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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.




Building the Right Environment to Support AI, Machine Learning and Deep Learning

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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