How to display number values with commas in form - ms-access

Question

In my Access query, I have the query using a VBA function to figure the value that goes in the query field.
In the form, if the stringval textbox has a value, then I want to compute it, but if not, it should remain empty (null).
Function GetValue(stringval, numval)
Dim result
stringval= stringval & ""
result= IIf(stringval<> "", numval* 1.5, Null)
GetValue = Int(result)
End Function
Now, I have a form that uses this query, and on the form is a textbox that displays the query value. I want the value to be formatted with commas in the numbers for easy reading. Everything I've tried so far does not show any commas.
I've tried:
used Standard for the Format > Formatfor the textbox (in properties)
putting #,###.### in the textbox Format value
putting #,##0.0## in the textbox Format value
changing Data > Text Format but it only gives me Plain Text and Rich Text - no option for numbers.
returning a double from the function
Note: if I don't use a custom VBA function, and write the formula directly into the query, then it does display commas. But when I move the formula into the function then the commas are lost.
What do I do?
[update]
I tried Gustav's solutions and since they didn't work for me, I added those as items to my "what I've tried" list above.
Also, if I look at the query in datasheet view, the number values sort alphabetically instead of by the size of the value. When I used the forumulae directly in the query instead of using functions, it sorted by the value of the number. I hope this is a clue.

Answer 1

Numbers carries no format. A format is applied when displayed only.
But be sure to return a Double if not Null:
Function GetValue(stringval, numval)
Dim result
If stringval & "" <> "" Then
result = Int(CDbl(numval) * 1.5)
Else
result = Null
End If
GetValue = result
End Function
Then apply your Format to the textbox
#,##0.0##
Or force a formatted string to be returned:
If stringval & "" <> "" Then
result = Format(Int(CDbl(numval) * 1.5), "#,##0.0##")
Else
result = Null
End If
and skip formatting of the textbox.

Answer 2

The solution is this: the function has to be declared as a double.
That allows the query's datasheet view to know it is displaying numbers - and so you can set the field's format to Standard for the comma to display. This also allows the form to know it has a number and it will display the comma there, too. I knew it had to do with double, but didn't realize before that the function needed to be declared as such.
Function GetValue(stringval, numval) as double '<----THIS!!!!
Dim result
If stringval & "" <> "" Then
result = numval * 1.5
Else
result = 0 `<--can't return null here; use nz function in control source for textbox
End If
GetValue = int(result) 'to remove decimals
End Function
The problem I was having was in some of my functions I need to return double or null, because I wanted textboxes to remain blank if they contained no data. Now, at least I know how to make the numbers generated by functions to display commas.
And here is how to deal with the fact that you can't return null as the value of a double. The function is originally from here.
Put this function in a module so it is public, and then in the control source for the textbox, instead of just putting the field value, put Zn(fieldvalue). This works like a charm (although using functions in the control source seems to have a delay on the form display). This way you can keep the underlying value as a double and still get commas to display in both the form and the query whilst keeping the field blank if necessary.
Public Function Zn(pvar)
' Return null if input is zero or ""
If IsNull(pvar) Then
Zn = Null
ElseIf IsNumeric(pvar) Then
If pvar = 0 Then
Zn = Null
Else
Zn = pvar
End If
Else
If Len(pvar) = 0 Then
Zn = Null
Else
Zn = pvar
End If
End If
End Function