Handled Exception

[sgrya1]

Hi,

In a parent subroutine with a Try/Catch I want to check if a file is open.
If the file is open then it seems to create an exception in the ?parent? routine which means that it doesn't continue with the code.
Is there any way to handle the exception in the subroutine so that it doesn't interfere with the parent?

And is the a term for a parent subroutine?

    Function IsFileOpen(ByRef FileName As String) As Boolean
        Dim blnRetVal As Boolean = False
        Dim fs As FileStream = Nothing
        Try
            fs = File.Open(FileName, FileMode.Open, FileAccess.Read, FileShare.None)
        Catch ex As Exception
                blnRetVal = True
            Finally
                If Not IsNothing(fs) Then
                    fs.Close()
                End If
            End Try
        Return blnRetVal
    End Function

[.paul.]

I found this...

Private Sub IsFileOpen(ByVal file As FileInfo)
    Dim stream As FileStream = Nothing
    Try
        stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None)
        stream.Close()
    Catch ex As Exception

        If TypeOf ex Is IOException AndAlso IsFileLocked(ex) Then
            ' do something here, either close the file if you have a handle, show a msgbox, retry  or as a last resort terminate the process - which could cause corruption and lose data
        End If
    End Try
End Sub

Private Shared Function IsFileLocked(exception As Exception) As Boolean
    Dim errorCode As Integer = Marshal.GetHRForException(exception) And ((1 << 16) - 1)
    Return errorCode = 32 OrElse errorCode = 33
End Function

[.paul.]

How your code reacts to an open file depends on the code you run after checking if the file is open. We'd need to see that code...

[sgrya1]

There's nothing in the parent routine that refers to the file.
The Exception occurs right at the point of checking if the file is open then instantly returns to the catch of the parent.

I have a problem with

[jmcilhinney]

Firstly, there's no justification for declaring that FileName parameter ByRef. For reference types, you only ever use ByRef if you want to assign a new object to the parameter inside the method and have that change affect the original reference. You're not assigning anything to the parameter in the method so there can't possibly be a reason to use ByRef. ByVal is the default for a reason.

As for your code, it has a number of shortcomings. Firstly, catching any and every exception and declaring that the file is open based on that is wrong. What if a FileNotFoundException is thrown? How does the fact that a file doesn't exist indicate that it is open? Even .paul.'s code is wrong in that it catches an Exception and then tests whether it is type IOException instead of just catching IOException in the first place. Even ignoring that, your code could be improved in a number of ways. This:

If Not IsNothing(fs) Then

should be this:

If fs IsNot Nothing Then

but there should be no reason to have to make that check at all. If you open the file with a Using statement then the FileStream will be implicitly closed if it is successfully created:

Private Function IsFileOpen(fileName As String) As Boolean
    Dim result = False
    
    Try
        Using fs = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None)
        End Using
    Catch ex As Exception
        result = True
    End Using
 
    Return result
End Function

[sgrya1]

There's nothing in the parent routine that refers to the file.
The Exception occurs right at the point of checking if the file is open then instantly returns to the catch of the parent.

I have a problem with

[jmcilhinney]

There's nothing in the parent routine that refers to the file.
The Exception occurs right at the point of checking if the file is open then instantly returns to the catch of the parent.

I have a problem with

That seems to suggest that you have defined your own class named Exception. That would explain why you're not catching the exception in that method. It is an IOException that is being thrown, which does inherit System.Exception, but you are actually catching something other than System.Exception and so you don't catch the exception that is thrown. In your code, change Exception to System.Exception and I expect that it will work as you expect. In that case, find that class named Exception and get rid of it or, if it actually does serve a useful purpose, rename it to something sensible.

[sgrya1]

jmcilhinney,

You've helped me many times before. Thank you!!
I can't check this right now but I'm sure it's the answer.

[sgrya1]

jmcilhinney,

You've helped me many times before. Thank you!!
I can't check this right now but I'm sure it's the answer.

[sgrya1]

Arr. Same thing.



It says system.IO.IOException so I don't think I've defined my own class for an exception.

[jmcilhinney]

Arr. Same thing.



It says system.IO.IOException so I don't think I've defined my own class for an exception.

That's a complete non-sequitur. What that is telling you is that a System.IO.IOException was thrown. That class inherits the System.Exception class so, if your code was actually catching System.Exception then it would catch an IOException. The fact that it's not means that you are not catching System.Exception. Nothing you have shown there is any indication that you haven't defined your own Exception class and, in fact, is evidence that you have. As far as I can tell, it's either that or something on your system is broken.

The way to find out what that class actually is to right-click on it and select Go To Declaration or the like. If you have declared that type in your own code then doing that will show you where. That said, as I already mentioned, you shouldn't be catching System.Exception in the first place. The point of your code is to determine whether a file is already open and it will only be an IOException that gets thrown in that situation so it should be only IOException that you are catching. As I stated quite clearly, catching System.Exception would result in your telling the user that a file that didn't exist was already open, which doesn't seem like any good application should be doing.

[techgnome]

It's possible that the OP has Imported a namespace that has its own Exception class that maybe interfering... so it may not necessarily be that he's defined a custom class Exception. I've been bit by that before.


-tg

[Arve K.]

You will also get an warning with that code, telling you that a return statement is missing.

Private Function IsFileOpen(fileName As String) As Boolean
    Try
        Using fs = IO.File.Open(fileName, IO.FileMode.Open, IO.FileAccess.Read, IO.FileShare.None)
        End Using
 
    Catch ex As Exception
        Return True
 
    End Try
 
    Return False <-----
End Function

[sgrya1]

Apologies all.
As jmcilhinney and techgnome has suggested I my project has a reference .dll file that overrules the exception.
Thank you all for pointing that out.