How to get the source item properties in the related item onafteradd event?

hi All,

I have an type(source item) called A.

A has relationship called B, which relates A to C(related item)

I wrote a VB server event and in related item(C) onafteradd to execute
I want to get the properties of the source item before the relationship item is saved (onafteradd) and make a judgment with the properties of the relationship item.

my question is:

I'm trying to find the source_id of the relationship(B) to get the source item(A).
But I can't find the source id of the relationship because it's a new relationship that hasn't been created yet.

Dim B_relid As string = Me.getProperty("id")
Dim B as Item = Me.newItem("B","get")
        B.setAttribute("select","source_id")
        B.setProperty("related_id",B_relid)
B = B.apply()

Dim A_id as String = B.getproperty("source_id")            
Dim A As Item = myInnovator.GetItemById("A", A_id)     ' error because A_id is empty

Dim A1 As string =A.getProperty("a1") 

Can anyone tell me the code how to get the source item property using VB?

Thank you!