SSRS Max date from Lookupset - date

Question

I've spent a long time looking around for a solution for this and not found quite what I want. Efforts to adapt existing solutions for different problems have also not worked!
I am using LookupSet to return a list of dates, then joining them to return a list:
=Join(LookupSet(Fields!cPatSer.Value,Fields!cPatSer.Value,Fields!DDate.Value,"PatD"))
I want to only show the most recent date from that list. Here is what I have tried so far:
The above function wrapped in a Max function (doesn't work because Join returns a string)
Using a split function to split the resultant string looking for the commas then using the max function
Doing both of the above but converting the output to Date objects using CDate and DateTime.Parse first as follows...
=Join(LookupSet(Fields!cPatSer.Value,Fields!cPatSer.Value,CDate(Fields!DDate.Value),"PatD"))
=Join(LookupSet(Fields!cPatSer.Value,Fields!cPatSer.Value,DateTime.Parse(Fields!DDate.Value),"PatD"))
Can anybody provide any pointers please?

Answer 1

I've found a solution using Custom Code. Oz Locke made a function that does various aggregates for integer data (links below), and I've amended this to work for dates instead.
In the Code property of the report, paste in this:
'Amended from Oz Locke's code:
'https://github.com/OzLocke/SSRSAggLookup/blob/master/AggLookup.vb
'Allows users to adjust the aggregation type of lookupsets in a cell
Function AggLookup(ByVal choice As String, ByVal items As Object)
'Ensure passed array is not empty
'Return a zero so you don't have to allow for Nothing
If items Is Nothing Then
Return 0
End If
'Define names and data types for all variables
Dim current As Date
Dim count As Integer
Dim min As Date
Dim max As Date
Dim err As String
'Define values for variables where required
current = CDate("01/01/1900")
count = 0
err = ""
'Calculate and set variable values
For Each item As Object In items
'Calculate Count
count += 1
'Check value is a number
If IsDate(item) Then
'Set current
current = CDate(item)
'Calculate Min
If min = Nothing Then
min = current
End If
If min > current Then
min = current
End If
'Calculate the Max
If max = Nothing Then
max = current
End If
If max < current Then
max = current
End If
'If value is not a number return "NaN"
Else
err = "NaN"
End If
Next
'Select and set output based on user choice or parameter one
If err = "NaN" Then
If choice = "count" Then
Return count
Else
Return 0
End If
Else
Select Case choice
Case "count"
Return count
Case "min"
Return min
Case "max"
Return max
End Select
End If
End Function
Then in the cell of your report, use this expression:
=code.AggLookup("max", LookupSet(Fields!cPatSer.Value,Fields!cPatSer.Value,Fields!DDate.Value,"PatD"))
https://itsalocke.com/aggregate-on-a-lookup-in-ssrs/
https://github.com/OzLocke/SSRSAggLookup/blob/master/AggLookup.vb