pure virtual" (abstract in C#).

The reason you cannot instantiate an abstract class is that it presumably has abstract members, with no implementation. So say your class looks like this:

abstract class Airplane_Abstract
{
    public abstract int GetSomeInteger();
}

Then assuming you could instantiate one of these, you could write code like this:

var airplane = new Airplane_Abstract();

// What would this be?
int integer = airplane.GetSomeInteger();

Granted, I don't believe you actually have to have abstract members in an abstract class. But the general idea of an abstract class is that it's one that cannot exist on its own; it must be further defined in a class which inherits from it. Abstract members are the most obvious illustration of why this would be; there could be other reasons.

Think of Shape, for example. This is a pretty common example of something that would make sense as an abstract class. You can't really instantiate just a Shape, can you? ("Create a shape." "What kind of shape?" "No kind. Just an abstract shape." Doesn't really work, does it?)