为什么已经从另一个项目添加了此引用，为什么还要添加相同的引用

Question

Here is a Question, Supposed that I have two projects:

1. Application(console)
2. DataAccess(class library).

In DataAccess project, there is a DataContext class inherited from DbContext

public class DataContext : DbContext {...}

In Application project, I has added DataAccess reference, and in main method I added following statement:

using (var context = new DataContext ()){...}

but Resharper show

The type DbContext is defined in an assembly not referenced.

Why do I need to add the same assembly again?

Why can't I get DbContext definition from DataAccess.dll?

(DataAccess project has added EntityFramwork.dll through NuGet)

Edit 1: When a type inherited another type(ex: DataContext inherited from DbContext), why compiler didn't include the definition of DbContext in the same dll, so in another project, we just only need to add reference to child type's dll ?

Answer 1

Because your class library (the .dll ) doesn't 'contain' its dependent assemblies. So your executing assembly needs to have references to the dependent assemblies too if it wants to use your class library.

(It's a bit the same as with config files, they aren't embedded either, so even if you have for example a connectionstring defined in the config file of the class library, you still need to define it into your start-up application too.)

If you really want to embed libraries into a single dll you can take a look at ILMerge but it's recommended to just include all the libraries you need in your start-up application.

Another good one (that I can recommend) is Costura.Fody . You just need to install it through NuGet, no configuration required whatsoever and when you build your application the next time, it will embed everything into your class library.

Answer 2

Because the type you want to reach is on that assembly. Because you want to be able to define a variable on that type. It is like:

DataContext dbContext = new DataContext();
dbContext.Users.ToList();

this is what you want to do in presentation layer using by DAL. But the problem is you still using entity framework. Lets create a function on service layer which has dependency on EntityFramework.

List<User> GetUsers()
{
DataContext dbContext = new DataContext();
dbContext.Users.ToList();
}

And now we can simply call this service and function without adding Entity Framework reference. That's because; In presentation layer, we have nothing to do with entity framework right now. We are just calling a function. But in your case you are just using entity framework with the Context from another project. That's why your code needs EF refence and mine doesnt. Thanx