I come from a C# background and have recently started learning C++. One of the things I've encountered is the pimpl idiom. I've done C# development for some large firms, but never came across it.
Maybe this is wrong, but my understanding is that it's necessary in C++ because of the use of header files and no partial class option.
But in C# we would build applications using class libraries all the time. If something changed in the library code, we would recompile it to a dll and reference the new dll in the application project.
I don't really understand why the same thing can't be done with C++. Pimpl just looks like an ugly hack to me.
Is the pimpl idiom used in c#?
That depends on what you mean by this idiom.
The idiom in question is essentially to separate the implementation details of a type into one class, and the public surface area into a wrapper class that simply holds onto a pointer to the implementation class.
This has two benefits.
First, in C++ it can lead to improvements in compile time because consumers of the public surface area need only to parse the header file containing the public surface area; they need not parse the header file containing the implementation details.
Second, it leads to a very clean separation of interface from implementation; the implementation can change utterly without there ever being any impact upon the consumer, because the consumer never sees the implementation details.
The first benefit is of no matter in C#. The C# compiler is fast. (In large part of course because the language was designed to be fast to compile.)
The second benefit is of considerable use in C#. In C# the idiomatic way to implement this pattern would be to make a private nested class that does the "real" work and a public class that is just a facade with the public surface area.
A technique I particularly like in C# is to make the public class the base class of the private nested class, give the base class a private constructor to prevent third party extension, and use the factory pattern to hand out instances of the private nested class.
I don't really understand why the same thing can't be done with C++.
Then I encourage you to attempt to write a C++ compiler that does so. You'll either succeed in creating a much faster C++ compiler, or you'll find out why the same thing cannot be done in C++. You benefit either way!