DbSet.Cast<TEntity>() Error: Cannot create a DbSet<IEntity> from a non-generic DbSet for objects of type 'Entity'

For this, I would actually suggest using reflection. In the constructor of your DbContext, you can set a property to the function pointer:

method = this.GetType().GetMethod("Set", new Type[0]).MakeGenericMethod(typeof(UserImplementation));

You can then invoke this using:

method.Invoke(this, new object[0]);

And this should return an object of type DbSet<UserImplementation> which the .Cast<>() method can then be invoked on.

replace

nonGeneric.Cast<IUser>();

by

Enumerable.Cast<IUser>(nonGeneric);