If there were no virtual and override keywords in C#, the methods would not be explicitly marked for virtual or overridden behavior, and the default method resolution would be based on the type of the variable at compile-time. In such a scenario, when you have a variable of a base class type referring to an instance of a derived class, the methods would be resolved based on the static (compile-time) type of the variable rather than the dynamic (runtime) type of the object.

Here's an example to illustrate the behavior without virtual and override keywords:

class B
{
    public void Test()
    {
        Console.WriteLine("Test method in class B");
    }
}

class C : B
{
    public void Test()
    {
        Console.WriteLine("Test method in class C");
    }
}

B a = new C();
a.Test(); // Output: "Test method in class B"

In this example, the Test() method in both classes B and C is not marked with virtual or override. When you create an instance of C and assign it to a variable of type B, the Test() method of class B is invoked at compile-time and runtime. The method resolution is based on the type of the variable (B), rather than the actual type of the object (C).

Without the virtual and override keywords, the methods are resolved based on the static type of the variable. This behavior is sometimes referred to as "static binding" or "early binding".

The virtual and override keywords are used to explicitly indicate to the compiler that a method is designed for polymorphic behavior, allowing the derived class to provide its own implementation. These keywords enable "dynamic binding" or "late binding" at runtime, where the method resolution is based on the actual type of the object, not the type of the variable.

The virtual keyword is used in the base class to indicate that a method can be overridden, and the override keyword is used in the derived class to indicate that the method overrides the base class implementation. These keywords provide the necessary mechanism for achieving polymorphism and dynamic method binding in C#.