Microsoft Visual Basic 2010 Step by Step eBook

Chapter 9 Trapping Errors by Using Structured Error Handling 245

To Do this

Test for specific error

conditions in an event

handler

Use the Catch statement and the appropriate Exception object. For example:

Try

PictureBox1.Image = _

System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")

Catch ex As System.IO.FileNotFoundException 'if File Not Found

MsgBox("Check pathname and disc drive")

Catch ex As OutOfMemoryException 'if Out Of Memory

MsgBox("Is this really a bitmap?", , ex.Message)

Catch ex As Exception

MsgBox("Problem loading file", , ex.Message)

End Try

Create your own

errors in a program

Use the Throw statement. For example, the following code generates

an exception and handles it:

Try

Throw New Exception("There was a problem")

Catch ex As Exception

MsgBox(ex.Message)

End Try

Write nested Try...

Catch error handlers

Place one Try... Catch code block within another. For example:

Try

PictureBox1.Image = _

System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")

Catch

MsgBox("Insert the disc in drive D!, then click OK!")

Try

PictureBox1.Image = _

System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")

Catch

MsgBox("File Load feature disabled")

Button1.Enabled = False

End Try

End Try

Exit the current Try or

Catch code block

Use the Exit Try statement in the Try or the Catch code block. For example:

If PictureBox1.Enabled = False Then Exit Try