C # differences between abstract and Virtual Methods

Virtual and abstract methods can be rewritten by the derived classes. What is the difference between them?

1. The virtual method must have an implementation part and provide the option to overwrite the method for the derived class;
The abstract method does not provide the implementation part. The abstract method is a method that forces the override of the derived class. Otherwise, the derived class cannot be instantiated.
For example:
// Abstract Method
Public abstract class animal
{
Public abstract void sleep ();
Public abstract void eat ();
}
// Virtual Method
Public class animal
{
Public Virtual void sleep (){}
Public Virtual void eat (){}
}

2. abstract methods can only be declared in abstract classes. abstract methods must be rewritten in Derived classes;
Virtual methods are not and do not need to be rewritten. In fact, if a class contains abstract methods, the class is also abstract and must be declared as abstract. For example:
Public class animal
{
Public abstract void sleep ();
Public abstract void eat ();
}
The compiler reports the following error:
Main. CS (10): 'vstest. Animal. Sleep () 'is abstract but it is contained in nonabstract class 'vstest. Animal'
Main. CS (11): 'vstest. Animal. Eat () 'is abstract but it is contained in nonabstract class 'vstest. Animal'
3. the abstract method must be rewritten in the derived class. This is similar to the interface, and the virtual method is not required. Abstract METHODS cannot declare method entities. Virtual methods can contain classes of abstract methods and cannot be instantiated. Classes containing virtual methods can be instantiated! For example:
Public abstract class animal
{
Public abstract void sleep ();
Public abstract void eat ();
}

Public class Cat: Animal
{
Public override void sleep ()
{
Console. writeline ("cat is sleeping ");
}
// We need implement animal. Eat () here
}

The compiler reports the following error: Main. CS (14): 'vstest. cat 'does not implement inherited abstract member' vstest. animal. eat () ', because we have not implemented all abstract methods in the abstract class.