abstract class MyBase {
    protected abstract void CustomLogic(); // Subclasses implement this
    public void PayloadMethod() {
        ... // Do somethig
        CustomLogic();
        ... // Do something else
    }
}

class Derived1 : MyBase {
    protected override void CustomLogic() {
        ... // Custom logic 1
    }
}

class Derived2 : MyBase {
    protected override void CustomLogic() {
        ... // Custom logic 2
    }
}

class Derived3 : MyBase {
    protected override void CustomLogic() {
        ... // Custom logic 3
    }
}

abstract class Foo
{
    public void Bar()
    {
        //  some code defined in the parent class

        BarCore(); // the customized part of it as defined in the child class
    }

    protected virtual void BarCore() { }
}

class BaseClass
{
    public void DoSomething()
    {
        // base class code

        // derived class code, modifiable by the derived class
        this.DoItSpecificallyForThatDerivedClass();
    }

    protected abstract void DoItSpecificallyForThatDerivedClass();
}

public class ADerivedClass : BaseClass
{
    protected override void DoItSpecificallyForThatDerivedClass()
    {
        // code specific to this instance and/or class
    }
}