title: AutoFixture with AutoNSubstituteCustomization: Set object count on ReadOnly IEnumerable<t> property
body: <p>My test requires that I have different counts of objects in an IEnumerable property of the main entity collection. I have been searching for documentation about this but can't find anything. Here is a sample of what I mean (note that the base entity is created using <code>AutoNSubstituteCustomization</code>)</p>
  <pre><code>IFixture fixture = new Fixture().Customize(new AutoNSubstituteCustomization() { ConfigureMembers = true });
  var t = fixture.CreateMany&lt;ITransaction&gt;(5)
  var service1 = Substitute.For&lt;ITransactionsSvc&gt;();
  service1.GetTransactions().ReturnsForAnyArgs(t);
  var service2 = Substitute.For&lt;IRequestsSvc&gt;();
  service2.GetRequest(default).ReturnsForAnyArgs(
    new Result(){
      TransactionId = t.First().Files.First().RequestId
    }
  );
  </code></pre>
  <p>Where <code>ITransaction</code> would look like</p>
  <pre><code>public interface ITransaction
  {
      long RequestId { get; }
      IEnumerable&lt;FileDef&gt; Files { get; }
      IEnumerable&lt;Comment&gt; Comments { get; }
  }
  </code></pre>
  <p>I know I could set <code>fixture.RepeatCount</code> to specify the global count but how can I have a different value for Files and Comments?</p>
  <p>I already tried using <code>t.With(x =&gt; x.Files, () =&gt; fixture.CreateMany&lt;FileDef&gt;(rnd.Next(1,5))</code> but it throws saying this is a readonly property.</p>
  <p>I also tried using NSubstitute <code>.Returns</code> on the <code>t.Files</code> property but for some reason, the type of <code>RequestId</code> got changed from Int64 to Task`1 when trying to read the value for <code>service2 ReturnForAnyArgs</code> response.</p>
  <p>I know I had to remove some of the complexity from the real case so that is stays concise so I hope I didn't remove too much and kept it understandable. If you need any precisions, feel welcome to ask.</p>
  <p><em>Sub-question</em>: is there any complete documentation on AutoFixture? On <a href="https://autofixture.github.io/docs/" rel="nofollow noreferrer">AutoFixture website</a> I was only able to find very introductory documentation.</p>