vb.net how to open mdichild in mdiparent? using ToolStrip - vb.net

Question

This is my code, why is it when i open my first form (MdiParent) and when i open the frmCashin form (mdiChild) the child form still exceeds the parents form
Private Sub ToolStrip_Click(sender As Object, e As EventArgs) Handles ToolStrip.Click
Dim formcashin As New frmPOSCashIn
Dim formcashout As New frmPOSTCashout
If CashIn.Selected Then
formcashin.MdiParent = Me.MdiParent
formcashin.Show()
ElseIf Cashout.Selected Then
formcashout.MdiParent = Me.MdiParent
formcashout.Show()
End If
End Sub
just like this in the picture (i use dual monitor), I am new in .net technology,
I can drag the child form to outside the parent form
UPDATE
If CashIn.Selected Then
Dim formcashin As New frmPOSCashIn
Dim formcashout As New frmPOSTCashout
Dim mainform As New frmMain
formcashin.MdiParent = mainform.MdiParent
formcashin.Show()
ElseIf Cashout.Selected Then
Dim formcashin As New frmPOSCashIn
Dim formcashout As New frmPOSTCashout
Dim mainform As New frmMain
formcashout.MdiParent = mainform.MdiParent
formcashout.Show()
End If

Answer 1

Based on the code you posted, I assume that frmMain represents your main form (which is the MDI parent form) and that frmPOSCashIn and frmPOSTCashOut are two MDI child forms. And based on the screenshot, the toolstrip seems to be included in frmMain.
To wrap up the comments by jmcilhinney and Jimi:
You should not explicitly create a mainform variable. You can just use Me, since you are already executing within a frmMain form instance.
Since your frmMain instance itself is the MDI container (its IsMdiContainer property is set to True), it itself should be used as the value for the MdiParent property of your child forms (whose IsMdiContainer properties are set to False). (The MDI architecture allows only one level of hierarchy. You cannot nest MDI container forms as MDI children in an(other) MDI container form. So you should never use the MdiParent property of an MDI container form.)
You can declare your MDI child form variables inside the If-block. And in each block, you should only declare the variable that you actually need.
Something like this:
Private Sub ToolStrip_Click(sender As Object, e As EventArgs) Handles ToolStrip.Click
If CashIn.Selected Then
Dim formcashin As New frmPOSCashIn
formcashin.MdiParent = Me
formcashin.Show()
ElseIf Cashout.Selected Then
Dim formcashout As New frmPOSTCashout
formcashout.MdiParent = Me
formcashout.Show()
End If
End Sub
If you are new to this stuff, I would suggest that you first check out some tutorials/walkthroughs in the Microsoft documentation or other sources on the Internet.