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Showing posts from June, 2008

ToolTip and "Cannot access a disposed object" exception

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The "Cannot access a disposed object" exception is a nightmare for .NET developers. ;) Sometimes, this exception causes the worst backslash for a development project. And looking at exception details, we will find that the call stack will point to

"at System.Windows.Forms.Control.CreateHandle()
 at System.Windows.Forms.Control.get_Handle() "

Or

"at System.Windows.Forms.Form.CreateHandle()
 at System.Windows.Forms.Form.get_Handle() "

Since, we do not know from where the problem stems, it becomes a trial-and-error method to find out the source of the problem. After some days of research, I found out the following facts about the problem. Here it goes.

Reason :

   This exception occurs only when we try to access any disposed object, as it says. But the problem is, we would have never written statements that explicitly access a disposed object! So the real reason would be some unseen code that accesses the disposed Form or Object. In my case, the black sheep …

Finding whether its Design Mode or Runtime Mode in VB.NET

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Sometimes, while creating controls, either by Inherited Controls or UserControls, we will be required to skip a piece of code depending upon the design time or runtime mode. This can be done easily by the following code inside a control.









If Me.Site IsNot Nothing AndAlso Me.Site.DesignMode = True Then
    'Designtime code here
Else
    'Runtime code here
End If

Using this code in a control, we can find the mode and avoid certain errors when in Design Mode. This is mostly used in overridden event methods in Inherited Controls.

Update requires a valid UpdateCommand when passed DataRow collection with modified rows

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This is an error, with which i was dumbfounded for weeks. The problem is simple. I have a DataSet and it automatically generates INSERT command. So i can add rows to the DataSet and update it easily. But whenever i change values in existing rows and update it via TableAdapter, I get this exception "Update requires a valid UpdateCommand when passed DataRow collection with modified rows". I found inside the generated code that only INSERT command is generated. Whatever i try, the DataSet designer would not create an UPDATE command nor a DELETE command for this TableAdapter.

Solution 1:

You can select the TableAdapter and write your own query for the update command in the property dialog box.

But this is not a good idea.

Solution 2: (Better one)

The problem lies within your table design. Check it out again and check whether you have created a primary key for the table. Mostly this will be the case, as I forgot to create one. Now after creating the primary key, you can recreate …