C＃LINQ使变量where语句

Question

I have a filter which applies to my results.

What i have now is the following:

Filter 1 = Any
Filter 2 = Atlas
Filter 3 = phoenix

Now when the filter is on any , i run the following query:

foreach (DetectorStatu status in loadOperation.Entities.OrderBy(d => d.Status).Take(25).Reverse())

When the filter is on Atlas :

foreach (DetectorStatu status in loadOperation.Entities.OrderBy(d => d.Status).Where(d => ((d as DetectorStatu).Detector as Detector).EnabledDetectorTypes.Count(t => t.DetectorTypeID == 1) > 0).Take(25).Reverse())

When the filter is on Phoenix :

foreach (DetectorStatu status in loadOperation.Entities.OrderBy(d => d.Status).Where(d => ((d as DetectorStatu).Detector as Detector).EnabledDetectorTypes.Count(t => t.DetectorTypeID == 2 || t.DetectorTypeID == 3) > 0).Take(25).Reverse())

And the actual query that they are calling:

[Query]
[OutputCache(OutputCacheLocation.Server, 60)]
public IQueryable<DetectorStatu> GetDetectorDatabacklog(int userID)
{
    User user = ObjectContext.Users
    .Include("TeamMemberships")
    .First(u => u.UserID == userID);

    var teamIDs = user.TeamMemberships.Select(t => t.TeamID).ToList();
    var siteIDs = ObjectContext.SiteMemberships.Where(t => teamIDs.Contains(t.TeamID)).Select(t => t.SiteID).ToList();

    var DetectorStatus = ObjectContext.DetectorStatus
        .Include("Detector")
        .Include("Detector.Track")
        .Include("Detector.Site")
        .Include("Detector.EnabledDetectorTypes")
        .Include("Detector.EnabledDetectorTypes.DetectorType")
        .Where(d => siteIDs.Contains(d.Detector.SiteID))
        .OrderBy(d => d.Status);

    return DetectorStatus;
}

Now my question:

Is there a way to make this dynamic/shorter?

Becuase right now i check the filters via a if statement, and then run all the code. However, i want to add 8 more filter. And to add 8 more if statements will make the code really big and very unclear.
And i'm basically adding 50 lines of code, while i just need to change 1 line of the query.

Answer 1

You can build a query in multiple steps:

IQueryable<DetectorStatu> query = loadOperation.Entities.OrderBy(d => d.Status);

Then you add the filter, depending on your settings:

if (<filter is Atlas>)
{
  query = query.Where(d => ((d as DetectorStatu).Detector as Detector).EnabledDetectorTypes.Count(t => t.DetectorTypeID == 1) > 0);
}
else if (<filter is Phoenix>)
{
  query = query.Where(d => ((d as DetectorStatu).Detector as Detector).EnabledDetectorTypes.Count(t => t.DetectorTypeID == 2 || t.DetectorTypeID == 3) > 0);
}

etcetera. Of course you must substitute the correct code for <filter is ...> .

and finally you have only one foreach statement:

query = query.Take(25).Reverse();
foreach (DetectorStatu status in query) { ... }

Just one query will be sent to the database, at the moment you enter the foreach . This will take into account all steps from the "OrderBy" until the "Reverse".

Answer 2

It's possible to define a System.Linq.Expression:

Expression<Func<DetectorStatu, bool>> filter = null;

if(any){
   filter = x => true;
}

if(Atlas){
   filter = d => ((d as DetectorStatu).Detector as Detector).EnabledDetectorTypes.Count(t => t.DetectorTypeID == 1) > 0
}

if(Phoenix){
   filter = d => ((d as DetectorStatu).Detector as Detector).EnabledDetectorTypes.Count(t => t.DetectorTypeID == 2 || t.DetectorTypeID == 3) > 0;
}

In the end:

foreach (DetectorStatu status in loadOperation.Entities.OrderBy(d => d.Status)
   .Where(filter)
   .Take(25).Reverse())

Answer 3

Where clause simply expects a predicate (a Func that returns bool), which you can pass as an argument to your function. So basically you just can pass different predicate for each of your "Filter" option and apply it on your IQueryable for Where clause