Throwing an Exception using ASP.NET 2.0 and VB .NET

This tutorial will show you how to throw exceptions using ASP.NET 2.0 and VB.NET

Throwing exceptions are easy to do in C# .NET.

In this example we will be throwing exceptions when trying to create a directory in the filesystem, so we will need the System.IO namespace. Our code will catch two exceptions if the user tries to create a directory that is 248 characters long or tries to specify an invalid character within the directory name (such as the asterisk character).

Imports System.IO

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We'll put our code in the btnSubmit_Click() event.

When the btnSubmit_Click() event fires it runs a try block. The try block does two things: it lets exceptions thrown during the try block's execution to be caught by the catch block(s) below and ensures that execution can't leave the try block without running a finally block. In this example, we are specifying that our catch blocks handles exceptions of the type PathTooLongException and ArgumentException. We are also adding an if block that will throw an ArgumentException if there is a "\" character in the directory name.

Try Directory.CreateDirectory(MapPath(".") & "\" & txtDir.Text) ' adding this line throws an ArgumentException even if the \ character is normally valid If txtDir.Text.Contains("\") Then Throw New ArgumentException("Directory name cannot contain the ""\"" character") End If Catch ex As PathTooLongException lblStatus.Text = "There was an PathTooLongException when creating the directory" lblStatus.Text &= " in " & MapPath(".") & "\" & Constants.vbCr Catch ex As ArgumentException lblStatus.Text = "There was a ArgumentException when creating the directory" lblStatus.Text &= " in " & MapPath(".") & "\" & Constants.vbCr lblStatus.Text += ex.Message + Constants.vbCr Catch ex As Exception lblStatus.Text = ex.Message End Try

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We have one textbox, a Submit button, and a label on the front end for user interaction. The front end .aspx page looks something like this:

<table width="600" border="0" align="center" cellpadding="5" cellspacing="1" bgcolor="#cccccc"> <tr> <td width="100" align="right" bgcolor="#eeeeee" class="header1"> Catching an Exception:</td> <td align="center" bgcolor="#FFFFFF"> Directory:<asp:TextBox ID="txtDir" runat="server"></asp:TextBox> <br /> <asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" /><br /> &nbsp;<asp:label ID="lblStatus" runat="server"></asp:label></td> </tr> </table>

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The flow for the code behind page is as follows.

Imports System.IO Partial Class _Default Inherits System.Web.UI.Page Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click Try Directory.CreateDirectory(MapPath(".") & "\" & txtDir.Text) ' adding this line throws an ArgumentException even if the \ character is normally valid If txtDir.Text.Contains("\") Then Throw New ArgumentException("Directory name cannot contain the ""\"" character") End If Catch ex As PathTooLongException lblStatus.Text = "There was an PathTooLongException when creating the directory" lblStatus.Text &= " in " & MapPath(".") & "\" & Constants.vbCr Catch ex As ArgumentException lblStatus.Text = "There was a ArgumentException when creating the directory" lblStatus.Text &= " in " & MapPath(".") & "\" & Constants.vbCr lblStatus.Text += ex.Message + Constants.vbCr Catch ex As Exception lblStatus.Text = ex.Message End Try End Sub Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load End Sub End Class

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