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c#entity-frameworkgenericslambdadbcontext
Ivan Talalaev
Ivan Talalaev

Reputation: 6494

Generic with lambda expression

I'm trying to create "some kind" of custom "DbContext" for very specific application which uses MongoDb. And I faced with one problem:

How did DbSet know of DbContext existence?

I mean, when I write 

public DbSet<Assignment> Assignments { get; set; }
public DbSet<User> Users { get; set; }

in my DbContext, how the DbSet instance gets the reference to the DbContext class? Via reflection?

But when I was digging into EntityFramework source code I faced another problem

public virtual DbSet<TEntity> Set<TEntity>() where TEntity : class => _setInitializer.Value.CreateSet<TEntity>(this);

In this expression I cannot understand what does lambda expression in "where" clause. Could someone enlighten me?

UPD
Every time when I call my DbSet property in DbContext

public class BlogPostContextContext : DbContext
{
   public DbSet<Blog> Blogs { get; set; }
   public DbSet<Post> Posts { get; set; }
}

The DbSet calls the DbContext and it produces necessary manipulations over the data. My question is: how DbSet gets the reference to the DbContext. I'm asking about underlying communication between DbSet and DbContext.

Thanks in advance. Excuse my english.
Ivan Talalaev

Upvotes: 1

Views: 595

Answers (2)

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149518

I cannot understand what does lambda expression in "where" clause.

The method declaration has two parts. The first is a generic type constraint:

where TEntity : class

This means that any type used in the place of TEntity needs to be a reference type.

The second part is an Expression-bodied function: 

=> _setInitializer.Value.CreateSet<TEntity>(this);

Which is a C# 6.0 feature. If your method is a one-liner, it can be declared like a Lambda Expression. It simply means that each time you call Set, it will invoke the one line "body" of the method. The compiler will turn it into a named method "behind the scenes".

How did DbSet know of DbContext existence?

It doesn't know about it directly. DbContext represents your database, DbSet represents a table in that database. When you're manipulating your DbSet, it implicitly holds a reference to it's underlying DbContext and adds it's entities via the context.

Upvotes: 3

Ivan Talalaev
Ivan Talalaev

Reputation: 6494

I found an answer for my last part of question. According to this and this EntityFramework 6 (not sure about new EF7)
DbSet keep reference _context to the DbContext and uses it to Add or Remove entities.

In turn DbContext some how (possibly via reflection) sets that references _context in all its encapsulated fields of type DbSet at time of cunstructor call.

Upvotes: 0

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