Use a Dictionary between different thread in VB.NET - vb.net

Question

I am working on a project that have a server, whenever a clients connects it stores its user_id, ip in the dictionary which i have used. When I store the information between the different threads it is working but when I try to access the dictionary from different threads, it's not working well.
Let me clear what my code should do: There is one main thread active and listening for clients, when a clients wants to connect, main thread creates a child thread and that thread gets connected with client, client sends it a user_id, this user_id (as key for dictionary) and Tcpclient (as Value) get stored in dictionary. Further one client sends a message for particular client to server, that message contains the user_id for receiver. Now this client is connected with a particular thread which will check the key(user_id) in dictionary and then send the data to that client but dictionary is not giving the correct value for connected clients it just returns false even that client is connected.
Below is my code example.
Imports System.Net
Imports System.Net.Sockets
Imports System.Text
Imports System.IO
Public Class Mainform
Dim _server As TcpListener
Dim _listOfClients As New Dictionary(Of String, TcpClient)
Public MessageQueue As New Dictionary(Of String, Queue(Of String))
'Dim ClientData As StreamReader
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Try
Dim port As Integer = 8888
_server = New TcpListener(System.Net.IPAddress.Any, port)
_server.Start()
Threading.ThreadPool.QueueUserWorkItem(AddressOf NewClient)
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
Private Sub NewClient()
Dim client As TcpClient = _server.AcceptTcpClient()
Dim toRec(100000) As Byte
Dim nss As NetworkStream = client.GetStream()
nss.Read(toRec, 0, toRec.Length)
Dim ClientData As String = Encoding.ASCII.GetString(toRec)
MsgBox("Client Connected: " & ClientData)
Dim too() As String
Dim status As Boolean = False
Dim sendto As String
Try
_listOfClients.Add(ClientData, client)
Threading.ThreadPool.QueueUserWorkItem(AddressOf NewClient)
While True
Dim thisClient As String = ClientData
Dim ns As NetworkStream = client.GetStream()
Dim toRecieve(100000) As Byte
ns.Read(toRecieve, 0, toRecieve.Length)
Dim txt As String = Encoding.ASCII.GetString(toRecieve)
'MsgBox(txt)
too = txt.Split("#") 'server receives data in single string where user_id and message is connected with '#'
sendto = String.Copy(too(0)) 'this is the user_id of receiver client
If _listofclients.ContainsKey(sendto) Then
Dim data As String
data = String.Join("#", too)
Dim buffer As Byte() = Encoding.ASCII.GetBytes(data)
Dim nets As NetworkStream = _listofclients(sendto).GetStream()
nets.Write(buffer, 0, buffer.Length)
End If
End While
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
End Class