COM and .NET: what’s wrong with this code?

Can you spot the mistake in this code? Here we implement a CCW-based .NET object. This COM class is exposed as from a certain process (say, a service) to other processes in the system. We implement a simple get/set accessor that receives an array of objects:

[ComVisible(true), GuidAttribute("61b5d373-4258-40a2-89ac-2783d8ac0e99")]
class MyComObject
   public object[]  Filter
      get { return objArray; }

         Debug.Trace("SET: new array received: '{0}'" + objArray.Length);
         objArray = value;

   private object[] objArray = new object[0];

This COM object can be potentially called from another process B, for example from a VB script. For example, something like this:

Dim formatsArray(2)
formatsArray(1) = "abc"
formatsArray(2) = "def"

Set myObject = CreateObject( "TEST.MyComObject" )
myObject.Filter = formatsArray

Comments (4)

  1. J. Bergman says:

    No ProgId Attribute, and shouldn’t you make a copy of the object array, incomming, I think the array is a variant array that needs some sort of conversion.

  2. Adi Oltean says:

    OK – let’s assume that the COM object is correctly registered… 🙂

  3. AT says:

    Are you kidding ?

    Debug.Trace("SET: new array received: ‘{0}’" + objArray.Length);

    Two bugs in single line 😉

    1. Using a "+" for int + string concatenation instead of "," for string formating. If you use "+" then remove {0}.

    2. Not checking for null reference then doing objArray.Length

  4. Adi Oltean says:

    Doh – you are right. Silly me, I should have come with a more complicated question 🙂

    Anyway, the main point I want to emphasise is this: when marshalling top-level COM pointers as IN parameters in a routine (including a setter), then it is possible to put NULL as the parameter value. Probably not from VBScript, but certainly from C++.

    In the example above, a NULL value for the objArray safe array is nicely transmitted to the server, causing an AV in this case

    Note that when you have an IN/OUT or OUT parameter, the NULL value (of a pointer to a SAFEARRAY reference) is not sent over the wire. The COM marshalling will return an error to the client directly.

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