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What's New in C# 3.0? Part 2 : Page 2

The release of Visual Studio 2008 updates C# to version, 3.0, with several key language enhancements and LINQ support. Part Two of this series will walk you through C#'s new LINQ support features and other time-saving enhancements.


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LINQ to Dataset
Besides manipulating data in memory, LINQ can also be used to query data stored in structures like datasets and datatables. The following example (in C#) shows how the Authors table within the pubs database is loaded onto a Dataset object and then queried using LINQ:

SqlConnection conn; SqlCommand comm; SqlDataAdapter adapter; DataSet ds = new DataSet(); //---load the Employees table into the dataset--- conn = new SqlConnection(@"Data Source=.\SQLEXPRESS;" + "Initial Catalog=pubs;Integrated Security=True"); comm = new SqlCommand("SELECT * FROM Authors", conn); adapter = new SqlDataAdapter(comm); adapter.Fill(ds); //---query for authors living in CA--- var authors = from author in ds.Tables[0].AsEnumerable() where author.Field("State") == "CA" select author;

To display the result, you can either bind the result to a DataGridView control using the AsDataView() method:

//---bind to a datagridview control--- dataGridView1.DataSource = authors.AsDataView();

Or, iteratively loop through the result using a foreach loop:


foreach (DataRow row in authors) { Console.WriteLine("{0} - {1}, {2}", row["au_id"], row["au_fname"], row["au_lname"]); }

If you want to query the authors based on their contract status, use the following query:

var authors = from author in ds.Tables[0].AsEnumerable() where author.Field<Boolean>("Contract") == true select author;

The earlier section mentioned the C# 3.0's new anonymous types feature. Using this feature, you can define a new type without needing to define a new class. Here's one good use of anonymous types. Consider the following statement:

Figure 1. An Anonymous Type: authors is an anonymous type with three fields.

//---query for authors living in CA--- var authors = from author in ds.Tables[0].AsEnumerable() where author.Field<string>("State") == "CA" select new { ID = author.Field<string>("au_id"), FirstName = author.Field<string>("au_fname"), LastName = author.Field<string>("au_lname") };

Here, you select all the authors living in the state of CA while simultaneously creating a new type consisting of three properties: ID, FirstName, and LastName. If you now type the word "authors", IntelliSense will show you that authors is of type EnumerableRowCollection <'a> authors, and 'a is an anonymous type containing the three fields (see Figure 1).

You can now print out the result using a foreach loop:

foreach (var auth in authors) { Console.WriteLine("{0} - {1}, {2}", auth.ID, auth.FirstName, auth.LastName); }



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