C# Partial Classes

Question

I currently have a solution with multiple projects that mostly use the same classes. As a result, it appeared to me that it would be a good idea to add a class library containing these classes in the solution instead of repeating the class in each project.

However, one project I have requires a few additional properties to some of the classes that are specific to that project and will not be used anywhere else. As a result, I thought that I should use partial classes to add these properties. So I have done something like this:

In the class library:

namespace MyClassLibrary
{
    public partial class Book
    {
        public string Title { get; set; }
        public string AuthorLast { get; set; }
        public string AuthorFirst { get; set; }
        public string Publisher { get; set; }
        public string Edition { get; set; }
        public string ISBN10 { get; set; }
        public string ISBN13 { get; set; }
    }
}

In the project (MyProject):

namespace MyClassLibrary
{
    public partial class Book
    {
        public string AdditionalProperty { get; set; }
    }
}

I have referenced MyClassLibrary in MyProject (a C# windows form app), however when I try to use this class in the codebehind for the form I receive the following error:

class MyClassLibrary.Book

Warning: The type 'MyClassLibrary.Book' in 'C:... (Project)' conflicts with the imported type 'MyClassLibrary.Book' in 'C:... (Class Library DLL)'. Using the type defined in 'C:...(project)'.

Any idea what I am doing wrong or if my whole approach is bad and I should be doing something else?

Answer 1

Partials are not for spanning assemblies. If you need to add to your class for a more specific type of usage, you should create a derived class:

public class MyFoo
{
    public string BasicProperty {get;set;}
}

public class MySpecificFoo : MyFoo
{
    public string AnotherProperty {get;set;}
}

In your project requiring the more specific type of MyFoo, utilize MySpecificFoo instead. Since it inherits/derives from MyFoo, it will have all of the properties and functionality of MyFoo, with the additional properties as well. This is part of Polymorphism, which is where real power of OOP lies.

Answer 2

In short, you can't use partial classes across projects. All the source must be compiled at the same time, and that's done per project.

Here's a full discussion on SO about this: Should you use a partial class across projects?