CommonULong64 OverflowException

Ahh, the fun that is VB...

First, yes, those values you have (4, and -176378626) are valid. We use signed types because of java's limitations with support for unsigned types.

You have the correct algorighm (high << 32 | low) but the issue is with VB's builtin types being 32 bits. So, when you have a high large number for your high value, the first shift-left of 32 bits results in an overflow since the default size is 32 bits. I don't believe you can cast a 32 bit value to a 64 bit value like you can with C so when I tinkered around with this, I had to create new 64 bit types and assign them manually. This should work for you:

Public Function Build64(ByVal value As Object) As UInt64
  Dim ui64High As UInt64 = value.high
  Dim ui64Low As UInt64 = value.low
  ui64High = ui64High << 32
  Return Convert.ToUInt64(ui64High Or ui64Low)
End Function

There may be a way to condense this but since I'm by no means a VB guru, this is the best I could come up with. I did verify that the outcome of this function is the same as it's C counterpart:

public UInt64 build64(CommonULong64 ul64)
{
  return (((UInt64)(UInt32)ul64.high) << 32) | ((UInt64)(UInt32)ul64.low);
}

Hope this helps...

-Joe