What exactly is C#'s dynamic type all about? Is it dynamic or is it just static typing under cover? And how does it change things like early binding, virtual and non-virtual?

This is a chapter of our ebook on C#, a work in progress.

Deep C#

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 Chapter List

1. Why C#?
   
   I Strong Typing & Type Safety
2. Strong Typing
      Extract  Why Strong Typing
3. Value & Reference
4.    Extract Value And Reference
5. Structs & Classes
      Extract Structs & Classes 
6. Inheritance
      Extract Inheritance
   
7. Interfaces & Multiple Inheritance
      Extract Interface
   
8. Controlling Inheritance
   II Casting & Generics
9. Casting - The Escape From Strong Typing
      Extract Casting I ***NEW!
   
10. Generics
11. Advanced Generics
12. Anonymous & Dynamic
    Typing III Functions
13. Delegates
14. Multicast Delegates
15. Anonymous Methods, Lambdas & Closures
    IV Async
16. Threading, Tasks & Locking
17. The Invoke Pattern
18. Async Await
19. The Parallel For
    V Data - LINQ, XML & Regular Expressions
20. The LINQ Principle
21. XML
22. LINQ To XML
23. Regular Expressions
    VI Unsafe & Interop
24. Interop
25. COM
26. Custom Attributes
27. Bit Manipulation
28. Advanced Structs
29. Pointers 

Extra Material

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C# is a language that started out in the style of Java with strong typing and very little extra. Over time it has added features in a careful and thoughtful way to include things that you might not associate with a strongly typed "traditional" object oriented language. For many this is what makes C# special. 

Back in version, 4.0, it added the unthinkable - late binding with dynamic typing. This provides the modern C# programmer with idioms that previously would have been thought out of the question. Dynamic typing goes directly against static strong typing that is supposed to catch so many errors are compile time that otherwise would only appear at run time. 

The worry is that by adopting such loose practice C# might become a mess by adopting models that go against its birthright of strict typing and object-oriented procedural code? 

What is interesting is the way that C# combines the best of both worlds - but see if you agree. 

On being dynamic in C#

Strong typing isn't necessarily the good idea it first seems to be - it has some advantages but so does dynamic typing.

It is also true that it is the biggest current divide between programmers and programming languages is into camps that swear allegiance to the strong typing principle or simply attempt to ignore type altogether because it is simpler.

C# started out as a young, clean, strongly-typed language but as it reaches its middle age it is displaying signs of being envious of the currently popular dynamic languages. This is made worse by the desire to meet the demand to be simpler, easier to use and more productive.

Some definitions

First it is worth saying what strict, static and dynamic typing are all about.

A language uses strict typing if it demands that every item of data has a type and can only be stored in a variable of that type.

Usually things are a little more complicated in that types form a hierarchy and a variable of a given type can store data of that type and types derived from it. Put simply a strictly typed language "worries" about type and generates type exceptions when it thinks you are trying to store data of the wrong type in a variable. 

Contrast this with a freely typed language that tries to make it possible for you to store any data in any variable and tries to make sense of what you want to do taking the actual type into account. Put simply a freely typed language tries to ignore data type as much as possible and generates a type exception only when you write something that cannot be reasonably achieved. the type free language motto is "if it can be made to work its ok".

A strongly typed language can also be statically or dynamically typed.

Static typing means that all data types can be worked out at compile time and any errors can be flagged at compile time. In other words you can work out the types of all data and all variables simply by reading the code.

A dynamic language allows a variable to determine its type at run time. In this case you can't necessarily work out the type of a variable by simply reading the code and not all type errors can be picked up at compile time.

Notice that there are two variations possible on dynamic typing. You could enforce a rule which fixes the type of a variable once it is known i.e. variables can't change their type once set. Or you could implement fully dynamic typed variables that can be used to store anything anytime and change there type according to what is stored in them.

You might at this point think that its all very simple but in an object oriented language like C# type actually plays a role in determining what happens in a program - it can change the flow of control and hence things are more subtle.