LINQ Links

The MSDN Site: http://msdn.microsoft.com/en-us/vbasic/aa904594.aspx 

Wikipedia: http://en.wikipedia.org/wiki/Language_Integrated_Query 

LINQ (Language Integrated Query) was delivered in .NET 3.5 and Visual Studio 2008. It addreses O/R mapping issues by making query operations like SQL statements part of the programming language. It also offers built-in support for querying in-memory collections like arrays or lists, XML, DataSets, and relational databases.It also allows you to develop queries in your native development language (with IntelliSense).

LINQ to XML: It is recommended that developers switch over from coding against the XmlDocument DOM to LINQ's XDocument

LINQ to SQL: Deprecated... use LINQ to Entities

LINQ to Entities: Entity Framework Learning Guide

Books:

• Essential LINQ (Microsoft .NET Development Series)
• Pro LINQ: Language Integrated Query in C# 2008 (Windows.Net)
• Programming Entity Framework
• LINQ in Action

LINQ to SQL - Dead or alive?

Microsoft claims that LINQ to SQL is not a dead technology, despite their focus on Entity Framework. From Redmond Developer News:

Tim Mallalieu, the program manager for both LINQ to SQL and the Entity Framework, ... says Microsoft anticipated the backlash but said both data access interfaces will be better off for this move. For those who don't think the Entity Framework is a suitable replacement, Mallalieu said stay tuned.

"There's some pretty nifty stuff we're doing in the beta 2 time frame that we are not speaking about as yet, I think it will give you a better experience and will reduce the anxiety that people have around the Entity Framework," Mallalieu said. "In terms of capabilities, I think will make the overall integrated experience of ASP.NET, Dynamic Data, MVC and these other things easier, we did a little bit of triaging and feedback, there is some valid feedback around complexity in the Entity Framework and we are doing things to address that. "

Especially in light of the fact that LINQ to SQL is specific to SQL Server, I think it's best to wait a while to see what happens with Entity Framework. Note that there are quite a few folks who aren't happy with the way it has turned out so far.

Fun with C# 3.0 (#3) - Lambda expressions

The addition of LINQ to C#/.NET has really improved the way that we can handle data in our applications. Here's an example.

Suppose we want to write a method to count the number of strings in a list which exceed a certain length. It's not to hard to do with C# 2.0; you iterate over the items, make the comparison, increment a counter and return it. But C# 3.0 makes it simple.

When the namespace System.Linq is included in your file, IEnumerable objects get a load of new extension methods. One of them is Where, which will return a sub-collection of items that match a filter. The filter is defined as a lambda expression that returns a bool. If the item causes the lambda expression to evaluate to true, then the item will be included in the sub-collection. Let's look at some code:

public int NumberLongerThan(List<string> list, int n)
{
   var subList = list.Where(
        (string str) => str.Length > n
   );
   return subList.Count();
}

Here we pass in a List<string> and a number. The subList will have the string items that are longer than n characters. Note that we don't have to define the type of subList; the compiler can infer it at compile-time.

Here is the code that uses the method.

List<string> list = new List<string>() { "abc", "def", "GH", "ijKL", "mnop", "QRSTUV", "Wxyz" };
Console.WriteLine(NumberLongerThan(list, 0));
Console.WriteLine(NumberLongerThan(list, 1));
Console.WriteLine(NumberLongerThan(list, 2));
Console.WriteLine(NumberLongerThan(list, 3));

This is the lambda expression:

(string str) => str.Length > n

It states that a string is passed in and returns a bool based on the test of the string Length. The Where() method takes an expression that expects a string and returns a bool. Since the compiler knows this, we can let it infer the type of the parameter and specify the expression like so:

str => str.Length > n

Also, note that if we have a named method that is of the correct delegate type (Func<T, bool>), we can pass it to Where(). Here, List<T> is List<string>, so the method must take a string parameter.

public bool TestString(string s) { return s.Length > 3; }

var filtered = list.Where(TestString);
//-- or --
Func<string, bool> f = TestString;
var list2 = list.Where(f);

The advantage of using the unnamed lambda expression is that we can reference the local variable n as a parameter in our test expression.

More info at Charlie Calvert's blog.