Check for input in optional form variable

I have a function with some arguments, and one of them is optional and of the "form" variable type.

Public Function MyFunction(Argument1 as Boolean, Optional TheForm as Form) as String
    If TheForm is null Then
        'Do something
    End If
End Function

"TheForm is null" always returns false, whether or not anything has been entered for that argument.
Is there an explicit way the to test whether something was entered for TheForm argument, without using error trapping, or using a variant variable type?  That should be possible, right??
Jolio81Asked:
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hnasrCommented:
Try:
Public Function MyFunction(Argument1 As Boolean, Optional TheForm As Form) As String
    If TypeName(TheForm) = "Nothing" Then
        'Do something
        MsgBox ("Optioanal argument ignored")
    End If
End Function

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PatHartmanCommented:
Try using IsEmpty() when referring to an object.
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Jolio81Author Commented:
Public Function MyFunction(Argument1 As Boolean, Optional TheForm As Form) As String
    MyFunction = "Is TheForm Empty? " & IsEmpty(TheForm)
End Function

In the Immediate window, I type:
debug.print myfunction(True)

The result I get is this:
Is TheForm Empty? False
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Dale FyeCommented:
Try IsMissing()  or Is Nothing

Public Function MyFunction(Arguement1 as boolean, Optional TheForm as Form) as String

MyFunction = "Is TheForm Missing:  " & isMissing(TheForm) & vbcrlf _
                     & "Is TheForm Nothing: " & (TheForm is nothing)

End function

I think you will find, that the Is Nothing syntax is what you are looking for.
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Jolio81Author Commented:
Fyed,

I tried this:

Public Function MyFunction(Argument1 As Boolean, Optional TheForm As Form) As String
    MyFunction = "Is TheForm nothing? " & TheForm Is Nothing
End Function

but it threw "Compile error: Type mismatch".  I also tried "TheForm = Nothing" but got the same.

IsMissing didn't work either; the function returned "Is TheForm missing? false"
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Jolio81Author Commented:
Works great!  Thanks hnasr!
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Dale FyeCommented:
Well, if you had included the parenthesis around the expression, as I did in my example, it would have worked properly.

MyFunction = "Is TheForm Nothing: " & (TheForm is nothing)

When you left out the parenthesis, Access assumed you wanted to contatenate the TheForm (which "is nothing" as it was not passed to the function) to the text "Is TheForm Nothing", which is why you got a type missmatch.  By including the parenthesis, you force evaluation of the Is Nothing test and then concatenate either True or False to the string.
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hnasrCommented:
Welcome!
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Jolio81Author Commented:
Dear Fyed,

Let us blame those at Microsoft for the willy-nilly behavior of the compiler.  The following does work properly without parentheses:

      debug.Print "Does not true = false? " & not true = false

and yet

      "Is TheForm nothing? " & TheForm Is Nothing

does not.  I think that's dumb.  Sorry, I would have definitely given you points.
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Dale FyeCommented:
@jolio81

you have to set some form of precedence when doing evaluation, what's the old adage

Parentheses, exponentiation, multiplication, division, addition, subtraction

in the case above, I'm sure the "=" and "Not" operators have priority over the "&"

doesn't appear that "xxxxx is Nothing" is higher in priority than "&", which is why I generally explicitly wrap this type of thing in parentheses.

No prob on the points, the point in my explanation was to help you understand the behavior.
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Jolio81Author Commented:
I appreciate the advice; I'll start making a habit of using parentheses.  Thanks Fyed!
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