DirectCast - Object not set to an instance of an object???

[flycast]

I am trying to deserialize a custom object that has been previously serialized and saved in a file. and I have the following code:

        If open.ShowDialog = Windows.Forms.DialogResult.OK Then
            Dim fs As FileStream = New FileStream(open.FileName, FileMode.Open)
            Try
                testing = New ItemInfo
                Dim formatter As New BinaryFormatter
                Dim obj As Object = formatter.Deserialize(fs)
                testing = DirectCast(obj, ItemInfo)
            Catch ex As Exception
                MsgBox(ex.Message)
            Finally
                fs.Close()
            End Try
        End If 

The problem is the line

testing = DirectCast(obj, ItemInfo) 

I am getting an exception:

Object not set to an instance of an object

I have verified that the variable testing and obj are actually instantiated. That leaves ItemInfo which is the name of my custom type (Not an instance of the class).

Since the variable testing and obj are instantiated it seems like it does not like the reference to the class ItemInfo.
What could I be doing wrong here?

[dday9]

Try bypassing the obj variable altogether.

testing = New ItemInfo 
Dim formatter As New BinaryFormatter 
testing = DirectCast(formatter.Deserialize(fs), ItemInfo)

See if that works.

Edit -

Also I wanted to let you know that I visited your website, visited the first website in your portfolio and it doesn't display correct in IE10, but it does in Mozilla. Just an FYI :]

[Shaggy Hiker]

The variable 'Testing' couldn't cause the problem, only obj could be causing this problem in the line you show. However, you state that you verified that obj was instantiated. How did you do this?

[Niya]

You wrapped that code in an exception handler so how do you know which line cause the exception ?

[flycast]

Shaggy Hiker:
I verified that obj was instantiated by using a breakpoint and following execution line by line.

Niya:
Same as above - following line by line using breakpoints

[flycast]

Additionally, If I serialize a string and then deserialize it it works fine.

Serialize:

        Dim test As String = "Hello"

        ' Persist to file
        Dim save As New SaveFileDialog
        If save.ShowDialog = Windows.Forms.DialogResult.OK Then
            Dim stream As FileStream = File.Create(save.FileName)
            Dim formatter As New BinaryFormatter()
            formatter.Serialize(stream, test)
            stream.Close()
        End If

Deserialize:

        Dim test As String = ""
        If open.ShowDialog = Windows.Forms.DialogResult.OK Then
            Dim fs As FileStream = New FileStream(open.FileName, FileMode.Open)
            Try
                Dim formatter As New BinaryFormatter
                Dim obj As Object = formatter.Deserialize(fs)
                test = CType(obj, String)
            Catch ex As Exception
                MsgBox(ex.Message)
            Finally
                fs.Close()
            End Try
        End If

[Shaggy Hiker]

That particular exception is only raised when some object is Nothing when it can't be Nothing. In the line in question, the object "testing" doesn't matter at all, since it's state doesn't matter in that line. The only other object in the line is Obj. If you stepped through the line and looked at Obj at the time of the exception and it was not Nothing, and the next step took you to the exception handler...that would be quite interesting, really. The test with the string doesn't really mean anything. All that would be testing is whether the code works as advertised, which it does.

The specific way you answered my question left just enough wiggle room that I'd like to nail that down before going on. After all, stepping through the code should work just fine...and could leave Obj at Nothing. The key point is this: When you stepped through the code, at the point execution reached this line:

testing = DirectCast(obj, ItemInfo)

did you look at what Obj held using Shift+F9, and did it hold a valid Object?

Second, if you then stepped one line further (or at least attempted to do so), did you immediately jump to the exception handler?

If you answer Yes to both of those, then the exception isn't in the line in question....and it IS quite interesting.

[flycast]

Hmmm...
Getting closer. This works:

                Dim obj As ItemInfo = formatter.Deserialize(fs)
                With testing
                    .CompileHistory = obj.CompileHistory
                    .Description = obj.Description
                    .FileLocation = obj.FileLocation
                    .ItemNumber = obj.ItemNumber
                End With

This did not:

                Dim obj As ItemInfo = formatter.Deserialize(fs)
                testing = obj

The exception is happening when I do the assignment. I have a custom event written in ItemInfo to get around the serialization error that happens when you have a custom class with events. Basically the serializer tried to serialize both the target and it's caller (in this case the form which cannot be serialized). The exception is being thrown in the AddHandler part the custom error

    <NonSerialized()> _
    Private Events As New System.ComponentModel.EventHandlerList

'Then later...

    Public Custom Event APropertyChanged As EventHandler
        AddHandler(ByVal value As EventHandler)
            Try
                Events.AddHandler("APropertyChanged", value)
            Catch ex As Exception
                MsgBox(ex.Message)
            End Try
        End AddHandler

        RemoveHandler(ByVal value As EventHandler)
            Try
                Events.RemoveHandler("APropertyChanged", value)
            Catch ex As Exception
                MsgBox(ex.Message)
            End Try
        End RemoveHandler

        RaiseEvent(ByVal sender As Object, ByVal e As System.EventArgs)
            CType(Events("APropertyChanged"), EventHandler).Invoke(sender, e)
        End RaiseEvent
    End Event

So value is nothing. I am not real strong on custom events and I am pretty confused about what is happening here.

[Shaggy Hiker]

So, the problem wasn't really in the line that showed the exception, but in the underlying class. It happens, but it's a bit rare.

In that first snippet, testing is only something because you created a New ItemInfo. You then set a few members in the With statement...then overwrite testing with obj. That's really quite odd. The variable testing holds the address of some NewInfo object, as does the variable obj. You set a few properties of the fist instance from the same properties in the second instance, then you throw out the first instance and replace it with the second instance and it works? That suggests that something happens to NewInfo as a side effect of accessing one of those four properties, because once you execute this line:

testing = obj

the instance that had been in testing is no longer accessible. Shared members of the class would be accessible, but not instance members. It sure doesn't look like you are setting any Shared members in the With statement, but maybe you are. The point is that in both the first and the second snippet, the result of the line:

testing = obj

is that whatever instance was being referenced by testing is now lost, and the variable testing now holds the address of the instance that was being referenced by obj. So, any changes to testing in the With statement don't matter because that instance is no longer accessible. Which means that accessing the properties must be changing obj in some way, since testing is no longer in the picture (unless you overloaded the = operator).

[flycast]

Shaggy:

You set a few properties of the fist instance from the same properties in the second instance, then you throw out the first instance and replace it with the second instance and it works?

No. IT was either/or. When I set a few property values of "testing" with the values from "obj" it worked. When I just assign testing = obj then it does not. When I do that then it adds events and that is when there is a nothing gumming up the works.

I didn't overload the = operator.

The root is when I do testing = obj then the class goes into the customer event and removes events handlers and then adds event handlers.

[Shaggy Hiker]

I'm a bit confused. Your first snippet in post #8, which you said worked, ended with the testing = obj line. In the second snippet you said that that line, alone, didn't work. Was that assignment not supposed to have been the final line in the first snippet? That would make more sense, certainly, but it's not what is shown.

[flycast]

Shaggy:
That line is missing it's comment - it should be commented out...my bad. It has been edited.

[Shaggy Hiker]

Ah, that changes things completely. It's a pretty interesting case. In all the classes that I have serialized, I don't recall ever serializing one that has an event. I can see why that would be a problem, since it would normally want to serialize all the data members, and the data members for an event would be the list of method pointers that get called when the event is raised, which would be bad, since they wouldn't retain any meaning.

I think that the exception is showing in the wrong place, which seems to be what you have found, too. What happens if you check whether Value Is Nothing in both the add and remove handlers, and take no action if it is?

[.paul.]

Classes with eventhandlers don't serialize correctly unless you remove the handlers. I suppose that applies when deserializing too...