The forward declarations in C++ are a way to provide metadata about the other pieces of code that might be used by the currently compiled source to the compiler, so it can generate the correct code.
That metadata can come from the author of the linked library/component. However, it can also be automatically generated (for example there are tools that generate C++ header files for COM objects). In any case, the C++ way of expressing that metadata is through the header files you need to include in your source code.
The C#/.Net also consume similar metadata at compile time. However, that metadata is automatically generated when the assembly it applies to is built and is usually embedded into it. Thus, when you reference in your C# project an assembly, you are essentially telling the compiler "look for the metadata you need in this assembly as well, please".
In other words, the metadata generation and consumption in C# is more transparent to the developers, allowing them to focus on what really matters - writing their own code.
There are also other benefits to having the metadata about the code bundled with the assembly as well. Reflection, code emitting, on-the-fly serialization - they all depend on the metadata to be able to generate the proper code at run-time.
The C++ analogue to this would be RTTI, although it's not widely-adopted due ot incompatible implementations.
