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IComparer Object reference not set to an instance of an object.

JRockFL
JRockFL asked
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Last Modified: 2012-08-14
I'm not familiar with IComparer, but I'm getting a null reference
What am I doing wrong?
Private _comparer As IComparer

                    If _comparer.Compare(_lastValue, e.Item.DataItem) <> 0 Then
                        'add a header if it was different from the previous item.

                        Dim item As New GroupHeader()

                        _groupTemplate.InstantiateIn(item)
                        item.DataItem = e.Item.DataItem
                        Me.Controls.Add(item)

                        item.DataBind()
                    End If
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Kevin CrossChief Technology Officer
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Commented:
To be clear, the error you are getting is caused by the fact that you have not instantiated an object here:

Private _comparer As IComparer

To fix this normally in a declaration, you would add the New keyword and provide any constructor arguments necessary like so:

Private _comparer As New IComparer 'Invalid for IComparer

However, if you put this in code, you will see this will not work as IComparer is an Interface and cannot be instantiated; therefore, when you go to use it as an object, it is indeed a null reference since you cannot instantiate an object of that type in the first place.

Hopefully that makes sense.

HTH
Kevin
Fernando SotoRetired
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Author

Commented:
Thank you for the replies

_lastValue is object
e.Item.DataItem is System.Data.DataRowView

Should this class implement the IComparer interface in order to compare those two objects?
Kevin CrossChief Technology Officer
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Commented:
You can write a custom Comparer to compare objects that have special rules.  If you are just comparing the string values, then you can do this as string already implements Comparer.

e.Item(0).ToString().CompareTo(_lastValue)
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