Regex as key in Dictionary in VB.NET - regex

Question

Is there a way to use Regex as a key in a Dictionary? Something like Dictionary(Of Regex, String)?
I'm trying to find a Regex in a list (let's say that there is no dictionary for the first time) by string, which it matches.
I can do it by manually iterating through the list of RegEx expressions. I'm just seeking for a method to do that more easily, such as TryGetValue from a Dictionary.

Answer 1

When you use Regex as the type for the key in a Dictionary, it will work, but it compares the key by object instance, not by the expression string. In other words, if you create two separate Regex objects, using the same expression for both, and then add them to the dictionary, they will be treated as two different keys (because they are two different objects).
Dim d As New Dictionary(Of Regex, String)()
Dim r As New Regex(".*")
Dim r2 As New Regex(".*")
d(r) = "1"
d(r2) = "2"
d(r) = "overwrite 1"
Console.WriteLine(d.Count) ' Outputs "2"
If you want to use the expression as the key, rather than the Regex object, then you need to create your dictionary with a key type of String, for instance:
Dim d As New Dictionary(Of String, String)()
d(".*") = "1"
d(".*") = "2"
d(".*") = "3"
Console.WriteLine(d.Count) ' Outputs "1"
Then, when you are using the expression string as the key, you can use TryGetValue, like you described:
Dim d As New Dictionary(Of String, String)()
d(".*") = "1"
Dim value As String = Nothing
' Outputs "1"
If d.TryGetValue(".*", value) Then
Console.WriteLine(value)
Else
Console.WriteLine("Not found")
End If
' Outputs "Not found"
If d.TryGetValue(".+", value) Then
Console.WriteLine(value)
Else
Console.WriteLine("Not found")
End If