以下是一种参考思路:
module shared
public str as string
end module

public class firstmain
inherits form
....
private sub button1_click..
dim frm as new form2
frm.showdialog()
me.textbox1.text=str
end sub
end class

public class form2
inherits form
...
Private Sub Form2_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
str = Me.TextBox1.Text
End Sub
end class