CODEWITHSUNDEEP
comvb6
MHJF
MHJF

Reputation: 127

How do I check a property of a COM object for a null value in VB6?

I'm working on an extension for a VB6 legacy application.

We've developed an extension in C#, which is exposed to VB6 via COM. One of the methods in the C# solution returns an object with several properties, some of which are themselves instances of a class defined in the project (not an actual snippet; unfortunately I'm not allowed to post the original code):

public class CustomType
{
   ...
}

public class ComAdapter
{
  public CustomType ExampleProperty 
  { 
    get { ... } // edited: this was not an auto-property and causing an exception
  }

  ... (more properties and some logic)
}

In Vb6, we receive an instance of the ComAdapter class. In some cases, the property Instance can be null - this is intended. This should trigger different behavior in the legacy app. The question is: how do I check if the ExampleProperty property is null?

I've tried all of the following:

Dim oAdapter as ComAdapter
... (code to retrieve instance ComAdapter)

If oAdapter.ExampleProperty Is Nothing Then (...)
If oAdapter.ExampleProperty Is Null Then (...)
If oAdapter.ExampleProperty Is Empty Then (...)
If IsNothing(oAdapter.ExampleProperty) Then (...)
If IsNull(oAdapter.ExampleProperty) Then (...)
If IsEmpty(oAdapter.ExampleProperty) Then (...)

All fail. I get "Object reference not set to an instance of an object" in all cases. I'm guessing this is related to the fact that I'm trying to check a property, but I don't know how to solve it.

Edit: I checked that oAdapter is properly set. There are several other properties aside from the one I'm trying to check which are actually accessible, so it doesn't seem likely that oAdapter is the problem.

Upvotes: 2

Views: 271

Answers (2)

Mark Hurd
Mark Hurd

Reputation: 10931

The first one is the correct syntax.

If oAdapter.ExampleProperty Is Nothing Then (...)

If you're getting "Object reference not set to an instance of an object", either oAdapter is Nothing (null) or ExampleProperty is not actually an auto-parameter as you suggest.

Upvotes: 2

Roger Rowland
Roger Rowland

Reputation: 26259

Perhaps try like this (obviously, I can't test this without your code):

Dim oAdapter as ComAdapter
... (code to retrieve instance ComAdapter)

Dim oCustomType as CustomType

Set oCustomType = oAdapter.ExampleProperty

if oCustomType Is Nothing Then (...)

Upvotes: 0

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