Dim u As UInteger = 0
Try
Do
u += 1
j = DirectCast(o.item(d), HTMLTableRow).cells
Loop
Catch ex As Exception
MsgBox("Access No." & u & " throws：" & ex.GetType.ToString & "：" & ex.Message)
End Try
This is the piece of code I used as a test - a dead loop, infinitely accessing the variable O (assigned in code before) and assigning it to the variable J with some operation (O and J are both MSHTML.IHTMLElementCollection type). Under the debug mode, I can run it normally until the counter u reaches its upper limit. However, under the release mode, after loop for 5000~6000 times (in each test the number is different) it will throw "NullReferenceException". Note that I've just accessed O, never changed it, why is the exception? Is this a bug of MSHTML the assembly? Moreover, if I make a minor change:
Dim u As UInteger = 0, v As Object
Try
Do
u += 1
v = DirectCast(o.item(d), HTMLTableRow)
Loop
Catch ex As Exception
MsgBox("Access No." & u & "throws：" & ex.GetType.ToString & "：" & ex.Message)
End Try
That is, to remove the ".cells", and then there will be no exceptions. What's going on here? (This cannot be used as a workaround because in my program the ".cells" must be accessed)
If I use TryCatch block to ignore the exception and just try again, it won't run normally any more - throwing the exception for each loop. There must be some qualitative changes.
OK, MSHTML, you win. I have to use the dumbest workaround. Just try...catch the exception and try again. After numerous tests, I got the following possible exceptions that must be handled:
COMException, when you can simply try the statement throwing this again.
UnauthorizedAccessException, when you can simply try again like the last one.
NullReferenceException, when you CANNOT simply try again as you'll again catch the same exception. You'll have to initialize a new HTMLDocument and reload the URL, then do the remaining work.
Wanting anyone with a more elegant solution. I swear I'll never use the hideous Assembly if possible.
Related
Before we even start: In researching this problem I've looked at dozens of posts here and elsewhere, and realize that VB is the worst for Bluetooth programming. However, this is for a client who has a massive legacy VB system and I have no choice.
According to MS documentation, the BluetoothLEDevice.FromIdAsync is supposed to return a BluetoothLEDevice object but on my system it returns a generic System.__ComObject that I can't cast to a BluetoothLEDevice. I have tried Cast and DirectCast but neither work. The sample C++ code I've looked at doesn't require any type of casting; the variable is declared and set using the BluetoothLEDevice.FromIdAsync function without any dramas.
Here is my test code.
Private Sub ConnectToDevice(di As DeviceInformation)
'
' di.id = "BluetoothLE#BluetoothLE48:5f:99:3c:fd:36-84:2e:14:26:9e:7b"
'
Debug.Print("Connecting To " & di.Name & " at " & Now.ToShortTimeString)
Dim genericObject As Object
Dim myDevice As BluetoothLEDevice
Try
genericObject = BluetoothLEDevice.FromIdAsync(di.Id)
Catch ex As Exception
Debug.Print(ex.Message)
End Try
If Not IsNothing(genericObject) Then Debug.Print("Using 'Object' yeilds " & genericObject.ToString)
Try
myDevice = BluetoothLEDevice.FromIdAsync(di.Id)
Catch ex As Exception
Debug.Print(ex.Message)
End Try
End Sub
And here is the output:
Connecting To TM-2021090161 at 2:08 PM
Using 'Object' yeilds System.__ComObject
Exception thrown: 'System.InvalidCastException' in testBTLE.exe
Unable to cast object of type 'System.__ComObject' to type 'Windows.Devices.Bluetooth.BluetoothLEDevice'.
And a screen shot of the returned genericObject:
The FromIdAsync needs to be run async which I couldn't do because I got an error stating it didn't have an awaiter. It turns out that I needed to NuGet the System.Runtime.Windowsruntime dll. I added the Await and it's working now.
Thanks to Andrew for pointing me in the right direction.
When converting onError/GoTo statements from VB6 into VB.Net, I've been told to use try/catch statements instead. Most of the VB6 error blocks utilize Microsoft.VisualBasic.ErrObject to provide the error code and description. For example:
CombinePDF_ERROR:
lErrorCode = Err
strErrorSource = Err.Source
strErrorDescription = Err.Description
bInProcess = False
strCombinePDFLastFile1 = strFile1
strCombinePDFLastFile2 = strFile2
ChDrive left$(strCurrentDir, 1)
ChDir strCurrentDir
Call CombinePDFUIUnload
Err.Raise lErrorCode, strErrorSource, strErrorDescription
End Sub
Does the Err (Microsoft.VisualBasic.ErrObject) get its information from the onError/GoTo statements? lErrorCode, strErrorSource, strErrorDescription aren't given values prior to this. How do I replicate this functionality in a try/catch? Catch an exception and messageBox the message? First time using VB6 or VB.Net. Thank you for your time.
That specific code in your question is basically like this Catch block below. The Err.Raise is equivalent to a Throw, and the Err object is roughly equivalent to an Exception object.
Catch ex
bInProcess = False
strCombinePDFLastFile1 = strFile1
strCombinePDFLastFile2 = strFile2
ChDrive left$(strCurrentDir, 1)
ChDir strCurrentDir
Call CombinePDFUIUnload
Throw ex
But that's just that one block. You need to check each VB6 error handler, work out what its doing, and work out the closest equivalent with Try Catch. You need to understand the VB6 On Error and the Err object, and also .Net Try...Catch and Exception object.
You are going to have a very hard time on this project if you don't know VB6 or VB.Net.
How do I handle this type of error or exception?
Try
If log.Trim = txtUSN.Text Then
MessageBox.Show("USN found: " & log)
Else
MessageBox.Show("USN not found: " & log)
End If
Catch ex as Exception
MessageBox.Show(ex.Message)
The message was "Object reference not set to an instance of an object."
This is the rest of the code:
Dim log As String
Dim sql As New SqlCommand
sql.Connection = MyConnection
sql.CommandText = "SELECT * FROM dbo.tblAcc WHERE USN = '" & txtUSN.Text & "' "
MyConnection.Open()
log = sql.ExecuteScalar
MyConnection.Close()
The simple answer is your trying to use an object that is nothing. If it's nothing you can't use it, hence "Object reference not set to an instance of an object."
As already mentioned in my comments above, the culprit is: log. I'm not sure where you have declared this or when your using it and how your using it, for all I know it's nothing. If you have more code that would be greatly appreciated as I can point out where it's nothing, until then here's how to get around your issue.
Try
If log IsNot Nothing Then
If log.Trim = txtUSN.Text Then
MessageBox.Show("USN found: " & log)
Else
MessageBox.Show("USN not found: " & log)
End If
Else
MessageBox.Show("Log is NOTHING!")
End If
Catch ex as Exception
MessageBox.Show(ex.Message)
End Try
**EDIT**
After you posted more code in a comment (please post code in the area, not in comment's) it seem's there are a few issue's. You have log defined as a string; when you do this set it to something like: String.Empty instead of nothing. Also you want to sse the ExecuteScalar method to retrieve a single value (for example, an aggregate value) from a database which could be an integer, long, single etc data types. In your query your selecting everything, you can't call ExecuteScalar to return that data... I would recommend looking up information about building queries and executing them, it's to long for me to get in depth with it here.
Happy Coding!
Make sure that (log) is not an empty string.
if not String.IsNullorEmpty(log) then
end if
I have a simple block of legacy code that sits inside of a loop that crawls through some potentially bad xml node by node and which needs to be refactored as it is not working as intended:
Try
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
writer.WriteRaw(strXMLfragment)
Catch ex As Exception
InvalidXML = True
End Try
What this block is meant to do is check for valid xml and then write the xml out. What it actually does is check for invalid xml and then write the xml out only if it is valid. So it needs to be fixed to work as intended.
My first attempt at a fix:
Try
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
'writer.WriteRaw(strXMLfragment)
Catch ex As Exception
InvalidXML = True
Finally
writer.WriteRaw(strXMLfragment)
End Try
This works on my test data but I am concerned that WriteRaw may throw an exception on other data. I haven't found a conclusive statement about what will cause WriteRaw to throw an exception and what will happen when code in a Finally block throws an exception.
So I tried rewriting it like this:
Try
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
Catch ex As Exception
InvalidXML = True
End Try
Try
writer.WriteRaw(strXMLfragment)
Catch
End Try
Frankly it looks ugly as hell. Is there a more elegant way to refactor this or will the first attempt be suitable?
When an excpetion is raised in a Finally block, nothing special happens: the exception is propagated out and up like any other exception, and code after the exception in this finally block will not be executed.
Your first attempt will fail if strXMLfragment is null or an empty string (or due to a already running asynchronous operation).
So if you really want to handle/swallow all exceptions, you'll have to use two Try blocks.
To make it cleaner, you might want to pull your first Try/Catch into it's own private function and make it reusable:
Private Function TryParseXml(ByVal xml as String) as Boolean
Try
XDocument.Parse(xml)
Return True
Catch ex As Exception
Return False
End Try
End Function
Then wrap your writer.WriteRaw call in it's own Try/Catch.
Dim myXml = "<temproot>" & strXMLfragment & "</temproot>"
If TryParseXml(myXml) Then
xmlFrag.LoadXml(myXml)
Else
Try
writer.WriteRaw(strXMLfragment)
Catch ex as Exception
' handle exception
End Try
End If
Yes, ultimately this is using two Try/Catch blocks. There is no real way around this as the only real way to determine if Xml is valid is to attempt to parse it and wait for it to blow up.
In the end I came up with this profoundly simple and elegant refactoring:
Try
writer.WriteRaw(strXMLfragment)
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
Catch ex As Exception
InvalidXML = True
End Try
With the WriteRaw line executing first it will always write out the XML unless it throws an exception. Then the LoadXml line can test for validity without interfering with writing the xml out. This way the InvalidXML flag is set as designed and there won't be any unexpected exceptions.
I made an application that our company uses to launch databases and updates them on the users machine, when needed.
I am having a slight problem when it comes to launching databases and the database starts up slow. When this occurs my application throws an exception, as I assuming its awaiting some kind of response back.
As of now the error thrown is: The system cannot find the file specified
I am trying to prevent this exception logging for cases like this(Slow Application), but still allow the logging if a real error occurs while opening a database.
Current Code I am using:
Private Sub OpenApplication()
If File.Exists(LocalPathString) Then ' File Found. Open the File.
Try
Dim ps As New Process
ps = Process.Start(LocalPathString)
Catch ex As Exception
ex.Source += " | " & LocalPathString
RaiseEvent ShowError(ex)
Finally
RaiseEvent CancelIt() ' Thread Complete. Close the ActionForm
End Try
Else
If LocalPathString = vbNullString Then
RaiseEvent CancelIt() ' No file exits. Cancel thread.
Else
RaiseEvent ShowError(New Exception("Database Not Located: " & LocalPathString))
End If
End If
End Sub
StackTrace:
System.Diagnostics.Process.StartWithShellExecuteEx(startInfo As ProcessStartInfo)
App.exe: N 00912
System.Diagnostics.Process.Start()
App.exe: N 00136
System.Diagnostics.Process.Start(startInfo As ProcessStartInfo)
App.exe: N 00049
SAMi.ActionClass.OpenApplication()
App.exe: N 00117
Maybe I'm missing something, but why don't you simply omit the logging if you found that specific exception?
Catch ex As Exception
ex.Source += " | " & LocalPathString
if not ex.Message.Contains("The system cannot find the file specified") Then
RaiseEvent ShowError(ex)
end if