Select Case & Try statement infinite loop - vb.net

Question

When trying to print a report in my vb.net project, I have some code to check whether all of the required fields have been filled in. If it isn't, a message box appears to notify the user. When they press 'OK', I need the program to stop executing the code to load the report. At the moment, it is stuck in an infinite loop, where it goes through a Try function and a SELECT CASE repeatedly instead.
What needs changing to stop this? I can't work out what the issue is, and why the following sections of code keep looping round one after the other
Try
Select Case checkwhat.ToUpper
Case "SUPPLIER"
If cmbSuppliers.Text.Trim = "" Then
MsgBox("Please select a supplier", MsgBoxStyle.OkOnly, "No Supplier Selected")
Return False
Exit Try
End If
Case "RB6B"
check("SUPPLIER")
If check("SUPPLIER") = True Then Else Exit Sub
createWorkTable("SUPPLIERS-TERRITORY-LS")
regReport("rTerritoryWTableCrosstabB.rpt", "", con, Me.MdiParent, cReport, True)
fillPms(cReport, "Sales by Territory by Supplier", "For " & cmbSuppliers.Text.Trim, "", "AOT02")

Answer 1

This is not a full answer but I can at least address some issues:
Select Case checkwhat.ToUpper
Case "SUPPLIER"
If cmbSuppliers.Text.Trim = "" Then
MsgBox("Please select a supplier", MsgBoxStyle.OkOnly, "No Supplier Selected")
Return False
Exit Try '<- Redundant
Else
' Perform other checks
Return True
End If
So this means you can at least get both return values assuming you enter the supplier case.
Now.. In your RB6B case, you run the check function twice.
Case "RB6B"
check("SUPPLIER")
If check("SUPPLIER") = True Then
The two options you have are:
Dim supplierValid as Boolean = check("SUPPLIER")
If supplierValid = True
Or just removing the first line
If check("SUPPLIER") = True Then
Okay. So following that:
If check("SUPPLIER") = True Then Else Exit Sub
Try and avoid using this style, because it can hide code paths and you may miss some functionality without realising. Two options shown below. One avoids additional nesting
If check("SUPPLIER") = True Then
createWorkTable("SUPPLIERS-TERRITORY-LS")
regReport("rTerritoryWTableCrosstabB.rpt", "", con, Me.MdiParent, cReport, True)
fillPms(cReport, "Sales by Territory by Supplier", "For " & cmbSuppliers.Text.Trim, "", "AOT02")
Else
Exit Sub
End If
Or
If check("SUPPLIER") = False Then
Exit Sub
End If
createWorkTable("SUPPLIERS-TERRITORY-LS")
regReport("rTerritoryWTableCrosstabB.rpt", "", con, Me.MdiParent, cReport, True)
fillPms(cReport, "Sales by Territory by Supplier", "For " & cmbSuppliers.Text.Trim, "", "AOT02")