在实体框架中调用 AsNoTracking 的位置是否重要

Question

Does it matter where the AsNoTracking method is called when writing an Entity Framework query? eg

var matchingCustomers = context.Customers.AsNoTracking().Where(n => n.city == "Milan").Skip(50).Take(100).OrderBy(n => n.Name).ToList();
var matchingCustomers = context.Customers.Where(n => n.city == "Milan").AsNoTracking().Skip(50).Take(100).OrderBy(n => n.Name).ToList();
var matchingCustomers = context.Customers.Where(n => n.city == "Milan").Skip(50).AsNoTracking().Take(100).OrderBy(n => n.Name).ToList();
var matchingCustomers = context.Customers.Where(n => n.city == "Milan").Skip(50).Take(100).AsNoTracking().OrderBy(n => n.Name).ToList();
var matchingCustomers = context.Customers.Where(n => n.city == "Milan").Skip(50).Take(100).OrderBy(n => n.Name).AsNoTracking().ToList();
var matchingCustomers = context.Customers.Where(n => n.city == "Milan").Skip(50).Take(100).OrderBy(n => n.Name).ToList().AsNoTracking();

I like adding it to the end of statements but before the ToList is called like this:

var matchingCustomers = context.Customers.Where(n => n.city == "Milan").Skip(50).Take(100).OrderBy(n => n.Name).AsNoTracking().ToList();

Answer 1

No it doesn't matter: (source)

A new query with NoTracking applied, or the source query if NoTracking is not supported.

So you either do it in the beginning and you expand the "new" query with the method chain, or you do it in the end and then get the "new" query. As long as you call it before the query is executed you're fine.

Answer 2

I think that

var matchingCustomers = context.Customers.Where(n => n.city == "Milan").Skip(50).Take(100).OrderBy(n => n.Name).ToList().AsNoTracking();

matters because you are trying to apply NoTracking to a data structure already in memory once EF has executed, and traked, the query.

When you use this fluent API; you are defining a query without executing it until you, of course, execute the query. ToList() will execute the query an bring the data to memory to transform it into a List<T> data structure.

Let's split the command to understand this:

• context.Customers --> Select [*] from Customers
• Where(n => n.city == "Milan") --> Select [*] from Customers where city == 'Milan'
• Skip(50).Take(100) --> Select [*] from Customers where city == 'Milan' OFFSET 50 ROWS FETCH NEXT 100 ROWS ONLY
• OrderBy name --> Select [*] from Customers where city == 'Milan' OFFSET 50 ROWS FETCH NEXT 100 ROWS ONLY ORDER BY name
• ToList() --> Execute the query and bring the data into memory with Tracking by default!
• AsNoTraking() --> Does nothing because EF already executed the query and tracked the data.