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One of the most common data-definition tasks is to create a table. Creating a table provides a repository for your application to store and retrieve data. You may also need to retire tables from use by dropping them. It is always a good idea to remove objects from an application that you no longer need because it leaves fewer objects for a hacker to attack.
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CHAPTER 11 PROGRAMMING ADO.NET
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A click to Button1 creates a table named DotNetTable in the database pointed at by the cnn1 SqlConnection object. The code inside the Button1_Click procedure appears next. There are two parts to the procedure, which are as follows: First, the code instantiates a SqlCommand object (cmd1). The cmd1 CommandText property includes the code for creating the DotNetTable. The Connection property equals cnn1 so that the table is added to the version of the Northwind database files within the current project. The table has three columns. The ContactID column with an IDENTITY property serves as the source for the table s primary key. The ContactName column holds up to 25 Unicode characters for a contact s name. The ContactEAddr column holds up to 60 Unicode characters for a contact s email address. After defining the cmd1 SqlCommand object, the code executes the command with the ExecuteNonQuery method. Before invoking the method, you must open the SqlConnection object for the SqlCommand object. The code to execute the cmd1 SqlCommand object occurs within the Try clause of a Try...Catch...Finally statement. Whether the attempt to execute succeeds or fails, the Finally clause will always close the cnn1 SqlConnection object. If the T-SQL for the CommandText property cannot execute, the Catch clause provides some feedback. For example, if the DotNetTable database table already exists, you ll receive a message alerting you about this. No special code is required for this feedback. It is a normal error message that SQL Server Express returns to the client application through ADO.NET. Dim cmd1 As New SqlCommand cmd1.CommandText = "CREATE TABLE DotNetTable (" & _ "ContactID int IDENTITY PRIMARY KEY, " & _ "ContactName nvarchar(25) NOT NULL, " & _ "ContactEAddr nvarchar(60) NOT NULL)" cmd1.Connection = cnn1 'Invoke the command Try cnn1.Open() cmd1.ExecuteNonQuery() MessageBox.Show("Command succeeded.", "Outcome", _ MessageBoxButtons.OK, MessageBoxIcon.Information) Catch ex As Exception MessageBox.Show(ex.Message) Finally cnn1.Close() End Try Figure 11-8 shows Form4 after two successive clicks to Button1. If there is not already a table named DotNetTable in the database pointed at by cnn1 before the first click, then the attempt to execute the SQL in the CommandText property for cmd1 succeeds. The procedure presents a message box like the top one on the right side of Figure 11-8. Notice that the icon and caption for the message box follow from the settings for the Show method in the Try clause of the preceding code
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segment. A second successive click to Button1 will fail because the DotNetTable will already exist from the prior click to the button. A native error message appears in the bottom-right pane to report this outcome. You can add any custom code you prefer to replace the message box generated in the Catch clause of the preceding code segment to display a more tailored message or even perform some remedial action.
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Figure 11-8. It is good programming practice to provide feedback to users after they perform an action. The Click event procedure for Button6 is nearly identical to that for Button1. Recall that the Button6_Click procedure drops the DotNetTable. The only item that needs to change from Button1_ Click to Button6_Click is the SQL string assignment to the cmd1 SqlCommand object. The following excerpt from the Button6_Click procedure shows the new SQL statement for the CommandText property along with the statements just before and after it. The rest of Button6_Click is identical to the rest of Button1_Click. Dim cmd1 As New SqlCommand cmd1.CommandText = "DROP TABLE DotNetTable" cmd1.Connection = cnn1
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