Type mismatch in variable assignment to generics based variable

Question

Does anyone know why C# doesn't seem to like the following bit of code?

The issue is that MyContext cannot be assigned to the _context variable due to its type mismatch, however MyContext derives (inherits?) from DbContext , so that should satisfy the generics where clause, surely?

public class UnitOfWork<TContext> : IUnitOfWork<TContext> where TContext : DbContext
{
    private TContext _context;

    public UnitOfWork()
    {
        _context = new MyContext(Common.ConfigurationProvider.DatabaseConnectionString);
    }
...

}

Answer 1

This is not allowed because where TContext : DbContext constraint promises that TContext will be of DbContext and not MyContext .

In other words TContext may be MyContext2 which have no inheritance relationship with MyContext . So compiler is not happy with that :(

If you're going to use specific context MyContext then IMO you don't need generics.

---

Edit: If you're sure that your context will be descendant of type argument(TContext) then you can play a trick to make it work,

 public UnitOfWork() { _context = (TContext)(object)new MyContext(...); } 

but, I will think twice before using this in production code. This will fail if MyContext is not TContext or descendant of it.

Answer 2

TContext可能是另一种类型继承DbContext与类型不相容MyContext 。