[Fact]
public void TestTargetMustNotHoldAReferenceToItemsAfterCallingClear()
{
    var item = new MarkedType();
    var weakReferenceToItem = new WeakReference<MarkedType>(item);
    var testTarget = new ListBuilder<MarkedType>().WithItems(item)
                                                  .Build();

    testTarget.Clear();

    item = null;
    GC.Collect();
    MarkedType retrievedItem = null;
    Assert.False(weakReferenceToItem.TryGetTarget(out retrievedItem));
    Assert.Null(retrievedItem);
}

public class MarkedType { }