vb.net Loop through public structure and pass to generics - vb.net

Question

I have inherited some god awful vb.net code and having trouble trying to work out the generics for looping through the current structure in place.
here is a snippet of the struct
Public Structure ApplicationDetails
Dim ID As Integer
Dim AgentID As Integer
Dim ApplicationDate As Date
Dim CompletedDate As Date
here is the madness to populate it
With ApplicationInfo
.ID = If(Not IsDBNull(DT(0)("ID")), DT(0)("ID"), Nothing)
.ApplicationDate = If(Not IsDBNull(DT(0)("ApplicationDate")), DT(0)("MortgageAmount"), Nothing)
.CompletedDate = If(Not IsDBNull(DT(0)("CompleteDate")), DT(0)("MortgageAmount"), Nothing)
now i want to do something like this:
For Each item In ApplicationInfo.GetType().GetProperties()
Dim thisType = item.GetType()
Dim name = item.Name
Dim value = DtItem(Of item.GetType())(0, name.ToString(), DT)
item.SetValue(item, value, Nothing)
Next
Private Function DtItem(Of T)(ByVal num As Integer, ByVal name As String, ByRef DT As DataTable) As T
Return If(Not IsDBNull(DT(num)(name)), DT(num)(name), Nothing)
End Function
but i am not sure on the syntax to set the value and when trying to get the type i get item.GetTYpe() is not declared. I know i must be on the right track, just missing a little something.

Answer 1

A number of issues.
First and foremost, SetValue only works on object types, not values, so you would have to change your Structure to a Class.
Public Class ApplicationDetails
End Class
Next, you are looping through properties but your "structure" only had fields. So you need to add properties:
Public Class ApplicationDetails
Private _ID As Integer
Property ID As Integer
Get
Return _ID
End Get
Set(ByVal value As Integer)
_ID = value
End Set
End Property
//' etc
End Class
Otherwise, you would have to work with GetFields.
I don't think Generics will work here since you are only dealing with objects and you don't know the type (despite reflection):
Private Function DtItem(ByVal num As Integer, ByVal name As String, ByRef DT As DataTable) As Object
Return If(Not IsDBNull(DT(num)(name)), DT(num)(name), Nothing)
End Function
Lastly, your reflection call is wrong. Try changing it to this:
For Each item As PropertyInfo In ApplicationInfo.GetType().GetProperties
Dim value As Object = DtItem(0, item.Name, _dt)
If item.CanWrite Then
item.SetValue(ApplicationInfo, value, Nothing)
End If
Next
I'm not sure doing this through reflection is gaining you anything. In your "madness" example, it looks like you might be trying to put in a MortgageAmount, which I assume is a decimal, into a date field. That might be needed to look at.

Answer 2

I think you need to rethink your functional decomposition there. You're not getting much out of moving one line into a function, and I don't think you'll be able to pass a type constructor to a generic the way you are attempting to do so. You're also confusing some of your type checks.
Try something along these lines instead:
For Each item In ApplicationInfo.GetType().GetProperties()
Dim theName As String = item.Name,
isNullable = Not item.PropertyType.IsValueType OrElse _
Nullable.GetUnderlyingType(item.PropertyType) IsNot Nothing
If item.CanWrite Then
If Not IsDBNull(DT(0)(theName))
item.SetValue(ApplicationInfo, DT(0)(theName), Nothing)
ElseIf isNullable
item.SetValue(ApplicationInfo, Nothing, Nothing)
End If
End If
Next
If your code is executed immediately after the object is initialized, the isNullable checks are extraneous and you could simply take no action as the properties will be initialized as null. Otherwise, I'd recommend performing the check so you don't attempt to assign Nothing to a value type, which is going to throw an exception. Alternatively, you could modify your structure to use nullable reference types, i.e.:
Public Structure ApplicationDetails
Property ID As Integer?
Property AgentID As Integer?
Property ApplicationDate As Date?
Property CompletedDate As Date?
End Structure
EDIT
As LarsTech points out, your structure members are not properties and this will not function as anticipated unless you modify your structure to indicate these fields are in fact properties, for which the compiler would automatically generate getters and setters.