C# in Depth

LINQ to DataSet335where defect.Field("Status") != Status.Closedselect defect.Field("Summary");foreach (string summary in query){Console.WriteLine (summary);}SelectsSummary fieldComing so soon after the nice, clean world of LINQ to SQL, listing 12.10 makes mefeel somewhat dirty. There’s hard-coded SQL, column names, and casts all over theplace. However, we’ll see that things are better when we have a typed dataset—and thiscode does get the job done. If you’re using both LINQ to SQL and LINQ to DataSet, youcan fill a DataTable using the DataTableExtensions.CopyToDataTable extensionmethod, but I wanted to keep to just one new technology at a time for this example.The first part B is “old-fashioned” ADO.NET code to fill the data table. I haven’tused an actual dataset for this example because we’re only interested in a singletable—putting it in a dataset would have made things slightly more complicated forno benefit. It’s only when we reach the query expression (C D and E) that LINQstarts coming in.The source of a query expression has to be enumerable in some form—and theDataTable type doesn’t even implement IEnumerable, let alone IEnumerable. TheDataTableExtensions class provides the AsEnumerable extension method (C), whichmerely returns an IEnumerable that iterates over the rows in the table.Accessing fields within a row is made slightly easier in LINQ to DataSet using theField extension method on DataRow. This not only removes the need to castresults, but it also deals with null values for you—it converts DBNull to a null referencefor you, or the null value of a nullable type.I won’t give any further examples of untyped datasets here, although there are acouple more queries in the book’s sample code. Hopefully you’ll find yourself in thesituation where you can use a typed dataset instead.12.3.2 Working with typed datasetsAlthough typed datasets aren’t as rich as using LINQ to SQL directly, they providemuch more static type information, which lets your code stay cleaner. There’s a bit ofwork to start with: we have to create a typed dataset for our defect-tracking systembefore we can begin using it.CREATING THE TYPED DATASET WITH VISUAL STUDIOThe process for generating a typed dataset in Visual Studio 2008 is almost exactly thesame as it is to generate LINQ to SQL entities. Again, you add a new item to the project(this time selecting DataSet in the list of options), and again you can drag and droptables from the Server Explorer window onto the designer surface.There aren’t quite as many options available in the property panes for typeddatasets, but we can still rename the DefectUser table, the DefectID field, along withthe associations. Likewise, we can still tell the Status, Summary, and UserType propertiesto use the enumeration types from the model. Figure 12.4 shows the designerafter a bit of editing and rearranging.EAccessesStatusfieldDLicensed to Rhona Hadida