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Data ErrorCode Errors HelpLink InnerException LineNumber Message Number Procedure
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CHAPTER 16 s HANDLING EXCEPTIONS
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Table 16-1. Continued
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Specifies the name of the computer running the instance of SQL Server that generated the exception. Specifies the name of the provider that generated the exception. Defines a string representation of the call stack when the exception was thrown. Specifies a numeric error code from SQL Server that represents an exception, warning, or no data found message. For more information, see SQL Server Books Online. Represents the method that throws the current exception.
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When an error occurs within SQL Server, it uses a T-SQL RAISERROR statement to raise an error and send it back to the calling program. A typical error message looks like this: Server: Msg 2812, Level 16, State 62, Line 1 Could not find stored procedure 'sp_DoesNotExist' In this message, 2812 represents the error number, 16 represents the severity level, and 62 represents the state of the error. You can also use the RAISERROR statement to display specific messages within a stored procedure. The RAISERROR statement in its simplest form takes three parameters. The first parameter is the message itself that needs to be shown. The second parameter is the severity level of the error. Any user can use severity levels 11 through 16. They represent messages that can be categorized as information, software, or hardware problems. The third parameter is an arbitrary integer from 1 through 127 that represents information about the state or source of the error. Let s see how a SQL error, raised by a stored procedure, is handled in VB .NET. You ll create a stored procedure and use the following T-SQL to raise an error when the number of orders in the Orders table exceeds 10: if @orderscount > 10 raiserror ( 'Orders Count is greater than 10 - Notify the Business Manager', 16, 1 ) Note that in this RAISERROR statement, you specify a message string, a severity level of 16, and an arbitrary state number of 1. When a RAISERROR statement that you write contains a message string, the error number is given automatically as 50000. When SQL Server raises errors using RAISERROR, it uses a predefined dictionary of messages to give out the corresponding error numbers.
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CHAPTER 16 s HANDLING EXCEPTIONS
Try It Out: Handling a Database Exception (Part 1): RAISERROR
Here, you ll see how to raise a database error and handle the exception. 1. Add a button to the Database tab page and change its Text property to Database Exception-1. Add a label to the right of this button, and for its Text property type Calls a stored procedure that uses RAISERROR. 2. Add a second button to the tab page, and change its Text property to Database Exception-2. Add a label to the right of this button, and for its Text property type Calls a stored procedure that encounters an error. 3. Add a third button to the tab page, and change its Text property to Database Exception-3. Add a label to the right of this button, and for its Text property type Creates multiple SqlError objects. The layout should look like Figure 16-7.
Figure 16-7. Database tab page 4. Using SSMSE, create a stored procedure in Northwind named sp_DbException_1, as follows: create procedure sp_DbException_1 as set nocount on declare @ordercount int Select @ordercount = count(*) From Orders if @ordercount > 10 raiserror ( 'Orders Count is greater than 10 - Notify the Business Manager', 16, 1 )
CHAPTER 16 s HANDLING EXCEPTIONS
5. Add the code in Listing 16-4 to the button3_Click method. Listing 16-4. button3_Click() Dim conn As SqlConnection = New SqlConnection _ ("Data Source=.\sqlexpress;" & _ "Integrated Security=True;" & _ "database=northwind") 'create command Dim cmd As SqlCommand = conn.CreateCommand 'Specify that a stored procedure is to be executed cmd.CommandType = CommandType.StoredProcedure cmd.CommandText = "sp_DbException_1" Try 'Open connection conn.Open() 'Execute stored procedure cmd.ExecuteNonQuery() Catch ex As System.Data.SqlClient.SqlException Dim str As String str = "Source: " + ex.Source.ToString str += ControlChars.NewLine + "Number: " + ex.Number.ToString str += ControlChars.NewLine + "Message: " + ex.Message str += ControlChars.NewLine + "Class: " + ex.Class.ToString str += ControlChars.NewLine + "Procedure: " + ex.Procedure str += ControlChars.NewLine + "Line Number: " + ex.LineNumber. ToString str += ControlChars.NewLine + "Server: " + ex.Server MessageBox.Show(str, "Database Exception") Catch ex As System.Exception Dim str As String str = "Source: " + ex.Source.ToString str += ControlChars.NewLine + "Exception Message: " + ex.Message MessageBox.Show(str, "General Exception") Finally If conn.State = ConnectionState.Open Then MessageBox.Show("Finally block Closing the connection", "Finally") conn.Close() End If End Try
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