How can I get a hashtable key name from a value in VB.NET? - vb.net

Question

I have a hashtable in VB.NET and I need to get the string value of a key from it's value. For example, if I do:
hashtable.add("string1","string2")
How would I get the value "string1" if I had "string2"?

Answer 1

You can't (at least not without simply looping through every value). Consider the fact that multiple keys can map to the same value:
hashtable.Add("string1", "string2")
hashtable.Add("string3", "string2")
Now given "string2" what would you expect to be returned?
If you really need to do a "reverse" lookup, then the simplest solution is to probably have two hashtable, one for the "forward" lookup and one for the "reverse" lookup.

Answer 2

As Dean / codeka says you can't strictly do this.
However you can do something like this as the Keys and Values of a Hashtable are in the same (unspecified) order:
Hashtable ht = new Hashtable();
ht.Add("string1", "str2");
ht.Add("string2", "str2");
List<string> keys = new List<string>(ht.Keys.OfType<string>());
string key = ht.Values.OfType<string>()
.Select((htI, i) => new { Key = keys[i], Value = htI })
.Where(htKVP => htKVP.Value == "str2")
.Select(htKVP => htKVP.Key)
.FirstOrDefault();
However, you would be better using a Dictionary<string, string> just because it is generically typed and lets you get to Linq easier.
Converted for VB.NET that is:
Dim ht as new Hashtable()
ht.Add("string1", "str2")
ht.Add("string2", "str2")
Dim keys as new List(Of String)(ht.Keys.OfType(Of String)())
Dim key as String = ht.Values.OfType(Of String)() _
.Select(Function(htI, i) New With { .Key = keys(i), .Value = htI }) _
.Where(Function(htKVP) htKVP.Value = "str2") _
.Select(Function(htKVP) htKVP.Key) _
.FirstOrDefault()
But again I'd start with:
Dim dt as New Dictionary(Of String, String)
You could add this as an extension method like so:
Imports System.Runtime.CompilerServices
Module StringExtensions
<Extension()>
Public Function FirstKeyForValue(ByVal Hashtable as ht, ByVal value As String) As String
return ht.Values.OfType(Of String)() _
.Select(Function(htI, i) New With { .Key = keys(i), .Value = htI }) _
.Where(Function(htKVP) htKVP.Value = "str2") _
.Select(Function(htKVP) htKVP.Key) _
.FirstOrDefault()
End Function
End Module

Answer 3

There is a much easier way than Matt's answer. You can perform linq on a hashtable.
Of course the below sample code can be modified to use the proper types used in your Hashtable variable as both Key and Value can be any type:
Public Class HashtableTest
Private Lookup As New Hashtable
Private Sub New()
Lookup("Key1") = "Value1"
Lookup("Key2") = "Value2"
End Sub
Public Sub Test()
MsgBox(GetKey("Value2"))
End Sub
Public Function GetKey(Value As String) As String
Dim FoundKey As String = ""
If Lookup.ContainsValue(Value) Then
FoundKey = (From Key As String In Lookup.Keys.Cast(Of String) Where Lookup(Key).ToString() = Value Select Key).FirstOrDefault()
End If
Return FoundKey
End Function
End Class