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CHAPTER 11 PROGRAMMING ADO.NET
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There is no use of the GetOrdinal method to permit the use of a column name instead of a column index value. You can use either approach depending on your personal preference. The Button2_Click procedure parses the Name column value with the GetString method instead of the GetSqlString method used in Button1_Click. Because the GetString method automatically returns a .NET Framework String data type, there is no need to invoke the ToString method as there is with the return value from the GetSqlString method. After the second part of Button2_Click finishes reading the first result set, the code tests the value of bolSecondRSStarted as the condition of an If...End If statement. If the value is False, the code writes another String constant to str1 to label the values from the second result set, and resets the value of bolSecondRSStarted to True. Therefore, the next time the code reaches the If...End If statement with bolSecondRSStarted as a condition, the flow bypasses the code within the If...End If statement.
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Don t forget the preceding tip about the need for a reference to System.Xml.dll when using a GetSqlXXX method. If your project does not have that reference, the GetSqlInt32 method for the rdr1 SqlDataReader will generate an error in the following listing. If you prefer not to add the reference to the System.Xml.dll, then you can replace the GetSqlInt32 method with the GetValue method.
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Dim str1 As String = "Product Categories" & ControlChars.CrLf Dim bolSecondRSStarted As Boolean Try Do Do While rdr1.Read() str1 += rdr1.GetSqlInt32(0).ToString & _ ", " & _ rdr1.GetString(1) & _ ControlChars.CrLf Loop If bolSecondRSStarted = False Then str1 += ControlChars.CrLf & "Product Subcategories" & ControlChars.CrLf bolSecondRSStarted = True End If Loop While rdr1.NextResult Catch ex As Exception MessageBox.Show(ex.Message) Finally rdr1.Close() cnn1.Close() End Try The code from the third part of Button2_Click is essentially the same as the third part of Button1_Click procedure. The only difference is the addition of a label for the message box in Button2_Click. MessageBox.Show( _ Microsoft.VisualBasic.Left(str1, str1.Length - 2), _ "Product Categories followed by Product Subcategories")
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Despite the similarity in the design of the third parts of Button1_Click and Button2_Click, the output from the Show method is strikingly different. The main distinction is that the output from Button2_Click has two result sets instead of just one. An excerpt from the message box displayed by Button2_Click appears in Figure 11-5. You can see all four rows from the first result set and the first six rows from the second result set.
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Figure 11-5. You can use a single ADO.NET DataReader object to return values from multiple result sets.
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Chances are that your main reason for using a SqlDataReader object won t be to display database values in a message box. Therefore, you may be thinking that the preceding two examples are somewhat artificial or at least not relevant to common development tasks. That would be a mistake. The main objective of the preceding two samples and their discussions is to give you a good foundation in understanding SqlDataReader objects and, more generally, any type of ADO.NET DataReader objects. Given that understanding, you can readily adapt SqlDataReaders for other purposes besides displaying results in a message box. The last sample in this section reinforces this notion by adapting the code sample to display the ProductCategoryID and Name column values from the ProductCategory table in a message box so that you can show the Name column values in a ListBox control. It is actually pretty common to want to display database results in Windows Form controls. Furthermore, this is a task that you are likely to want to perform quickly because your main objective for having a ListBox control is to make a selection from it, not to populate it. One of the best features about ADO.NET DataReader objects is that they offer the fastest way for ADO.NET developers to retrieve data from a database. Because the object of the last SqlDataReader sample is to read values from the ProductCategory table, it makes sense to adapt the first SqlDataReader solution, which also reads values from the same database table. However, please understand that this sample readily generalizes to any situation in which you need to display a column of values from a database table in a ListBox control. All you need to do is make two changes, as follows: 1. Revise the Connection object for the SqlCommand object from which you derive the SqlDataReader. Make the SqlConnection object point at the database you want to use. 2. Then, specify a new CommandText property value with a SQL string that extracts the column of values that you want to display in the ListBox control. The sample for populating a ListBox control with product category names needs just the first two parts of the Button1_Click procedure. The third part, which displays a message box, is no longer necessary. Furthermore, the first part, which creates a SqlDataReader, is useful for this sample without any changes. To adapt the Button1_Click procedure, all you need to do is modify the second part of the sample. The Button3_Click procedure shows the completed adapted sample.
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