Using DbContext Set() instead of exposing on the context

问题

Are there any differences when doing the following:

public class UsersContext : DbContext
{
    public DbSet<User> Users { get; set; }
}

versus using the Set<T> method of the context:

public class UsersContext : DbContext
{
}

var db = new UsersContext();
var users = db.Set<User>();

These effectively do the same thing, giving me a set of Users, but are there any big differences other than you are not exposing the set through a property?

回答1:

The Users property is added for convenience, so you don't need to remember what all of your tables are and what the corresponding class is for it, you can use Intellisense to see all of the tables the context was designed to interact with. The end result is functionally equivalent to using Set<T>.

回答2:

You get a benefit with the former method when using Code-First migrations, as new entities will be detected as such automatically. Otherwise, I'm quite certain they are equivalent.

回答3:

This is how I set my generic dbSet, works just fine

DbContext context = new MyContext();
DbSet<T> dbSet = context.Set<T>();

It is the generic version of something more explicit, such as

DbContext context = new MyContext();
DbSet<User> dbSet = context.Set<User>();

Either way, they are the same (when T is User)

回答4:

I think there is some difference. Let me use the example as in the question. Assume i want to do a Any based on User.FirstName and User.LastName (User table has more fields)

Method1: UsersContext.Users.Any(u => u.FirstName.ToLower() == userObj.FirstName && u.LastName.ToLower() == userObj.LastName);

Method2: (UsersContext.Set(typeof(User)) as IQueryable<User>).Any(u => u.FirstName.ToLower() == userObj.FirstName && u.LastName.ToLower() == userObj.LastName);

I checked in sql profiler the query fired in Method1 is:

    exec sp_executesql N'SELECT 
CASE WHEN ( EXISTS (SELECT 
    1 AS [C1]
    FROM [dbo].[User] AS [Extent1]
    WHERE (((LOWER([Extent1].[FirstName])) = (LOWER(@p__linq__0))) AND ((LOWER([Extent1].[LastName])) = @p__linq__1)
)) THEN cast(1 as bit) WHEN ( NOT EXISTS (SELECT 
    1 AS [C1]
    FROM [dbo].[User] AS [Extent2]
    WHERE (((LOWER([Extent2].[FirstName])) = (LOWER(@p__linq__0))) AND ([Extent2].[LastName] = @p__linq__1)
)) THEN cast(0 as bit) END AS [C1]
FROM  ( SELECT 1 AS X ) AS [SingleRowTable1]',@p__linq__0 nvarchar(4000),@p__linq__1 nvarchar(4000)',@p__linq__0=N'jack',@p__linq__1=N'saw'

From Method2:

    SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[FirstName] AS [FirstName], 
[Extent1].[LastName] AS [LastName], 
[Extent1].[Email] AS [Email], 
.......other fields......
FROM [dbo].[Users] AS [Extent1]

The table has 40000 records and Method1 takes around 20 ms while Method2 takes around 3500 ms.

回答5:

One difference is that the Set method takes any type include non entities and it won't throw exceptions, only returning an empty set of that type. So you could mess up if you typed the wrong class name.

回答6:

I think there is no such difference between two approaches except that Set<User>() is more suitable for implementing data access patterns like Repository pattern because of the generic nature of the Set<T>() method.

来源：https://stackoverflow.com/questions/13710883/using-dbcontext-sett-instead-of-exposing-on-the-context

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