LINQ 和 class 中的结果集映射

Question

here is the code

        var customers = db.ExecuteQuery<Customer>(@"SELECT CustomerID, CompanyName, ContactName, ContactTitle, 
   Address, City, Region, PostalCode, Country, Phone, Fax
   FROM   dbo.Customers
   WHERE  City = {0}", "London");

foreach (Customer c in customers)
   Console.WriteLine(c.ContactName);

code execute sql and retun a customer record. my question how result can be stored in customer class automatically....which i do not understand. if u see this line of code db.ExecuteQuery<Customer> from here we can understand that customer result will be return and customer data will be stored in customer class. how automatically data can be stored & assign to right property in customer class because a customer class CustomerID property name could be CustID....then what will happen.

the line db.ExecuteQuery<Customer> is very confusing for me and i just do not understand a new customer instance will be created with return customer data....so plzz discuss in detail.

Answer 1

ExecuteQuery just runs arbitrary SQL (after applying string.Format -esque parameterization) and then materializes the IDataReader results as a sequence of objects.

Note that the mappings between mapped types and the db-columns/type-members is well defined by the data-context ( usually via attributes on the members, but not always), but IIRC ExecuteQuery does not apply these mapping (I'm happy to be corrected here). You can inspect this via db.Mapping.GetMetaType(typeof(Customer)) .

The fundamentals here are no different to if you executed

var cust = db.Customers.ToList();

except the db.Customers version can apply more mappings etc.

The materialization itself (ie creating the object and setting members) is an example of reflection and meta-programming; most ORMs and micro-ORMs will work pretty much the same there: inspect the type ( Customer above) and inspect the fields in the reader - and then build some code on-the-fly (usually cached) that will create the new objects and set the members.