从接口继承时如何使用派生类

Question

I tried to create my DbContext like this:

public interface IMyDbContext
{
    IQueryable<Product> Products { get; set; }
}
public class MyDbContext: DbContext, IMyDbContext
{
    public DbSet<Product> Products { get; set; }
}

I don't want to use IDbSet in the interface because that class is used in another projects which does not use EntityFramework.

Visual Studio said that MyDbContext does not implement the member IQueryable<Product> , but DbSet<Product> is derived from IQueryable<Product> : https://msdn.microsoft.com/en-us/library/gg696460(v=vs.113).aspx

Could you please tell me why? And how can I fix this issue?

Thank you a lot.

Answer 1

Interface implementation has to exactly match the interface definition. So you can't have a property typed as IQueryable<T> on the interface and implement it with property typed as DbSet<T> on the class.

Use explicit interface implementation to work around that:

public interface IMyDbContext
{
    IQueryable<Product> Products { get; }
}

public class MyDbContext: DbContext, IMyDbContext
{
    public DbSet<Product> Products { get; set; }
    IQueryable<Product> IMyDbContext.Products { get { return Products; } }
}

As you can see, I removed the setter from the interface. That's because otherwise you'd need extra type checks, because there is plenty of classes that implement IQueryable<Product> but can't be assigned to DbSet<Product> .