CRTP or Curiously Recurring Template Pattern is often a very good topic to confuse people. If you are one of the people who got the grasp of it completely, well done! However, if you are not one of those people, this post is for you. I will try to explain my perception about CRTP.

So, without further introduction, let’s jumpt into the definition of CRTP. In C#, CRTP can be represented as a base class with difinitions similar to:

public abstract class Base<T> where T : Base<T>
{
// some other meaningful things...
}


Well, it isn’t that hard to read. But, wait… isn’t that… I mean… the type T… it is defined by itself, how… why… (after reasonable amount of time later)… okay, so T is something like:

But, wait again… what does it even mean… is that possible… how does it even compile… why…

Don’t worry if you are stuck somewhere above. I know the feeling. It is difficult to understand from the definition. The complex recurring relation became clear to me when I started looking at the problems it was trying to solve. Let’s have a look at that. But, before you start reading further, forget about CRTP and focus on the problem first.

Imagine you are creating an abstract base class which will have a common behavior for all the derived classes. Let’s say, you want to have a method named Copy, which is supposed to create a copy of the instance. However, as each derived class may be different, you want all the derived classes to implement the Copy method. One way of achieving that can be:

public abstract class Base
{
public abstract Base Copy();
}

public class Derived : Base
{
public string DerivedClassProperty
{
get { return "Yay!!!"; }
}

public override Base Copy()
{
var copy = MAGICALLY_COPY_ME();
return copy;
}
}


With the above code, if I want to copy the instance of a derived class and want to use any derived class property or method on the copied instance, I will need to downcast it manually to the derived type. Here is what I mean:

var derived = new Derived();
var copy = derived.Copy();

// Cannot perform the operation below
// Console.WriteLine(copy.DerivedClassProperty);

var copyCast = (Derived)copy;
Console.WriteLine(copyCast.DerivedClassProperty);


As you can tell, the manual downcasting looks messy. There are situations where such manual casting will lead you to problems. I prefer avoiding it as much as I can. However, in this case, the base class does not really know about the type of the derived class. So, we can’t really avoid the manual casting. Or, can we?

What if we pass the derived class type to the base class as a generic type parameter? Let’s try that:

public abstract class Base<T>
{
public abstract T Copy();
}

public class Derived : Base<Derived>
{
public string DerivedClassProperty
{
get { return "Yay!!!"; }
}

public override Derived Copy()
{
var copy = MAGICALLY_COPY_ME();
return copy;
}
}


With the slight change in the class definition, we can feel that it is already starting to look pretty. We no longer need to do the downcasting and we can use the copied instance as:

var derived = new Derived();
var copy = derived.Copy();

Console.WriteLine(copy.DerivedClassProperty);


Wonderful. But, there is a slight problem. The generic type parameter, that we are passing to the base class, can actually be anythng- as we have not yet added any restriction. So, there is nothing stopping me from writing a derived class like:

public class PeculiarDerived : Base<int>
{
public string WhoAmI
{
get { return "The answer to life, universe and everything."; }
}

public override int Copy()
{
return 42;
}
}


The above code compiles with no problem. It even runs perfectly. So, what’s the problem?

Well, the design we have been creating so far, kind of assumes that the type parameter T is derived from the Base type. In our minimalistic example above, it is not causing any problem if the assumption is violated. However, in real life, it will not be the case. You will often have some base class methods being called on any derived instance. To illustrate, let’s add something to our original requirement. That is, the base does some mythical operation on the copied instance before returning it. So, we will actually delegate the creation of the copy to the derived classes and do the required mythical operation from the base class. One possible implementation of that may look like:

public abstract class Base
{
public Base Copy()
{
var copy = this.DuplicateInternal();
copy.MythicalMumboJumbo();
return copy;
}

protected abstract Base DuplicateInternal();

private void MythicalMumboJumbo()
{
// abra-ka-dabra
}
}

public class Derived : Base
{
protected override Base DuplicateInternal()
{
return MAGICALLY_COPY_ME();
}
}


Ops, we forgot to add the generic type trick that we have discovered. Let’s try to add that.

public abstract class Base<T>
{
public T Copy()
{
var copy = this.DuplicateInternal();
copy.MythicalMumboJumbo(); // doesn't compile
return copy;
}

protected abstract T DuplicateInternal();

private void MythicalMumboJumbo()
{
// abra-ka-dabra
}
}

public class Derived : Base<Derived>
{
protected override Derived DuplicateInternal()
{
return this;
}
}


Wait, it does not compile this time. The line copy.MythicalMumboJumbo() is not compiling to be exact. After some thinking, it appears that the Base class does not know anything about the generic type T. And, as the variable copy is of type T, compiler is not sure there exists any method called MythicalMumboJumbo() in this type. We have to provide some hint to the compiler so that the type T becomes meaningful. What we actually want is the type T to be a derived type from Base. Simplest way of writing that is where T : Base. However, the Base has a generic type parameter which is the derived type itself. For example, when we create derived class, we define it as public class Derived : Base<Derived> { ... }. Similar reasoning can be used in writing generic type restriction. We can write the generic type constraints as where T : Base<T>. It may help if you think T as Derived.

So, with this understanding, our base class definition becomes:

public abstract class Base<T> where T : Base<T>
{
public T Copy()
{
var copy = this.DuplicateInternal();
copy.MythicalMumboJumbo();
return copy;
}

protected abstract T DuplicateInternal();

private void MythicalMumboJumbo()
{
// abra-ka-dabra
}
}


Well, now if you look carefully, this is actaully the CRTP in C#. I like to follow one very simple convention for CRTP. That is, I name the generic type parameter TDerived rather than just T. That gives me (or, the user of the code) an indication that the generic type parameter is expecting the derived type and it is implemented as CRTP.

The final signature of the base class may look like:

public abstract class Base<TDerived> where TDerived : Base<TDerived>
{
public TDerived Copy()
{
var copy = this.DuplicateInternal();
copy.MythicalMumboJumbo();
return copy;
}

protected abstract TDerived DuplicateInternal();

private void MythicalMumboJumbo()
{
// abra-ka-dabra
}
}


I hope, I was able to explain how I understand CRTP, without getting stuck in the never ending loop. If you apply CRTP in real life often (where applicable), you may have an epiphany- “Ohh…. that’s what CRTP is all about.”.

Generally, whenever you need the derived class information in the base class, you are probably looking for CRTP.

Here are some situations where you may apply CRTP.

Abstract data structures

Something like linked list. May be, the base class only knows about the parent, but keeps the children implementation for the derived classes.

public abstract class AbstractSinglyLinkedList<TDerived>
{
private TDerived parent;

{
this.parent = parent;
}

public TDerived Parent
{
get
{
return this.parent;
}
}
}

{

public BinaryTree(BinaryTree parent, int data) : base(parent)
{
this.data = data;
}

public BinaryTree Left { get; private set; }

public BinaryTree Right { get; private set; }

{
this.Left = new BinaryTree(base.Parent, left);
}

{
this.Right = new BinaryTree(base.Parent, right);
}
}


Fluent methods

Have you seen the nice fluent methods in different libraries where you can call methods after methods in a chain? Well, this is one way of implementing fluent methods:

public interface IFluentOperations<TDerived>
where TDerived : IFluentOperations<TDerived>
{
TDerived FluentlyDoSomething();
TDerived FluentlyDoSomethingElse();
TDerived EnoughFluentForNow();
}

public class MagicOperations : IFluentOperations<MagicOperations>
{
public MagicOperations FluentlyDoSomething()
{
// do something
return this;
}

public MagicOperations FluentlyDoSomethingElse()
{
//do something else
return this;
}

public MagicOperations EnoughFluentForNow()
{
return this;
}
}


And, then, you use it like a boss:

var op = new MagicOperations();
op
.FluentlyDoSomething()
.FluentlyDoSomethingElse()
.EnoughFluentForNow();


Template method pattern

You may have heard about (or, even used) a design pattern named template method pattern. If you need to return the actual instance from template method, CRTP can help you with the return type. Here is how it may look like:

public abstract class AlgorithmBase<TDerived>
where TDerived : AlgorithmBase<TDerived>
{
public TDerived Run()
{
this.Step1();
this.Step2();
// ...
this.StepN();
this.FinalStep();

return (TDerived)this;
}

protected abstract void Step1();
protected abstract void Step2();
// ...
protected abstract void StepN();
protected abstract void FinalStep();
}


Although these are different use cases of CRTP, but they all have a single requirement in common. That is, the base class requires the knowledge of the derived classes. If you face any such situation, I am sure that you can use CRTP quite comfortably now.

If you have any working example, feel free to add it in the comment. I would be curious to see your recurring template. :)