C# in Depth

334 CHAPTER 12 LINQ beyond collections12.3 LINQ to DataSetSeeing all the neat stuff that LINQ to SQL can achieve is all very well, but most developersare likely to be improving an existing application rather than creating a new onefrom the ground up. Rather than ripping out the entire persistence layer and replacingit with LINQ to SQL, it would be nice to be able to gain some of the advantages ofLINQ while using existing technology. Many ADO.NET applications use datasets,whether typed or untyped 4 —and 4 LINQ to DataSet gives you access to a lot of the benefitsof LINQ with little change to your current code.The query expressions used within LINQ to DataSet are just LINQ to Objects queries—there’sno translation into a call to DataTable.Select, for example. Instead,data rows are filtered and ordered with normal delegate instances that operate onthose rows.Unsurprisingly, you’ll get a better experience using typed datasets, but a set ofextension methods on DataTable and DataRow make it at least possible to work withuntyped datasets too. In this section we’ll look at both kinds of datasets, starting withuntyped ones.12.3.1 Working with untyped datasetsUntyped datasets have two problems as far as LINQ is concerned. First, we don’t haveaccess to the fields within the tables as typed properties; second, the tables themselvesaren’t enumerable. To some extent both are merely a matter of convenience—wecould use direct casts in all the queries, handle DBNull explicitly and so forth, as well asenumerate the rows in a table using dataTable.Rows.Cast. Theseworkarounds are quite ugly, which is why the DataTableExtensions and DataRow-Extensions classes exist.Code using untyped datasets is never going to be pretty, but using LINQ is far nicerthan filtering and sorting using DataTable.Select. No more escaping, worryingabout date and time formatting, and similar nastiness.Listing 12.10 gives a simple example. It just fills a single defect table and prints thesummaries of all the defects that don’t have a status of “closed.”Listing 12.10Displaying the summaries of open defects from an untyped DataTableDataTable dataTable = new DataTable();using (var connection = new SqlConnection(Settings.Default.SkeetySoftDefectsConnectionString)){string sql = "SELECT Summary, Status FROM Defect";new SqlDataAdapter(sql, connection).Fill(dataTable);}var query = from defect in dataTable.AsEnumerable()BCFills tablefromdatabaseMakes tableenumerable4An untyped dataset is one that has no static information about the contents of its tables. Typed datasets, usuallygenerated in the Visual Studio designer, know the tables which can be present in the dataset, and the columnswithin the rows in those tables.Licensed to Rhona Hadida