How do I pass a value from one TextBox to another TextBox in a different form? - vb.net

Question

Currently I have a TextBox on the first form called txtuserid and I want to pass the value of this to another TextBox called USERIDTextBox on a second form.
But when I try to run my code below, nothing gets passed to the TextBox on the second form. So I'm just wondering how I can pass this value from one form to another form?
Here is my code:
Private Sub cmdlogin_Click(sender As Object, e As EventArgs) Handles cmdlogin.Click
Try
If cn.State = ConnectionState.Open Then
cn.Close()
End If
cn.Open()
cmd.CommandText = "select userid,state from registration where userid= " & _
"'" & txtuserid.Text & "' and state='" & txtpw.Text & "'"
Dim dr As OleDb.OleDbDataReader
dr = cmd.ExecuteReader
If (dr.HasRows) Then
While dr.Read
' My Problem:
' This code shows the 2nd form but the USERIDTextBox value doesn't change?
Dim obj As New Sale
obj.USERIDTextBox.Text = txtuserid.Text
obj.Show()
End While
Else
MsgBox("Invalid username or PW")
End If
cn.Close()
Catch ex As Exception
End Try
End Sub

Answer 1

As a general rule, it's not a good idea to try accessing another object/forms controls directly. Instead, a better way to do it would be to pass the text in the 1st form's TextBox to a custom constructor on the 2nd form (the Sale one). Then the constructor on the 2nd form will be responsible for setting the value of the TextBox .
Here is an example of one way you could do this:
Sale.vb
Public Class Sale
Dim secondFormInputText As String
Public Sub New(inputTextFromFirstForm As String)
InitializeComponent()
' Set the class variable to whatever text string was passed to this form
secondFormInputText = inputTextFromFirstForm
End Sub
Private Sub Sale_Load(sender As Object, e As EventArgs) Handles MyBase.Load
' Set the textbox text using this class variable
USERIDTextBox.Text = secondFormInputText
End Sub
End Class
Login.vb
Private Sub cmdLoginExample_Click(sender As Object, e As EventArgs) Handles cmdLogin.Click
Dim obj As New Sale(txtuserid.Text)
obj.Show()
End Sub
So now instead of setting the Sale form's TextBox directly, you can pass the text on the 1st form to the constructor of the 2nd form. The constructor can then save the text it received to a class variable that the rest of the 2nd form can use.
One of the main benefits of this, is that if in the future you change your TextBox to a RichTextBox or possibly another control that might not even have a Text property, you won't have to go updating every single piece of code that tries to set the textbox value directly.
Instead you can change the TextBox to some other control, update the Sales form once with whatever changes you need to work with the new control, and none of the code on the other forms should need to be changed.
Edit:
Even though this question was specifically about how to pass a textbox value from one form to another form, you may also like to read the comments under your question. In particular, Plutonix had some very helpful advice on how you can improve your database code which might be of use to you.