Why is there no SingleOrDefaultAsync for IQueryables?

问题

The following code does not compile because SingleOrDefaultAsync() is not a suitable extension for GetAppointments(). I was just wondering why ...

public IQueryable<Appointment> GetAppointments()
{
        return Context.Appointments;
}

public async Task<Appointment> GetAppointmentAsync(int appointmentId)
{
        return await GetAppointments().SingleOrDefaultAsync(a => a.ID == appointmentId);
}

I am using EF 6.0.0. And please ignore what I am doing here exactly. I just tried to make things easier than they actually are in my project.

回答1:

Make sure you have added System.Data.Entity namespace to your usings. This is an extension method, and it will not be available until you add appropriate namespace.

回答2:

I fixed it by adding using Microsoft.EntityFrameworkCore;

来源：https://stackoverflow.com/questions/24259703/why-is-there-no-singleordefaultasync-for-iqueryables