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This article will explain in detail how LINQ implements subquery and delayed execution programming. The editor thinks it is very practical, so I share it with you for reference. I hope you can get something after reading this article.

LINQ subquery

A LINQ subquery is a query that contains the Lambda expression of another query. The following example uses a LINQ subquery to sort the last name of basketball stars:

String [] players = {"Tim Ducan", "Lebrom James", "Kobe Byrant"}; IEnumerable Q = players.OrderBy (m = > m.Split (). Last ())

Where Last is a LINQ subquery and Q represents an external query.

In a LINQ subquery, you can use any feasible C # expression syntax on the right side of the Lambda expression. LINQ subquery is just a simple C # expression, which means that all the rules applicable to LINQ subqueries can be derived from Lambda expressions.

The following query gets a sequence of characters in a character array whose length is equal to the minimum length:

String [] names = {"James", "Jack", "Landy", "C.Y", "Jay"}; IEnumerable Q = names .Where (n = > n.Length = = names.OrderBy (N2 = > n2.Length) .Select (N2 = > n2.Length). First (); foreach (var s in q) {Console.WriteLine (s); / / C.Y, Jay}

For LINQ subqueries, you can refer to external Lambda parameters or iterative variables (in composite queries). For example, in the above example, if the expression used by OrderBy is changed to (n = > n.Length) instead of N2, you will get an error message:

A local variable named'n 'cannot be declared in this scope because it would give a different meaning to' n', which is already used in a 'parent or current' scope to denote something else.

For this example, we can see the corresponding composite query writing:

IEnumerable Q = from n in names where n.Length = = (from N2 in names orderby n2.Length select n2.Length) .First () select n

The external iterative variable n is visible within the scope of the LINQ subquery, so we cannot reuse it as an iterative variable within the LINQ subquery.

LINQ subqueries are executed when the corresponding Lambda expression is executed, depending on the external query, or from the outside to the inside. Local queries fully follow this model, but interpretive queries (such as LINQ to SQL) follow only conceptually.

We can also use a more concise way to write the previous query:

IEnumerable Q = from n in names where n.Length = = names.OrderBy (N2 = > n2.Length) .First () .Length select n

If you use the Min aggregate function, you can further simplify:

IEnumerable Q = from n in names where n.Length = = names.Min (N2 = > n2.Length) select n

In fact, because n2.Length recalculates each time during an external query loop, which may cause efficiency problems in some cases, to avoid this problem, we can separate the LINQ subquery:

Int len = names.Min (n = > n.Length); IEnumerable query = from n in names where n.Length = = len select n

Subqueries and deferred execution

Returning a single element or aggregate operator in a LINQ subquery, such as first or Count, does not force the external query to execute immediately, which means that the external query still has the ability to delay execution. This is because the LINQ subquery is called indirectly-through a proxy (delegate) if it is a local query or through an expression tree (expression tree) if it is an interpretive query.

An interesting phenomenon is that when your LINQ subquery contains a Select expression, if it is a local query, you are actually diverging it into a sequence of queries-and each has the ability to delay execution. This effect is transparent because it can significantly improve efficiency.

This is the end of this article on "how LINQ implements subquery and delayed execution programming". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it out for more people to see.

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