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After adding a table, such as DotNetTable, to a database, you normally want to add rows to it. GroupBox1 in Form4 contains two TextBox controls and a Button control to help accomplish this task. Users can specify the ContactName and ContactEAddr column values for a new row in the TextBox1 and TextBox2 controls within the GroupBox1 control. Clicking Button2 adds the designated column values to DotNetTable. SQL Server Express automatically populates the ContactID column value because of its IDENTITY property setting. Figure 11-9 shows Form4 just before the addition of a new row to the DotNetTable with Rick Dobson and rickd@cabinc.net as the ContactName and ContactEAddr column values. If this is the first attempt to add a row to the table, the ContactID column value will be 1. Users can successively add new rows to the DotNetTable after clicking Insert. Clicking the Insert button clears the TextBox controls to ready them for the insertion of a new row.
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Figure 11-9. It s easy to collect values for a database with TextBox controls, but remember to verify your input values before submitting them to your database server. When you accept input from TextBox controls, it s good programming practice to include some code to validate the input. Failing to verify incoming data values can expose your application to hacker attacks. In addition, verifying incoming data can sometimes allow you to detect faulty input. The Button2_Click procedure has code to verify both ContactName and ContactEAddr column values. The following excerpt from the Button2_Click procedure shows the code for verifying ContactName and ContactEAddr column values. Several Dim statements precede an If...ElseIf... Else...End If statement. The Dim statements define a SqlCommand object, as well as several quantities used to verify email addresses. The expressions for verifying the ContactName and ContactEAddr column values are conditions for the If and ElseIf clauses of the If...ElseIf...Else...End If statement. The If condition has four criteria for verifying ContactName column values. A ContactName cannot contain a semicolon (;). This is a common character used in hacker attacks for input to databases. In addition, most names do not have a semicolon within them. A ContactName with an equal sign (=) is rejected for the same reasons that semicolons are rejected. A third criterion for a ContactName value is that its number of characters is greater than zero. If the ContactName field length equals 0, then a user did not input a value into TextBox1. The DotNetTable definition requires an entry for ContactName. The last ContactName criterion rejects a ContactName column value with greater than twenty-five characters. The nvarchar(25) setting for ContactName in the table s CREATE TABLE statement limits ContactName column values to no more than twenty-five characters. The ElseIf condition has three criteria for verifying email addresses as ContactEAddr column values. All email addresses must have an @ sign. The int_at quantity will be 0 for ContactEAddr values that do not contain an @ sign.
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Every email address must have at least one period (.) after its @ sign, such as @cabinc.net. The int_period quantity is 0 if this condition is not satisfied. Finally, the length of an email address must be at least eight characters (c@ccc.cc). Email addresses with less than eight characters are therefore not valid.
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It is popular among advanced developers to use regular expressions for verifying email addresses (and other String values). However, regular expressions are still not widely used by most developers, and using them makes your code difficult to maintain. In addition, much more familiar Visual Basic String expressions can sometimes allow more precise validation criteria.
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The Else clause in the If...ElseIf...Else...End If statement assigns CommandText and Connection property values to the cmd1 SqlCommand object declared and instantiated at the top of the excerpt. The CommandText property value is a SQL string that uses the Text property values of TextBox1 and TextBox2 to construct an INSERT statement for the DotNetTable. The Else clause assigns the cnn1 SqlConnection object instantiated at the top of the Form4_Load procedure to the Connection property for the cmd1 SqlCommand object. Dim cmd1 As New SqlCommand Dim int_at As Integer = InStr(Me.TextBox2.Text, "@") Dim int_period As Integer = InStr(int_at, Me.TextBox2.Text, ".") Dim int_len As Integer = Len(Me.TextBox2.Text) If InStr(Me.TextBox1.Text, ";") > 0 Or _ InStr(Me.TextBox1.Text, "=") > 0 Or _ (Len(Me.TextBox1.Text) = 0) Or _ (Len(Me.TextBox1.Text) > 25) Then MessageBox.Show("Invalid Name. Please fix name.") Exit Sub ElseIf Not (int_at > 0 And int_period > 0 And int_len >= 8) Then MessageBox.Show("Invalid email address.", "Please fix it") Exit Sub Else cmd1.CommandText = "INSERT DotNetTable " & _ "(ContactName, ContactEAddr) VALUES " & _ "('" & Me.TextBox1.Text & "', '" & Me.TextBox2.Text & "')" cmd1.Connection = cnn1 End If After exiting the If...ElseIf...Else...End If statement with valid ContactName and ContactEAddr column values, the procedure is ready to execute the cmd1 SqlCommand that inserts a row into DotNetTable. The final code segment from Button2_Click, which appears next, accomplishes this task. Before invoking the ExecuteNonQuery method for the SqlCommand object, it is necessary to open the cnn1 SqlConnection that the SqlCommand object uses to interact with the SQL Server Express database. If the ExecuteNonQuery method completes successfully, the code clears TextBox1 and TextBox2. Clearing the TextBox controls gives visual confirmation that something happened. In this case, the something is the insertion of the new row. Reasons that the ExecuteNonQuery might fail could include a paused or stopped SQL Server Express instance, or a moved or even deleted database file.
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