Learn LINQ to SQL (vi): Exploring features

Deferred execution

IQueryable query = from c in ctx.Customers select c;

Such a query syntax does not cause the statement to execute immediately, it is just a description, corresponding to a SQL. Statements are executed only when they are needed, such as:

IQueryable query = from c in ctx.Customers select c;

　　　　　　　 foreach (Customer c in query)

　　　　　　　　　　　 Response.Write(c.CustomerID);

If you perform a two-foreach operation, the execution of the SQL statement will be captured two times:

IQueryable query = from c in ctx.Customers select c;

　　　　　　　 foreach (Customer c in query)

　　　　　　　　　　　 Response.Write(c.CustomerID);

　　　　　　　 foreach (Customer c in query)

　　　　　　　　　　　 Response.Write(c.ContactName);

Corresponding SQL:

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

FROM [dbo].[Customers] AS [t0]

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

FROM [dbo].[Customers] AS [t0]

For such a requirement, it is recommended that you first save the query results using the ToList () method, and then query the collection:

IEnumerable<Customer> customers = (from c in ctx.Customers select c).ToList();

　　　　　　　 foreach (Customer c in customers)

　　　　　　　　　　　 Response.Write(c.CustomerID);

　　　　　　　 foreach (Customer c in customers)

　　　　　　　　　　　 Response.Write(c.ContactName);

The advantage of deferred execution is that we can stitch query syntax like concatenation SQL and then execute:

var query = from c in ctx.Customers select c;

　　　　　　　 var newquery = (from c in query select c).OrderBy(c => c.CustomerID);

DataLoadOptions

var products = from p in ctx.Products select p;

　　　　　　　 foreach (var p in products)

　　　　　　　 {

　　　　　　　　　　　 if (p.UnitPrice > 10)

　　　　　　　　　　　　　　　 ShowDetail(p.Order_Details);

　　　　　　　 }

　　　 private void ShowDetail(EntitySet<Order_Detail> orderdetails)

　　　 {}