Can We Directly Assign a Base Class Object to a Derived Class Reference in C#?

Can a base class object in C# be directly assigned to a derived class reference?

In object-oriented programming languages ​​such as C#, class inheritance establishes a parent-child relationship between classes, and derived classes inherit the properties and methods of their base classes. This raises a question: Can a base class object be assigned directly to a derived class reference using explicit type conversion?

Many programmers encounter this problem when trying to use typecasting for type conversion within a class hierarchy. However, it is important to understand that this type of assignment is not allowed in C#. The reason lies in the inherent nature of class references and their relationship to the actual objects they represent.

In object-oriented programming, a reference is a pointer to an actual object in memory. A derived class reference holds a pointer to a derived class instance. Assigning a base class object to a derived class reference means that the reference will point to an object that does not have all the properties and methods expected by the derived class. This can cause runtime errors and unexpected behavior.

Consider the following example:

class Animal { public void Eat() { } }
class Dog : Animal { public void Bark() { } }

Animal animal = new Animal();
Dog dog = (Dog)animal; // 错误：InvalidCastException

Here, trying to assign the base class object animal to the derived class reference dog will result in an InvalidCastException because the animal object does not have the Bark() method present in the Dog class.

In order to achieve correct type compatibility, you must ensure that the object referenced by the derived class reference is actually an instance of the derived class. This can be accomplished by creating an instance of the derived class and assigning it to the reference, rather than trying to convert the base class object into a derived class reference.

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