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Problem:  The problem we are having is in VB6 we defined many properties of
type Variant for the sole purpose of being able to store Null.  These
properties are typically used to store database fields.  So they will either
store say a number or null, or a string or null, or a decimal or null, etc.
Hence, the properties were setup as Lets, not Sets, in VB6.  When these
properties are upgraded to .NET they become Objects.  When .NET compiles
this project for COM interop and creates the TLB these properties become
Sets (PropPutRef in the IDL) because their datatype is Object.  We have not
found a way to tell .NET to make this a LET (PropPut) when building the TLB.

Because .NET makes these properties Sets our existing VB6 code breaks
because it doesn't have  Set <some property> = Whatever.  They simply say
<some property> = Whatever.  If we were to go through all our VB6 code and
put Sets on them it still breaks because we are not puting objects into
these properties, we put things like strings and integers in.  Sets don't
work with these types.

Our Solution:  The only work around to this perplexing issue is to manually
fix the IDL and build our the TLB ourselves.  We get the IDL, change the
PropPutRef to PropPut, fix all the other IDL syntax errors created by
OleView.exe and use MIDL.exe to build the TLB based on our new IDL file.  We
have some classes with 50 or more variant properties defined as Lets.  This
is going to be a very cumbersome task.
One final thing.  We are manually building our interfaces in .NET for all of
our classes we expose through COM.  A fix that involves a manual interface
would be of no concern.  Really, any fix would be better then what we're
currently doing.
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Here is a link to the documentation defining this behavior.  How do I
override it!!!!   Manually building the .TLB seems ridiculous.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cp
coninteroperatingwithunmanagedcode.asp

Thanks...