LINQ Union objects with same Base Class

I'm trying to create a list of 开发者_如何学运维all my objects from several lists of objects using Union.

Return Chart.AnnotativeNodes.Union( _
         Chart.DecisionNodes.Union( _
           Chart.EndNodes.Union( _
             Chart.StartNodes.Union(Chart.WorkCenterNodes))))

The above line gets an error because I can't union List(of AnnotativeNode) with List(of DecisionNode). Each list defined like List(of EndNode) or List(of StartNode), but each class inherits from the base type Node.

Is there a possible way to union these to get a result of IEnumerable(of Node)?

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If you're using .NET 4, you should be able to do it like this:

Return Chart.AnnotativeNodes.Union(Of Node) _
           (Chart.DecisionNodes.Union(Of Node) _
               (Chart.EndNodes.Union(Of Node) _
                   (Chart.StartNodes.Union(Of Node)(Chart.WorkCenterNodes))))

That should work due to generic covariance in .NET 4. Otherwise, you could just call Cast(Of Node) on each of the collections.

I suspect your code can be written more readably though, as:

Return Chart.AnnotativeNodes.Union(Of Node)(Chart.DecisionNodes) _
                            .Union(Of Node)(Chart.EndNodes) _
                            .Union(Of Node)(Chart.StartNodes) _
                            .Union(Of Node)(Chart.WorkCenterNodes)

If you need the union-ing to happen in a particular order, you can mess with it - I haven't bothered, as Union is meant to be a set operation in the first place.