C# How can you call a method of base class in derived class?

In C#, you cannot directly call a private method (or any member) from a base class in a derived class. Private members are not accessible outside the class in which they are defined, even in derived classes. However, if you want to call a method that's marked as protected or internal (or any other access modifier that allows visibility to derived classes), you can call it from the derived class.

Here's an example:

class BaseClass
{
    protected void SomeMethod()
    {
        Console.WriteLine("Some method in base class");
    }
}

class DerivedClass : BaseClass
{
    public void CallBaseMethod()
    {
        // You can call the protected method from the derived class
        SomeMethod();
    }
}

In this example, SomeMethod() is marked as protected in the BaseClass. The DerivedClass inherits from BaseClass, and in DerivedClass, you can call the protected method SomeMethod().

More appropriately, in C#, you can call a method from a base class in a derived class using the base keyword. 

This is particularly useful when you want to invoke a method that has been overridden in the derived class, or to ensure that the base class implementation is executed. But if method is not overridden then without base keyword, the method can be invoked in the derived class.

Here’s a simple example to illustrate how to do this:

// Base class
public class BaseClass
{
    public void Display()
    {
        Console.WriteLine("Base class method called.");
    }
}

// Derived class
public class DerivedClass : BaseClass
{

    public void Display()
    {
        Console.WriteLine("DerivedClass method.");
    }
    public void CallBaseMethod()
    {
        // Calling the base class method
        base.Display();
    }
}

// Example usage
class Program
{
    static void Main()
    {
        DerivedClass derived = new DerivedClass();
        derived.CallBaseMethod(); // Output: Base class method called.
    }
}

This mechanism allows for clear and controlled access to the methods of the base class from the derived class.