VB.Net Sort a List using property name

Question

I have a list containing Client objects. I want to sort the list by using it's property name ascending or descending, which I already have in the code in viewstate : ViewState("PropertyName") and ViewState("Order")

Dim objList As List(Of Client) = Session("ClientList")  
objList.Sort(ViewState("PropertyName") + " " + ViewState("Order"))    
datarepeater.datasource = objList

How can I achieve this ?

Do you really need to store the property-name in the ViewState? Then i would use a DataTable instead where you can use the column-name and LINQ to order it. — Tim Schmelter
– Tim Schmelter, Commented Mar 7, 2014 at 14:31
If I go that route then I have to convert my list into a datatable which I want to avoid. — user3129969
– user3129969, Commented Mar 7, 2014 at 14:45

Steven Doggart · Accepted Answer · 2014-03-07 15:42:52Z

4

Normally, if you knew the property with which you wanted to sort, you would be able to just do something like this:

clients.Sort(Function(x, y) x.Name.CompareTo(y.Name))

In the above example, I am, of course sorting on the Name property (I don't know what properties the Client class has, I'm just using it as an example.

However, since you don't know which property you are going to use until run-time, you'll need to do something more complicated. If you really want to use the actual property names of the class, you could use Reflection to retrieve the value of the property dynamically, for instance:

clients.Sort(Function(x, y)
                 Dim xProperty As PropertyInfo = x.GetType().GetProperty(ViewState("PropertyName").ToString)
                 Dim yProperty As PropertyInfo = y.GetType().GetProperty(ViewState("PropertyName").ToString)
                 Dim xValue As Object = xProperty.GetValue(x)
                 Dim yValue As Object = yProperty.GetValue(y)
                 Return xValue.ToString().CompareTo(yValue.ToString())
             End Function)

To reverse the sort order, just multiply the return value by -1, or switch which object you are comparing to what. For instance:

If ViewState("Order") = "Ascending" Then
    Return xValue.ToString().CompareTo(yValue.ToString())
Else
    Return yValue.ToString().CompareTo(xValue.ToString())
End If

edited Mar 7, 2014 at 15:42

answered Mar 7, 2014 at 15:03

Steven Doggart

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5 Comments

user3129969 Over a year ago

Thanks Steven however I get an Option stric disallows converting from object to String in the ViewState("PropertyName").SO I added a ToString. After that I get error msg "Overload resolution failed because no accessible "GetValue" accepts this number of arguments. I didn't add any arguments. Here is what I have: objList.Sort(Function(x, y) Dim xValue As String = x.GetType().GetProperty(ViewState("PropertyName").ToString).GetValue(x).ToString() Dim yValue As String = y.GetType().GetProperty(ViewState("PropertyName").ToString).GetValue(y).ToString() Return xValue.CompareTo(yValue) End Function)

user3129969 Over a year ago

On this site, is it possible to post instead of "adding a comment" it would make my message easier to read.

Steven Doggart Over a year ago

No, unfortunately, the only way to post properly formatted code is to edit your original question to include the new code. I think the reason why you are getting that error is because you are targeting an older version of the framework. The overload that takes just one parameter wasn't added until .NET Framework version 4.5. In that case, just add Nothing as the second parameter, for instance: xProperty.GetValue(x, Nothing).

user3129969 Over a year ago

Thanks I can't believe I missed that....Yes I am using Framework 4.0. Thanks again for the solution. I really have to get used to LINQ and LAMBDA's. Just hard to change a 15 year for...each "coding reflex"...

Steven Doggart Over a year ago

Yes, well, you could also use a delegate to a regular named method or create a class that implements IComparer. The same principles apply. The lambda expressions can be very handy for this sort of thing, though.

Dave Johnson · Accepted Answer · 2015-04-21 13:31:00Z

1

You can also use Linq (using the same general method proposed by Steven Doggart):

sorted = lst.OrderBy(Function(x) x.GetType().GetProperty(_strColumnName01).GetValue(x)). _
             ThenBy(Function(x) x.GetType().GetProperty(_strColumnName02).GetValue(x)).ToList()

In my particular example, I needed to sort by two columns, but for just a single column:

sorted = lst.OrderBy(Function(x) x.GetType().GetProperty(_strColumnName01).GetValue(x)).ToList()

answered Apr 21, 2015 at 13:31

Dave Johnson

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