Run-time type vs compile-time type in C#

Question

What is the difference between a run-time type and a compile-time type in C# and what implications exist regarding virtual method invocation?

Answer 1

Lets say we have two classes A and B declared as follows:

internal class A
{
    internal virtual void Test() => Console.WriteLine("A.Test()");
}

internal class B : A
{
    internal override void Test() => Console.WriteLine("B.Test()");
}

B inherits from A and overrides the method Test which prints a message to the console.

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What is the difference between a run-time type and a compile-time type in C#

Now lets consider the following statement:

A test = new B();

• at compile time: the compiler only knows that the variable test is of the type A. He does not know that we are actually giving him an instance of B. Therefore the compile-type of test is A.

• at run time: the type of test is known to be B and therefore has the run time type of B

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and what implications exist regarding virtual method invocation

Consider the following code statement:

((A)new B()).Test();

We are creating an instance of B casting it into the type A and invoking the Test() method on that object. The compiler type is A and the runtime type is B.

When the compiler wants to resolve the .Test() call he has a problem. Because A.Test() is virtual the compiler can not simply call A.Test because the instance stored might have overridden the method.

The compile itself can not determine which of the methods to call A.Test() or B.Test(). The method which is getting invoked is determined by the runtime and not "hardcoded" by the compiler.