VB.NET Windows App - Optional Sub Parameter byRef Object default no reference

Hello,

I have a VB.NET Sub routine with 3 parameters.  The last is " ByRef Optional ilsList As ImageList".

What the routine does internally varies, depending on whether it is called with an ImageList reference.  I therefore need to be able to check if this parameter was called.

I guess I need to assign a DEFAULT value to this reference... not sure how to do this.  I assumed maybe assigning NULL would work and allow me to do an ISREFERENCE() check against the ilsList object.  But this doesn't seem to work.

Any ideas?

Thanks in advance!