These three concepts are new for anyone that comes from VO.

An application can use code that is located in the application itself, or it can use code from other assemblies. An assembly is simply a DLL (the concept of an entirely linked library as in VO does not exists anymore). Such an assembly can be: - one of the assemblies from the .NET framework, these are located in the GAC - the Global Assembly Cache. - one of the other assemblies, located in the GAC - somewhere on your disk. In this case both Visual Studio and XIDE need to copy it to the BIN directory - in your current project (XIDE) or solution (Visual Studio). Then the DLL file is already in your BIN folder

Please note that your referenced assembly could need other assemblies, either from the GAC or from another location.

All code in assemblies, included in your own executable, is organized in namespaces. To be found, you need to either define the complete path included the namespace, like in

System.IO.File.WriteAllText( "c:\temp\HelloWorld.txt", "Hello World" )

or to include the needed namespace at the top of your source code, and then use the class directly:

using System.IO
.....
File.WriteAllText( "c:\temp\HelloWorld.txt", "Hello World" )

If you use your own assemblies, you should look at the sources, but if it is either a .NET Framework assembly or a 3rd party assembly, please look at the documentation. For the File class, the MSDN says:

Namespace:   System.IO
Assembly:  mscorlib (in mscorlib.dll)