<p>I am pulling data out of an old-school ActiveX in the form of arrays of doubles. I don't initially know the final number of samples I will actually retrieve.</p>

<p>What is the most efficient way to concatenate these arrays together in C# as I pull them  out of the system?</p>

You can't append to an actual array - the size of an array is fixed at creation time. Instead, use a
```
List<T>
```
which can grow as it needs to.
Alternatively, keep a list of arrays, and concatenate them all only when you've grabbed everything.
See
Eric Lippert's blog post on arrays
for more detail and insight than I could realistically provide :)