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No, you cannot have two partial classes referring to the same class in two different assemblies (projects). Once the assembly is compiled, the meta-data is baked in, and your classes are no longer partial. Partial classes allows you to split the definition of the same class into two files.
All parts of a partial class should be in the same namespace. Each part of a partial class should be in the same assembly or DLL, in other words you can't create a partial class in source files of a different class library project.
The source file name for each part of the partial class can be different, but each partial class's name must be the same. The name of all parts of a partial class should be the same. All parts of a partial class should be in the same assembly.
Inheritance cannot be applied to partial classes.
From MSDN -Partial Classes and Methods:
All partial-type definitions meant to be parts of the same type must be defined in the same assembly and the same module (.exe or .dll file). Partial definitions cannot span multiple modules.
No. Partial classes are a purely language feature. When an assembly is compiled, the files are combined to create the type. It isn't possible to spread the files out into different assemblies.
Depending on what you want to do, though, you might be able to use extension methods to accomplish what you need.
No it is not possible. When the assembly is compiled the class needs to be finished.
While other answers do provide the unpleasant "No" that anyone landing on this page didn't want to see or hear, I was struck by another thought that hasn't been mentioned here yet. If partial classes were allowed across assemblies, one would get access to private members of existing types that were not written by him, thus allowing him to manipulate them in ways that were not intended by the original author, thus jeopardizing the functionality of all inheriting classes too.
Not only that, those classes in other assemblies (and their children) would need to be recompiled to make it work. Thus it is logically not possible to allow splitting a class over different assemblies.
You can use extension methods when you want to add a method to a class in a different dll. The one drawback of this method is that you cant add static methods.
The question is why would you want to make a partial class in another assembly? You can define abstract classes and interfaces across assemblies, maybe you need to look into that.
You probably just want to create a Wrapper class within you own library, around the class in the 3rd part library. Then add whatever functionality to the wrapper class.
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