c# linq conditional union

hi I have the following code and would like to only make each union if a condition is true. I Know I could write a select of if else but wanted to know if there is a slicker Linq way!?

        //join the开发者_如何学编程 list into one and sort by seqnumber
      SegmentList = Alist.Cast<BaseSegment>()
     .Union(BList.Cast<BaseSegment>()).Union(CList.Cast<BaseSegment>())
     .OrderBy(item => item.SegSeqNumber).ToList();

So given the above if ATest =true how do I only iclude Alist like wise if BTest && CTest are true how do I include only BList and Clist

Thanks

---

To do it in a LINQ style way with your checkboxes, something like:

SegmentList = Alist.Where(i => checkbox1.IsChecked).Cast<BaseSegment>()
.Union(BList.Where(i => checkbox2.IsChecked).Cast<BaseSegment>())
.Union(CList.Where(i => checkbox3.IsChecked).Cast<BaseSegment>())
.OrderBy(item => item.SegSeqNumber).ToList();

would work. But I don't think it is either very understandable or efficient.

---

Something like this?

SegmentList = Alist.Cast<BaseSegment>()
                   .Union(includeB ? BList.Cast<BaseSegment>() : Enumerable.Empty<BaseSegment>())
                   .Union(includeC ? CList.Cast<BaseSegment>() : Enumerable.Empty<BaseSegment>())
                   .OrderBy(item => item.SegSeqNumber)
                   .ToList();

This is not identical to your original (it will remove duplicates from Alist no matter what) but should be what you want.

For any more than 2 conditional unions, you would probably want a different query, something like:

var listsByCb = new Dictionary<CheckBox, MyListType>
                {{ aListBox, aList}, {bListBox, bList}, {cListBox, cList}};

var segmentList = listsByCb.Where(kvp => kvp.Key.Checked)
                           .SelectMany(kvp => kvp.Value.Cast<BaseSegment>())
                           .Distinct();
                           .OrderBy(item => item.SegSeqNumber)
                           .ToList();

---

try something like

SegmentList = Alist.Cast<BaseSegment>().Where(z=>ATest)
     .Union(BList.Cast<BaseSegment>().Where(x=>Btest)).Union(CList.Cast<BaseSegment>().Where(c=>Ctest))
     .OrderBy(item => item.SegSeqNumber).ToList();

where class in each case will return all elements if corresponding test is true and will return no element otherwise. its completely untested though

---

Use a Where() clause for that, like following:

    //join the list into one and sort by seqnumber
  SegmentList = 
     Alist.Cast<BaseSegment>().Where(a => ATest(a))
     .Union(
        BList.Cast<BaseSegment>(.Where(b => BTest(b))
     .Union(CList.Cast<BaseSegment>().Where(c => CTest(c))
     .OrderBy(item => item.SegSeqNumber).ToList();

By the way, do you really need the last ToList()?