Catching Exceptions

In <script runat="server" /> block or codebehind:

Private Sub Page_Load(Source As Object, E As EventArgs)

Dim _Connection As New

➥SqlConnection("Server=localhost;Database=pubs;uid=sa;pwd=;")

Dim _Command As New SqlCommand("Select * From Authors", _Connection)

Try

'_Connection.Open commented to cause an exception

'_Connection.Open()

DG1.DataSource = _Command.ExecuteReader(CommandBehavior.CloseConnection)

DG1.DataBind()

Catch _Error As Exception

MessageLabel.Text = _Error.Message

Finally

_Connection.Close()

End Try

End Sub

You can also provide multiple Catch statements, as shown here:

Private Sub Page_Load(Source As Object, E As EventArgs)

'Error has been produced in the connection

'string by writing "Server=localhos" (no 't')

Dim _Connection As New

➥SqlConnection("Server=localhos;Database=pubs;uid=sa;pwd=;")

Dim _Command As New SqlCommand("Select * From Authors", _Connection)

Try

_Connection.Open()

DG1.DataSource = _Command.ExecuteReader(CommandBehavior.CloseConnection)

DG1.DataBind()

Catch _SqlError As SqlException

MessageLabel.Text = _SqlError.Message

Catch _Error As Exception

MessageLabel.Text= _Error.Message

Finally

_Connection.Close()

End Try

End Sub

Comments

The Try/Catch/Finally block is the best approach towards error handling. It provides a

structured error-handling technique that was not available in previous versions of Visual

Basic.The Try block is provided with the code that is to be run.The Catch block is for

catching the exceptions raised by any line of code in the Try block.The Finally block

is executed under both conditions-either when the Try section successfully executes or

when an error occurs and is passed to the Catch block. It is for this reason that the

Close() method of the connection object has been coded in the Finally block, so as to

ensure that under both cases (whether successful or unsuccessful), the connection to the

data store is closed.

Multiple Catch statements can also be provided to capture different types of errors

raised by objects. In the second code listing, the exceptions that would be related to

SqlClient objects are going to be captured by the Catch statement having an object of

the SqlException class. All other exceptions are caught by the second catch statement

provided in the Try block.

The order of the Catch statements is very important. Make sure you put the more

generalized Catch exceptions after the more specific exceptions. For example, in the second

code listing, SqlException is a subclass of Exception, and so is more specific.Thus,

its Catch block occurs above the more generic exception's Catch block. If the order

were reversed, the SqlException Catch block would never be reached, because the

exception block would always catch a SqlException that was thrown.

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