C# COM Interop returning object of null causes 0x800A01A8 Object required

Question

I'm writing COM wrappers that return an object, or null if the object does not exist. When calling from VBScript, returning null throws the error "Object required: 'ComObj.Prop2'" Code 800A01A8...

C#

public class testCOM
{
    public object Func(int i)
    {
        if (i == 1) return new object();
        if (i == 2) return DBNull.Value;
        return null;
    }
}

VBScript

set ComObj = CreateObject("ClassLibrary1.testCOM")

set TestObj = ComObj.Func(1) 'This Works
set TestObj = ComObj.Func(2) 'Throws "Object required: 'ComObj.Func(...)'" Code 800A01A8
set TestObj = ComObj.Func(3) 'Throws "Object required: 'ComObj.Func(...)'" Code 800A01A8
set TestObj = Nothing        'This is what I want to occur with Func(2) and Func(3)

I tried returning DBNull.Value, which is supposed to marshal as VT_NULL, but no luck...

I really don't want to write a wrapper object similar to Nullabe<> with .HasValue and .Value... Another option I don't like is to create a Nothing object (inside no namespace) so I can do:

If TestObj Is Nothing And TestObj = "Nothing" Then
    'TestObj was nothing or "Nothing"
End If

If I use the Nothing object, then my COM object has to reutrn type Object instead of the expected typed object, making the code a bit harder to read though functionally the same for my purposes.

What is the best way to return a null object to VBScript via COM?

score 0 · Answer 1 · edited Jul 16 '12 at 01:10

0

Check the signature in your interface and make sure it's correct. If I do the following:

public object test(int i) {
    object retval = null;
    if (i == 1) {
        retval = new Object();
    } else if (i == 2) {
        retval = DBNull.Value;
    }
    return retval;
}

with an interface signature of:

[DispId(1001)]
object test(int i);

and then, in VBScript, do:

' myObject is my instantiated COM class
WScript.Echo TypeName(myObject.test(1))
WScript.Echo TypeName(myObject.test(2))
WScript.Echo TypeName(myObject.test(3))

I get back:

Object
Null
Empty

edited Jul 16 '12 at 01:10

Kijewski

25,517
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answered Jul 16 '12 at 01:07

basiphobe

590
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I appreciate the response. Unfortunately, your suggestion does not assign the returned object/value to a variable. Instead, it passes it directly to another function, TypeName. If your vbscript was: – FuncL Sep 17 '12 at 21:53
If your vbscript attempted `set TestObj = myObject.test(2)` in order to `Echo TypeName(TestObj)`, it would error with Code 800A01A8. Apparently, `set` requires an object to be set, so I cannot return null, which is not an object. If you leave off the 'set', it works. I have resorted to creating a Nothing class in the root namespace and testing `If TypeName(TestObj) = "Nothing" Then` – FuncL Sep 17 '12 at 22:32
Couldn't you just test with IsNull before attempting to Set it? – basiphobe Sep 19 '12 at 12:39
As you might guess from my keeping up with this thread, I've accepted the returning an instance of an global::Nothing object. But, to respond to using IsNull, I assume you mean `If Not IsNull(ComObj.Func(2)) Then Set TestObj = ComObj.Func(2)`. Unfortunately, that would execute the function twice, doubling the work. However, You did give me an idea. I can create `Function Wrapper(obj) If TypeName(obj) = "Empty" Then Set Wrapper = Nothing Else Set Wrapper = obj End If End Function`. Now, this should work: `Set TestObj = Wrapper(CommObj.Func(2)))`. Thanks for the help! – FuncL Oct 26 '12 at 02:00

score 0 · Answer 2 · answered Mar 22 '13 at 05:50

Adding MarshalAs helps:

[return:MarshalAs(UnmanagedType.IDispatch)]
public object Func(int i)
{
    if (i == 1) return new object();
    if (i == 2) return DBNull.Value;
    return null;
}

I'm not sure why though. And if you have an interface then this attribute should be applied on interface level.

C# COM Interop returning object of null causes 0x800A01A8 Object required

2 Answers2