Microsoft ADO .NET 4 Step by Step - p 10 potx

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

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

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 Expression 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

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

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

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 DataTable 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 bebe-tween 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 DataSet 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 DataSet 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

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