How to query an arraylist using Linq that stores dictionaries - sql

Question

The following code reads query results from oracle data reader and stores each record in a dictionary and appends the dictionaries to an array list :
Dim dr As OracleDataReader = cmd.ExecuteReader()
'loop oracle data records and store them to dictionaries
'append dictionaries to an array list
Dim arr As New ArrayList
While dr.Read
Dim dict As New Dictionary(Of String, Object)
For count As Integer = 0 To (dr.FieldCount - 1)
dict.Add(dr.GetName(count), dr(count))
Next
arr.Add(dict)
End While
How do I write a LINQ query that can be used to retrieve values from the dictionaries stored in the array list? Please help. I've been searching and have not got any good answers

Answer 1

First of all, don't use ArrayList, ever. It is there for backwards compatibility but has no usage. I can make answer short - there is no use of LINQ with ArrayList. Use generic List(Of T) and LINQ to search values in it. No need for Dictionary either. This is the old style. We used Dictionary because it has key
I see, you trying to create your table structure but no need for this. First of all, there is System.Data.DataTable, which can be queried on client.
Or use this technique
Public Class User
Public Property Id As Integer
Public Property Name As String
Public Property Email As String
Public Property Country As String
End Class
Private Function LoadUsers() As List(Of User)
Dim uList As New List(Of User)()
' Some Code goes here
While dr.Read()
Dim u As New User()
u.Id = dr("Id")
u.Name = dr("Name")
u.Email = dr("Email")
u.Country = dr("Country")
uList.Add(u)
End While
. . . . . . .
Return uList
End While
' somewhere in class set member variable
_users = LoadUsers()
' And then you can search for info using LINQ
Public Function FindByCountry(ByVal country As String) As List(Of User)
Return _users.Where(Function(u) u.Country.Equals(country, StringComparison.OrdinalIgnoreCase))
End
The downside of this approach - you need Find function for each field. But what if you can pass a function itself. See- you have Name, email, Country - all strings. Here what you can do
Class Client
Sub SearchStrings(ByVal searchOption String, Byval searchValue As String)
Dim f As Func(Of User, boolean)
If searchOption = "Name" Then
f = Function(u as User)(u.Name.Equals(searchValue , Stringcomparison.OrdinalIgnoreCase))
ElseIf searchOption = "Country" Then
f = Function(u as User)(u.Country.Equals(searchValue , Stringcomparison.OrdinalIgnoreCase))
ElseIf searchOption = "Email" Then
f = Function(u as User)(u.Email.Equals(searchValue , Stringcomparison.OrdinalIgnoreCase))
Else
. . . .
End If
dataGrd.DataSource = myRepository.FindByString(f)
End Sub
End Class
' In your repository class
public sub FindByString(ByVal f as Func(Of String, Boolean)) As List(Of User)
_users.Where(f).ToList()
End sub
' use this to search single user
public sub FindByInteger(ByVal f as Func(Of Integer, Boolean)) As User
_users.SingleOrDefault(f)
End sub
The bottom line - drop what you do and use modern and efficient techniques. And above are just couple of them