Combine LINQ queries

Question

I\'m in the process of making a small query thing for a set of results of files.

public class f_results
    {
        public String name { get; set; }
               


        

Answer 1

At least I suggest to remove the .ToList() invokes everywhere. As LINQ has a deferred invocation, it will iterate once, even you have:

var foundfiles = from p in foundfiles where p.size >= sz select p ;
foundfiles = from p in foundfiles where p.mdate >= test select p

Update (in that case order by should be put after all filters)

But if you write:

var foundfiles = (from p in foundfiles where p.size >= sz orderby p.size descending select p).ToList() ;
foundfiles = (from p in foundfiles where p.mdate >= test select p).ToList();

It will iterate two times - and that might be a serious performance problem.

But I don't think the code will look much simpler if you make this code as a single query.

Also, why are you catching all exceptions? You shouldn't do that.

Answer 2

You could generate your filters beforehand, then apply them all at once - you would only have to iterate your initial enumeration once, something like this (shortened):

IEnumerable<f_results> foundfiles = new List<f_results>();
var filters = new List<Func<f_results, bool>>();

if (fsize.Text.Trim() != "")
{
    long sz = long.Parse(fsize.Text);
    filters.Add(x => x.size >= sz);
}

if (adate.Text.Trim() != "")
{
    DateTime test = DateTime.Parse(adate.Text);
    filters.Add(x => x.adate >= test);
}

foreach (var filter in filters)
{
    var filterToApply = filter;
    foundfiles = foundfiles.Where(filterToApply);
}
finalResults = new BindingList<f_results>(foundfiles);

More importantly don't call ToList() until you have processed all filters, otherwise you keep iterating through your full result list over and over.

Answer 3

OK - I wanted to semi answer my own question..

I can combine to a single query, the following works very well.. Ideal? Possibly not!

Am going to now look at BrokenGlass' suggestion which looks nice and tidy!

   Boolean flag_size = false;
    Boolean flag_adate = false;
    Boolean flag_cdate = false;
    Boolean flag_mdate = false;
    Int64 sz=0;
    DateTime adatetest=DateTime.Now;
    DateTime cdatetest = DateTime.Now;
    DateTime mdatetest = DateTime.Now;
    String mop = mdateop.Text;
    String aop = adateop.Text;
    String cop = cdateop.Text;

    if (fsize.Text.Trim() != "")
    {
        try
        {
            sz = Int64.Parse(fsize.Text);
            flag_size = true;
        }
        catch { }
    }

    if (adate.Text.Trim() != "")
    {
        try
        {
            adatetest = DateTime.Parse(adate.Text);
            flag_adate = true;
        }
        catch
        { }
    }
    if (cdate.Text.Trim() != "")
    {
        try
        {
            cdatetest = DateTime.Parse(cdate.Text);
            flag_cdate = true;
        }
        catch
        { }
    }
    if (mdate.Text.Trim() != "")
    {
        try
        {
            mdatetest = DateTime.Parse(mdate.Text);
            flag_mdate = true;
        }
        catch
        { }
    }


    foundfiles = new BindingList<f_results>(totalresults.Find(fname.Text, true));


            List<f_results> y = (from p in foundfiles.AsParallel() 
                                 where  (!flag_size || (flag_size && p.size >= sz)) &&
                                        (!flag_mdate || (flag_mdate && mop == ">" && p.mdate >= mdatetest) || (flag_mdate && mop == "< " && p.mdate >= mdatetest)) &&
                                        (!flag_adate || (flag_adate && aop == ">" && p.adate >= adatetest) || (flag_adate && aop == "< " && p.adate >= adatetest)) &&
                                        (!flag_cdate || (flag_cdate && cop == ">" && p.cdate >= cdatetest) || (flag_cdate && cop == "< " && p.cdate >= cdatetest))
                                 orderby p.size descending 
                                 select p).ToList();

            foundfiles = new BindingList<f_results>(y);