Is C# really become a dynamic programming language? Now

Well, a dynamic programming language postpones until runtime most of the checks that would otherwise have been needed at compile time. This reduces development time because you can be much more 'cavalier' with such things as numeric conversions and whether objects have particular properties or methods. In effect, you can make things up as you go along!

The drawback is that you tend to have more runtime errors and performance is not as good because the runtime needs to do more work. 

If C# were to become a fully dynamic language, then it would inherit these advantages and disadvantages.

The question would then be whether this would represent a net gain compared to static typing. The latter approach picks up as many errors as possible at compile time, which increases development time, but suffers from fewer runtime errors and provides better performance as there's less for the runtime to do.

The C# design team evidently didn't think that going completely dynamic would be a good idea and thought it preferable to give the language some dynamic programming abilities whilst leaving it as essentially a statically typed language.

My own view is that dynamic languages are OK for quickly throwing together small applications but that you tend to lose the plot if you attempt something more ambitious.

You may recall that Microsoft recently attempted to make Javascript more suitable for larger applications (and more acceptable to C++, C# and VB.NET programmers) by introducing TypeScript which adds optional strict typing and better OOP concepts to the language. The jury's still out on whether this has been successful or not.