ac＃列表中的FindAll，但搜索词不同

Question

    List<DTOeduevent> newList = new List<DTOeduevent>();
foreach (DTOeduevent e in eduList.FindAll(s => 
       s.EventClassID.Equals(cla) 
    && s.LocationID.Equals(loc)
    && s.EducatorID.Equals(edu))) 
       newList.Add(e);

cla, loc, edu can be (null or empty) or supplied with values--

basically how can I simply return the original list (eduList) if cla, loc, edu are all null

or search by loc, search by loc, edu, search by edu, cla -- etc........

my sample code only makes a new list if all 3 vars have values--

is there an elegant way to do this, without brute force if statements?

Answer 1

List<DTOeduevent> newList = eduList.FindAll(s => 
       (cla == null || s.EventClassID.Equals(cla))
    && (loc == null || s.LocationID.Equals(loc))
    && (edu == null || s.EducatorID.Equals(edu)));

Assuming the values are Nullable value types or classes. If they're strings, you could replace cla == null with String.IsNullOrEmpty(cla) .

Answer 2

IEnumerable<DTOeduevent> newList = eduList;
if (cla != null)
{
  newList = newList.Where(s => s.EventClassID == cla);
}
if (loc != null)
{
  newList = newList.Where(s => s.LocationID == loc);
}
if (edu != null)
{
  newList = newList.Where(s => s.EducatorID == edu);
}

newList = newList.ToList();

Due to deferred execution, the Where statements should all execute at once, when you call ToList ; it will only do one loop through the original list.

Answer 3

I would personally lean towards something that encapsulated the logic of what you seem to be doing here: checking that a found id is equal to some search id. The only wrinkle is how to get that check for null or empty in there first.

One way to do that is by using a static extension method:

public static class DtoFilterExtensions
{
    public static bool IsIdEqual(this string searchId, string foundId) {
        Debug.Assert(!string.IsNullOrEmpty(foundId));
        return !string.IsNullOrEmpty(searchId) && foundId.Equals(searchId);
    }

}

I would also lean towards using LINQ and IEnumerable<> as Domenic does, even though you could make it work with List.FindAll just as easily. Here would be a sample usage:

    public void Filter(string cla, string loc, string edu) {
        var startList = new List<DTOeduevent>();
        var filteredList = startList
            .Where(x => x.classId.IsIdEqual(cla) && x.locationId.IsIdEqual(loc) && x.educatorId.IsIdEqual(edu));
        Show(filteredList.ToList());
    }

In your own code of course you have got that start list either in a member variable or a parameter, and this assumes you have got some method like Show() where you want to do something with the filtered results. You trigger the deferred execution then, as Domenic explained with the ToList call (which is of course another extension method provided as part of LINQ).

HTH,
Berryl