Function 'DisplayArray' doesn't return a value on all code paths. A null reference exception could occur at run time when the result is used - vb.net

Question

I keep getting a error and because of this my code wont work. The error is "Function 'DisplayArray' doesn't return a value on all code paths. A null reference exception could occur at run time when the result is used". I can't get rid of this error. I'm new to programming, can anyone help?
Private Function DisplayArray() As String
Dim j As Integer = 0
ReDim Preserve Array(UpperSub)
Dim AddNum As Double = 0.0
txtAddNum.Focus()
If Double.TryParse(txtAddNum.Text, AddNum) Then
If AddNum > 100 Then
MessageBox.Show("Number must be below 100")
ElseIf AddNum < 0 Then
MessageBox.Show("Number must be above 0")
Else
Array(UpperSub) = CDec(AddNum)
UpperSub = UpperSub + 1
End If
Else
MessageBox.Show("Value has to be a number")
End If
txtAddNum.Clear()
txtDisplay.Clear()
For j = 0 To UpperSub - 1
txtDisplay.Text = txtDisplay.Text _
& CStr(Array(j)) & ControlChars.NewLine
Next
txtNumberOfScores.Text = CStr(UpperSub)
End Function

Answer 1

A Function is designed to return a result, using the Return keyword. Your Function does not have a Return statement. Since none of the possible code paths (determined by branching on the If statements as well as the flow from beginning to end) return a value, you're getting this part of the error message: "Function 'DisplayArray' doesn't return a value on all code paths."
The second part of the error message means that if you tried to assign the return value of the Function to a variable, like this:
Dim result As String = DisplayArray()
You'd get a null value, as nothing is returned from the function.
The simplest solution is to change from a Function to a Sub. Subs in VB.NET do not return a value. So change
Private Function DisplayArray() As String
To
Private Sub DisplayArray()
And
End Function
To
End Sub
Note that the As String() in the Function declaration says this method will return a value that is a String, and the Sub has no return value (again, because it doesn't return a value).
To make this a Function that returns a value, you'll have to return at least one value from the method. Here's an example:
Private Function DisplayArray() As String
Dim j As Integer = 0
ReDim Preserve Array(UpperSub)
Dim AddNum As Double = 0.0
txtAddNum.Focus()
If Double.TryParse(txtAddNum.Text, AddNum) Then
If AddNum > 100 Then
MessageBox.Show("Number must be below 100")
Return String.Empty
ElseIf AddNum < 0 Then
MessageBox.Show("Number must be above 0")
Return String.Empty
Else
Array(UpperSub) = CDec(AddNum)
UpperSub = UpperSub + 1
txtAddNum.Clear()
txtDisplay.Clear()
For j = 0 To UpperSub - 1
txtDisplay.Text = txtDisplay.Text _
& CStr(Array(j)) & ControlChars.NewLine
Next
Return CStr(UpperSub)
End If
Else
MessageBox.Show("Value has to be a number")
Return String.Empty
End If
End Function
Essentially, if the validation fails, an empty string is returned. If the validation passes, the rest of the code is executed and the string value of UpperSub is returned.
You could then assign it to the TextBox like this:
txtNumberOfScores.Text = DisplayArray()
The above is a simple example based on your posted code, intended to show you how to return values from a Function. Adjust it to fit your needs (or use a Sub instead). Given that you want to update the display of the txtDisplay with the array and txtNumberOfScores as well you should do fine with a Sub.