Extension Methods vs Instance Methods vs Static Class [closed]

Question

I'm a little bit confused about the different ways to use methods to interact with objects in C#, particularly the major design differences and consequences between the following:

1. Invoking an instance method
2. Using a static class on a POCO
3. Creating an extension method

Example:

public class MyPoint
{
    public double x { get; set; }
    public double y { get; set; }

    public double? DistanceFrom(MyPoint p)
    {
        if (p != null)
        {
            return  Math.Sqrt(Math.Pow(this.x - p.x, 2) + Math.Pow(this.y - p.y, 2));
        }
        return null;
    }
}

If you could accomplish the desired outcome by simply placing a method in a class definition, why would a POCO combined with either a static helper class or an extension method be preferable?

Ostiole

And if you don't have the class definition? Now you need either a static helper class, or extension methods. Extension methods allow you to extend classes you don't own the source to. They also allow you to remove some of the less-common functions to put them somewhere else you can still get to if you need to, but are not by default included. – Vlf

@EBrown, Interesting, I had never thought about it that way. – Ostiole

You can also define extension methods on interfaces. – Rennes

It's one of the nice things of extension methods: say you wish to extend class A by including method DoSomethingCooler(), but the definition for class A exists in namespace EBrown.NamespaceB in a separate DLL you do not own the source-code to, you can define an extension method to class A that allows you to call A.DoSomethingCooler() just as if you owned the source. The disadvantage: private members are still private. – Vlf

Answer 1

You asked, "If you could accomplish the desired outcome by simply placing a method in a class definition, why would a POCO combined with either a static helper class or an extension method be preferable?"

The answer is that it depends on the situation, and if the methods in question are directly related to your class' primary concern (see single responsibility principle).

Here are some examples of where it might be a good idea to use each type of approach/method (using your code sample as a starting point).

1. Instance Methods

//This all makes sense as instance methods because you're 
//encapsulating logic MyPoint is concerned with.
public class MyPoint
{
    public double x { get; set; }
    public double y { get; set; }

    public double? DistanceFrom(MyPoint p)
    {
        if (p != null)
            return  Math.Sqrt(Math.Pow(this.x - p.x, 2) + Math.Pow(this.y - p.y, 2));            
        return null;
    }
}

2. Static Class Methods - An error logging example.

    //Your class doesn't directly concern itself with logging implmentation;
    //that's something that is better left to a separate class, perhaps
    //a "Logger" utility class with static methods that are available to your class.
    public double? DistanceFrom(MyPoint p)
    {
        try
        {
            if (p != null)
                return  Math.Sqrt(Math.Pow(this.x - p.x, 2) + Math.Pow(this.y - p.y, 2));            
            return null;
        }
        catch(Exception ex)
        {
             //**** Static helper class that can be called from other classes ****
             Logger.LogError(ex);

             //NOTE: Logger might encapsulate other logging methods like...
             //Logger.LogInformation(string s)
             //...so an extension method would be less natural, since Logger
             //doesn't relate to a specific base type that you can create an
             //extension method for.
        }
}

3. Extension Methods - An XML serialization example.

//Maybe you want to make it so that any object can XML serialize itself
//using an easy-to-use, shared syntax.
//Your MyPoint class isn't directly concerned about XML serialization,
//so it doesn't make sense to implement this as an instance method but
//MyPoint can pick up this capability from this extension method.
public static class XmlSerialization
{
    public static string ToXml(this object valueToSerialize)
    {
        var serializer = new XmlSerializer(valueToSerialize.GetType());
        var sb = new StringBuilder();
        using (var writer = new StringWriter(sb))
            serializer.Serialize(writer, valueToSerialize);

        return sb.ToString();
    }
}

//example usage
var point = new MyPoint();
var pointXml = point.ToXml(); //<- from the extension method

The rule of thumb is:

1. If the method relates to a class' primary concern, put it in an instance method.
2. If you have a generic utility that might be useful to multiple classes, consider putting it in a static class' method.
3. If you have a situation similar to 2, but related to a single base type, or you think the code would look cleaner/more concise without having to separately reference a static class, consider an extension method.

Lyophobic

All-in-all, I think this is the best answer. I have an issue with your static class example, but it's probably just an issue with the example itself. If there is business logic or something that doesn't have to do with the Point itself, then it should be in a separate class. Of course, if possible, it should be an extension method. Static classes like Helper classes are way overused. Whatever it is a member of, that's where it should be (whether on the object or an extension). – Mollie

You left out the this before the first parameter in your ToXml extension method. – Rennes

Yeah, I think where the static class has the advantage over an extension method is if the static class' responsibility doesn't directly relate to a specific type. In the "Logger" example, you might have several methods like "Logger.LogError(Exception ex)" and "Logger.LogInformation(string message)". An extension method isn't a natural fit in this case, as you want logger to encapsulate logging logic, but there's not a natural-feeling, single type to extend. – Lyophobic

@Lyophobic - Fantastic answer with solid examples to go with it. Thank you! – Ostiole

Happy to be of assistance! – Lyophobic

Answer 2

Instance/Static Methods

Accessible members: public, protected, private (cannot be accessed if inherited)

Definition: Same class/struct/interface (can be split across files with the partial keyword)

Called as: object.Method()

In this context, I mean that Static Methods are methods defined within the class they manipulate. That is, they are defined alongside the other class objects. (Static methods defined within the MyPoint class in your example code.)

We all know (or should know) what these are and the benefits to them, I won't go in to much detail except to say:

Instance methods have access to all private, protected, and public members of the class. As do static methods.

In most cases, if you have a large number of methods and/or properties to add, or they significantly change the operation of the object, you should inherit the original object (if possible). This gives you access to all public and protected members of the class/struct/interface.

Static Helper Class Methods

Accessible members: public

Definition: Any class/namespace

Called as: HelperClass.Method(object)

By Static Helper Class Methods I am implying that the actual definition of the static methods referred to by this section is not within the actual class definition. (I.e. a class like MyPointHelpers or similar, using your code example.)

Static Helper class methods only have access to the public members of an object (much like Extension methods, I wrote this section after the extension method section).

Static Helper classes and Extension Methods are closely related, and in many instances are the same. Therefore I'll leave the benefits to them in the Extension Methods section.

Extension Methods

Accessible members: public

Definition: Any class/namespace

Called as: object.Method()

Extension methods only have access to the public members of an object. Though they appear to be members of the class, they are not. This limits what they are useful for. (Methods that require access to any of the private or protected members should not be extensions.)

Extension methods serve three huge benefits in my opinion.

1. Say you are developing class A, and class A has about 7 methods in it. You also know that you would like to develop a few methods that you won't always need, but that would be handy if you ever do. You could use extension methods for this. These methods would be abstracted away in another class that you can include (by class, thanks to C# 6.0) later if you ever need them. Uncommon methods that you know you want to use later, but you know you don't always need.

2. Say you are developing programme A, and you are using a class from DLL Something.Other.C, that you do not own the source to. Now, you want to add a method that interacts with class Something.Other.C in a way that would make sense with an instance or regular static method, but you haven't the source so you can't! Enter extension methods, where you can define a method that appears to be a member of class Something.Other.C, but is actually part of your code.

3. Say you develop your own library that you use with a lot of your own applications, and you realize in the midst of developing application X that you could really use a method Y on class A again. Well, instead of modifying the definition of class A (because that's a lot more work, and you don't use method Y anywhere except application X), you can define an extension method Y, on class A that is only present in application X. Now your overhead of the method is limited strictly to application X. Application Z doesn't need to have this extension method.

Performance

As far as performance, this would depend on the methods, what they do, and how they do it. You would be subject to whatever public properties/methods/fields were on the objects you are altering, and would need to gauge performance on that. (If calling public Value instead of private value causes some significant validation overhead, an instance method makes more sense as it can work with the private fields or properties.)

Vlf

"Static Helper class methods only have access to the public members of an object" - static methods also have access to private members of the type in which they are defined. Often used for factory methods (e.g. SymmetricAlgorithm.Create(), MySingleton.Instance – Cellophane

@Cellophane If the static methods are defined in the object's class, that is. If they are defined, in say, MyPointHelpers then that is a false statement. – Vlf

@Cellophane I did clarify what I mean when I use the term static in each context. – Vlf

This is a fantastic answer - I wish I could have accepted it as well. Thank you so much for the clarification! – Ostiole

@EBrown I have about three more hours before I'm allowed to upvote again. Don't worry though - I'm going to go on an upvote rampage on this question. – Ostiole

@Ostiole Right on man, let me know if I left anything out that you would like an answer to. – Vlf

Thanks for the words on performance! – Exteroceptor

Answer 3

In short:

1. Instance methods
   • Instance properties are like nouns. e.g. cat.Color = Color.Blue;
   • Instance methods are like verbs. They should result in an action that is related to the type of class. e.g. cat.Meow();
   • This type of method is very common in C#.
2. Static methods
   • Think of this as a helper method.
   • Static methods typically perform an action that is related to the class type... not a specific instance.
   • Static methods have to be defined when you create the class.
   • Example: File.Open(String, FileMode) is a static method that returns a FileStream. There is no need to have an instance of a file here.
3. Extension methods
   • Think of this as a helper method.
   • Extension methods are defined after-the-fact... for existing third-party classes that you cannot change, but wish you could.
   • For example: it is not uncommon to see people writing extension methods for the DateTime class.
   • It is not uncommon to see a debate about when/where an extension method should be used.

REFERENCES

• What's a “static method” in C#?
• File.Open

Pumpernickel

Extension methods can also be used to define standard behavior for instances. e.g. the System.Linq.Enumerable class. – Flare

Answer 4

The biggest differences are:

• you can define extensions to objects you cannot change
• an instance method can access private variables, where static methods/extensions can not

Static methods and extensions are basically the same: Visual Studio allows you to call extension methods as if they were an instance method, but in the end it's just a static method with an instance passed to it.

I don't know about performance, and I don't think you should worry about that too much. Maybe instance methods are a bit faster if you access alot of properties, since the current instance is already on the stack (but I wouldn't know for sure if the compiler works that way).

Personally I add methods to classes if they truely belong to the behavior of that class, and use extension methods to perform operations that have a wider scope, such as myClass.ConvertToOtherType().

Loya