What do you mean "I have added ClosedXML.dll"? You don't "add DLL" (DLL is no really significant in .NET, no more then a naming convention for some of executable modules
, .NET operates with assemblies
), you need to reference assembly
to build an assembly dependent on ClosedXML or anything else. If this is Visual Studio, you have the "Add Reference" item in your project tree, a part of "Solution Explorer" tree view. If you build in other ways, there are always the options to reference assemblies.
The namespace is
, in your case,
. How could you possibly miss the namespace? Please see:
Namespace does not add any modularity or something like that; they only introduce extended "fully-qualified" naming to top-level types. You can use
clause or not using it at all: it cannot change anything in assembly dependencies.