66 Microsoft ADO.NET 4 Step by Step Selecting and Sorting DataRow Objects: Visual Basic Note This exercise uses the “Chapter 4 VB” sample project and continues the previous exercise in this chapter. 1. Open the source code view for the TableExaminer form. Locate the ActCriteria_Click event handler. This routine collects user-supplied selection and sorting expressions; then uses them to obtain a set of DataRow instances from a DataTable. Most of the code exists to ensure that the user provides valid expressions. 2. Locate the Try Catch statement just after the “Apply the filter and sorting list” com- ment. In the Try block, add the following statement: results = workTable.Select(CriteriaFilter.Text, CriteriaSorting.Text) This line performs the actual row selection, returning the optionally sorted DataRow instances or an empty array when the selection expression doesn’t match any of the table’s rows. 3. Run the program. On the Lookup By Criteria tab, provide expressions that will return a list of students with improving grades, sorted by name. Enter ScoreTrimester3 > ScoreTrimester1 OR ScoreTrimester3 > ScoreTrimester2 in the Filter Criteria field, and StudentName in the Sorting List field. Click Lookup. The matching rows appear in a separate window. Chapter 4 Accessing the Right Data Values 67 Performing Case-Sensitive Lookups The Select method ignores character casing by default when comparing string values. For instance, the following expression will match joe, Joe, JOE, or any other mixed-case variation on the name: FirstName = 'joe' To enforce case-sensitive matches on all searches instead, set the table’s CaseSensitive property. C# someTable.CaseSensitive = true; Visual Basic someTable.CaseSensitive = True Using Expression Columns In Chapter 2, “Building Tables of Data,” you learned how to add columns to a table that would each hold data values of a specific type. These static columns define the core data within a table. The DataTable class also supports expression columns, fields that expose a cal- culated result based on the data in other row columns. For instance, if your table of orders includes a Subtotal column and a Tax column, you could add an expression column named Total that calculated the sum of Subtotal and Tax. 68 Microsoft ADO.NET 4 Step by Step To add an expression column to a table, create a standard DataColumn object, fill in its ColumnName and DataType properties, and then assign a string expression that performs the custom calculation to the Expression property. C# // Syntax using a DataColumn object. DataColumn orderTotal = new DataColumn(); orderTotal.ColumnName = "Total"; orderTotal.DataType = typeof(decimal); orderTotal.Expression = "Subtotal + ISNULL(Tax, 0)"; someTable.Columns.Add(orderTotal); // Syntax using Add arguments only. someTable.Columns.Add("Total", typeof(decimal), "Subtotal + ISNULL(Tax, 0)"); Visual Basic ' Syntax using a DataColumn object. Dim orderTotal As New DataColumn orderTotal.ColumnName = "Total" orderTotal.DataType = GetType(Decimal) orderTotal.Expression = "Subtotal + ISNULL(Tax, 0)" someTable.Columns.Add(orderTotal) ' Syntax using Add arguments only. someTable.Columns.Add("Total", GetType(Decimal), "Subtotal + ISNULL(Tax, 0)") The expression field uses the same elements from Table 4-1 that you used with the DataTable.Select method. To view the full documentation for this expression, access the Visual Studio online help entry for “DataColumn.Expression Property.” Note The documentation for the Ex pression property discusses “aggregate functions.” These are covered in Chapter 6. After being added to your table, you can query expression columns in Select statements or examine them with standard ADO.NET code just like static columns. Expression columns are not calculated until you attempt to access them. If there is anything wrong with the expres- sion, such as including references to non-existent columns, the code accessing the column will throw an exception. Chapter 4 Accessing the Right Data Values 69 Adding Expression Columns to a DataTable : C# Note This exercise uses the “Chapter 4 CSharp” sample project and continues the previous exer- cise in this chapter. 1. Open the source code view for the TableExaminer form. Locate the ActExpression_Click event handler. This routine defines up to three expression columns based on column names, data types, and calculation expressions supplied by the user. It then adds these columns to the application’s sample DataTable. Most of the code exists to ensure that the user provides valid column definitions. 2. Locate the try catch statement just after the “Add the expression column” comment. In the try block, add the following statement: workTable.Columns.Add(nameField.Text.Trim(), Type.GetType("System." + typeField.SelectedItem.ToString()), expressionField.Text); This code adds the expression columns to the sample table, passing the column name, the data type from the System namespace, and the field expression. 3. Run the program. On the Add Expression Columns tab, fill in the Name, Type, and Expression fields with the desired custom columns. To create a column that calcu- lates the average annual score for each student, in the first row of fields, set Name to YearAverage, select Decimal in the Type field, and enter (ScoreTrimester1 + ScoreTrimester2 + ScoreTrimester3) / 3 in the Expression field. 4. Expression columns can reference other expression columns. Create a column that calculates a letter grade for the YearAverage column. In the second row of fields, enter LetterGrade in the Name field, select String in the Type field, and enter IIF(YearAverage >= 3.5, 'A', IIF(YearAverage >= 2.5, 'B', IIF(YearAverage >= 1.5, 'C', IIF(YearAverage >= 0.5, 'D', 'F')))) in the Expression field. Click Build to see the results. 70 Microsoft ADO.NET 4 Step by Step Adding Expression Columns to a DataTable : Visual Basic Note This exercise uses the “Chapter 4 VB” sample project and continues the previous exercise in this chapter. 1. Open the source code view for the TableExaminer form. Locate the ActExpression_Click event handler. This routine defines up to three expression columns based on column names, data types, and calculation expressions supplied by the user. It then adds these columns to the application’s sample DataTable. Most of the code exists to ensure that the user provides valid column definitions. 2. Locate the Try Catch statement just after the “Add the expression column” comment. In the Try block, add the following statement: workTable.Columns.Add(nameField.Text.Trim, Type.GetType("System." & typeField.SelectedItem.ToString), expressionField.Text) This code adds the expression columns to the sample table, passing the column name, the data type from the System namespace, and the field expression. 3. Run the program. On the Add Expression Columns tab, fill in the Name, Type, and Expression fields with the desired custom columns. To create a column that calcu- lates the average annual score for each student, in the first row of fields, set Name to YearAverage, select Decimal in the Type field, and enter (ScoreTrimester1 + ScoreTrimester2 + ScoreTrimester3) / 3 in the Expression field. 4. Expression columns can reference other expression columns. Create a column that calculates a letter grade for the YearAverage column. In the second row of fields, enter LetterGrade in the Name field, select String in the Type field, and enter IIF(YearAverage >= 3.5, 'A', IIF(YearAverage >= 2.5, 'B', IIF(YearAverage >= 1.5, 'C', IIF(YearAverage >= 0.5, 'D', 'F')))) in the Expression field. Click Build to see the results. Chapter 4 Accessing the Right Data Values 71 Summary This chapter introduced different ways to access records previously added to a DataTable instance. The DataTable.Rows.Find method uses the table’s primary key value(s) to return a single DataRow instance, similar to the way that .NET’s Generic.Dictionary class returns an object based on a lookup key. The DataTable.Select method also performs a row lookup, but uses a SQL–like Boolean expression, expanding the search criteria to all columns in the table, not just the primary key. This method also provides a way to sort the results. Expression columns let you add real-time calculated values to each DataRow in your table. Like the other columns in the table, expression columns are strongly typed. Because they are calculated only when accessed, their values refresh automatically whenever any of their de- pendent column values change. Chapter 4 Quick Reference To Do This Access a DataRow by its primary key Add DataColumn instances to a DataTable. Add one or more of those DataColumn instances to the table’s PrimaryKey property. Add relevant DataRow objects to the DataTable. Call the table’s Rows.Find method, passing it the desired row’s primary key value(s). Locate DataRow objects with a SQL-like query Add DataColumn instances to a DataTable. Add relevant DataRow objects to the DataTable. Build a query expression string (see the “DataColumn. Expression Property” entry in online help). Call the table’s Select method, passing it the query expression. Perform a case-sensitive or case-insensitive DataRow lookup Set the DataTable object’s CaseSensitive Boolean property. Call Rows.Find or Select methods with string search content. Add calculated columns to a DataTable Add standard DataColumn instances to a DataTable. Create a new DataColumn instance for the calculated column. Assign the DataColumn object’s ColumnName and DataType fields. Build a column expression string (see “DataColumn. Expression Property” entry in online help). Set the DataColumn object’s Expression property to the expression string. Add the DataColumn to the DataTable. 73 Chapter 5 Bringing Related Data Together After completing this chapter, you will be able to:  Join multiple DataTable instances into a DataSet  Establish parent-child relationships between tables of data  Understand the types of table constraints available in ADO.NET  Build relationships that auto-correct linked rows when needed The D ataTable class provides ADO.NET’s core data-management functionality. But many of the tools that build and interact with DataTable content do so through a higher level of abstraction: the DataSet. Instead of relying on a single table’s worth of DataRows, a DataSet links multiple tables together, making it possible to generate data queries based on the rela- tionships between the tables and their data. In this chapter, the DataSet object takes center stage. You will discover how a DataSet becomes more than the sum of its DataTable parts. By combining data tables, relationship definitions be- tween those tables, and column-specific constraints that help ensure data integrity between the tables, ADO.NET provides new views on data that would be complicated to achieve with solitary data tables. Note The exercises in this chapter all use the same sample project, a tool that shows the related records between two DataTable instances. Although you will be able to run the application after each exercise, the expected results for the full application might not appear until you complete all exercises in the chapter. Collecting Tables into Sets ADO.NET includes a System.Data.DataSet class that defines a collection of tables, their rela- tionships, and related field constraints. To establish a data set in your program, create a new DataSet object, optionally passing it a set name. C# DataSet someSet = new DataSet("SetName"); Visual Basic Dim someSet As New DataSet("SetName") 74 Microsoft ADO.NET 4 Step by Step Adding a name to a standalone DataTable instance might be inconsequential, but some table-related features in ADO.NET do enable access to a DataTable object by its table name. For example, the D ataSet class includes a Tables property that, as expected, holds a collection of individual DataTable instances. You access tables within the collection either by name or by an index number. To add a new DataTable to a DataSet, write the following: C# someSet.Tables.Add(someTable); Visual Basic someSet.Tables.Add(someTable) You can also pass a string to the Add method, which creates a new named table object with- out columns or rows. You can add as many data tables as you want to the Tables collection. At this point, they are still treated as individual tables; adding them to the collection of tables does not automatically endow them with relationship features. Note A DataSet can contain two tables with the same name as long as their namespace values differ. Chapter 7, “Saving and Restoring Data,” discusses these namespaces. Also, if two tables share a common name (and namespace) but differ in the casing of those names (“CUSTOMERS” versus “customers”), the DataSet will treat them as distinct tables. When querying these tables, you must provide the same casing as the original table names, or else the query will fail. However, if a table name has no duplicate within a DataSet, its name in queries can be case- insensitive. The D ataSet includes some properties and methods that replicate the functionality of the contained tables. These features share identical names with their table counterparts. When used, these properties and methods work as if those same features had been used at the table level in all contained tables. Some of these members that you’ve seen before include the following:  Clear  CaseSensitive  AcceptChanges  RejectChanges  EnforceConstraints  HasErrors Chapter 5 Bringing Related Data Together 75 Adding Tables to a DataSet : C# 1. Open the “Chapter 5 CSharp” project from the installed samples folder. The project in- cludes three Windows.Forms classes: FlightInfo, FlightDetail, and LegDetail. 2. Open the source code view for the FlightInfo form. Locate the BuildSampleDataSet function. This routine creates the main DataSet used in the application. 3. Just after the “Add the two tables to the data set” comment, add the following statements: result = new DataSet("FlightSample"); parentTable = BuildFlightTable(); childTable = BuildLegTable(); result.Tables.Add(parentTable); result.Tables.Add(childTable); These lines create two tables that share a common value: the flight ID number. In the flight table the field is named ID, whereas it is called FlightID in the leg table. A later example in this chapter will establish the relationship between the two tables. Adding Tables to a DataSet : Visual Basic 1. Open the “Chapter 5 VB” project from the installed samples folder. The project includes three Windows.Forms classes: FlightInfo, FlightDetail, and LegDetail. 2. Open the source code view for the FlightInfo form. Locate the BuildSampleDataSet function. This routine creates the main DataSet used in the application. 3. Just after the “Add the two tables to the data set” comment, add the following statements: result = New DataSet("FlightSample") parentTable = BuildFlightTable() childTable = BuildLegTable() result.Tables.Add(parentTable) result.Tables.Add(childTable) These lines create two tables that share a common value: the flight ID number. In the flight table the field is named ID, whereas it is called FlightID in the leg table. A later example in this chapter will establish the relationship between the two tables. . 66 Microsoft ADO. NET 4 Step by Step Selecting and Sorting DataRow Objects: Visual Basic Note This exercise uses the “Chapter 4 VB” sample project and continues the previous exercise. IIF(YearAverage >= 0.5, 'D', 'F')))) in the Expression field. Click Build to see the results. 70 Microsoft ADO. NET 4 Step by Step Adding Expression Columns to a DataTable : Visual. DataSet("SetName") 74 Microsoft ADO. NET 4 Step by Step Adding a name to a standalone DataTable instance might be inconsequential, but some table-related features in ADO. NET do enable access