More efficient error catching? - vb.net

Question

So I am at an internship for school that is having me code a program using VB.NET. One of the forms has three combo boxes that must have a choice selected in each before moving on. If the user missed one of the comboboxes I am trying to bring it to their attention to go back and make a selection before continuing. I have an if statement, which works exactly how I want it to:
If cboYear.SelectedIndex = -1 Then
warningString = warningString + "Year" & vbNewLine & "Vendor" & vbNewLine & "Report"
txtYear.ForeColor = Color.Red
TextBox7.ForeColor = Color.Red
txtReport.ForeColor = Color.Red
MessageBox.Show(warningString)
ElseIf cboVendorName.SelectedIndex = -1 Then
warningString = warningString + "Vendor" + vbNewLine & "Report"
txtYear.ForeColor = Color.Black
TextBox7.ForeColor = Color.Red
txtReport.ForeColor = Color.Red
MessageBox.Show(warningString)
ElseIf cboReport.SelectedIndex = -1 Then
warningString = warningString + "Report" & vbNewLine
txtYear.ForeColor = Color.Black
TextBox7.ForeColor = Color.Black
txtReport.ForeColor = Color.Red
MessageBox.Show(warningString)
Else
Main.Show()
Me.Hide()
End If
(warningString is a generic string that says something like "Please fill in the following:")
So like I said this works exactly how I want it to, and its pretty easy to follow my logic here. My question is how can I make this code more efficient? I have tried arrays to hold combobox selections, and I have tried a for loop and a select case to try and streamline the code to no avail. The array always breaks out and returns a NullReferenceException before it gets to my Select Case or For loop to see if there is a value in the combobox or not.
If there is someone out there who can help walk me through the logic of getting some kind of loop to work for the above, that would be awesome. I'm not asking for working code or anything like that, however if there is a solution to be found and I get there myself I will post it here for future reference for people. Who knows, this might even be the most efficient way already?
Edit: Plutonix has been gracious enough to have offered some comments on my question, and he brought up a point about the if statements not being mutually exclusive. That is because the comboboxes are being populated by datasets, and each CBO in order determines what information from the datasets will populate the next one. So, depending on what year is selected in the 'Year' combobox, determines which vendors populate the 'Vendor' combobox, and that determines which reports are populated in the 'Report' combobox.

Answer 1

Since each test is doing something different a loop would be clumsy. Even if you only enable B once A is validated, you might need to allow for them to UnDo a previous selection. The code can be condensed though:
Dim IsValid As Boolean = True
txtYear.ForeColor = Color.Black
TextBox7.ForeColor = Color.Black
txtReport.ForeColor = Color.Black
If cboYear.SelectedIndex = -1 Then
warningString = warningString & "Year" & vbNewLine & "Vendor" & vbNewLine & "Report"
txtYear.ForeColor = Color.Red
TextBox7.ForeColor = Color.Red
txtReport.ForeColor = Color.Red
IsValid = False
End If
If cboVendorName.Enabled AndAlso cboVendorName.SelectedIndex = -1 Then
warningString = warningString & "Vendor" & vbNewLine & "Report"
TextBox7.ForeColor = Color.Red
txtReport.ForeColor = Color.Red
IsValid = False
End If
If cboReport.Enabled AndAlso cboReport.SelectedIndex = -1 Then
warningString = warningString & "Report" & vbNewLine
txtReport.ForeColor = Color.Red
IsValid = False
End If
' See note
If IsValid = False Then
MessageBox.Show(warningString)
Else
' DONT use default form instances!
Main.Show()
Me.Hide()
End If
The IF changed a bit: I dont know how you tell which ones are active, the code above is using .Enabled to see if each has been set up for use. That way you wont report errors on CBO 2 and 3 when validating #1.
I would make it a function which Returns IsValid and let the calling code manage the forms to show...and dont use default form instances.