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How to avoid the "System.InvalidOperationException: The client projection contains a reference to a constant expression" error?

I am writing a C# .NET 5 application that will act as a backend for an Angular frontend, providing CRUD APIs. The purpose of the app is managing a flight school.
I am writing the API methods that will return the list of pilots and the single pilot, that is https://myapp/api/pilots and https://myapp/api/pilots/{id}.
I have three methods here:
GetPilots() for the complete list
GetPilot(long idPilot) for the detail
an auxiliary method GetFlightTime(long idPilot) to return the total flight time of each pilot doing the sum of the flight durations.
My problem: the detail method works because I first call the auxiliary function and then I use the result in the returned viewmodel. But GetPilots() doesn't work and returns the error System.InvalidOperationException: The client projection contains a reference to a constant expression of 'QTBWeb.Models.PilotsRepository' through the instance method 'GetFlightTime'. This could potentially cause a memory leak; consider making the method static so that it does not capture constant in the instance.
Is this because I am calling the GetFlightTime method inside the LINQ expression? I don't understand the make the method static suggestion. How can I reformat the code to make it work?
Thanks!
public IEnumerable<PilotViewModel> GetPilots() // THIS METHOD RETURNS ERROR
{
return _context.Pilots
.Select(pilot => new PilotViewModel
{
Id = pilot.Id,
Name = pilot.Name,
FlightMinutes = GetFlightTime(pilot.Id) // THE PROBLEM IS HERE
})
.ToList();
}
public PilotViewModel GetPilot(long idPilot) // THIS METHOD WORKS
{
var _flightMinutes = GetFlightTime(idPilot);
return _context.Pilots
.Select(pilot => new PilotViewModel
{
Id = pilot.Id,
Name = pilot.Name,
FlightMinutes = _flightMinutes
})
.Where(pilot => pilot.Id == idPilot)
.FirstOrDefault();
}
public int GetFlightTime(long idPilot)
{
return _context.Flights
.Where(flight => flight.pilot == idPilot)
.Select(flight => flight.Duration).Sum();
}

A good way to solve this would be to make sure that your Pilot class has a collection of Flights, serving as the other side of the one-to-many map you have as Flight.Pilot.
You can then use this collection to calculate the sum, without having to query the database for every looped instance of Pilot.
Your code would look something like this:
public IEnumerable<PilotViewModel> GetPilots()
{
return _context.Pilots
.Include(pilot => pilot.Flights) // Include Flights to join data
.Select(pilot => new PilotViewModel
{
Id = pilot.Id,
Name = pilot.Name,
FlightMinutes = pilot.Flights.Sum(flight => flight.Duration)
});
}
public PilotViewModel GetPilot(long idPilot)
{
return _context.Pilots
.Include(pilot => pilot.Flights) // Include Flights to join data
.Where(pilot => pilot.Id == idPilot) // Notice how we filter first
.Select(pilot => new PilotViewModel
{
Id = pilot.Id,
Name = pilot.Name,
FlightMinutes = pilot.Flights.Sum(flight => flight.Duration)
})
.FirstOrDefault();
}

Join LINQ-query with predicate

I've some problems with a LINQ query in C#.
I have in the database the same tables that have the same structure.
So, today, I've been troubling with my LINQ query.
More details, I want to join some tables using predicates.
I have a function that has two parameters.
The first parameter is some kind of Context (For example, it may be ProductContext, CarContext, CatContext and etc).
The second parameter is a List<something> that I will join with my first parameter - Context.
I do not want a set of methods.
I've added the sample:
public Element[] GetByIds( MyPredicateContext, Guid[] ids)
{
return
from id in ids
join element in MyPredicateContext on id equals element.Id
select
new Element
{
Id = element.Id,
Description = element.JobDescription,
};
}

If the query is correct, one basic issue that I can see is the return type is Element array whereas you are trying to return IEnumerable. Maybe doing a .ToArray() on the result set might solve the problem.

Why not
return MyPredicateContext.Where(element=>ids.Contains(element.Id))
.Select(e=>new Element()
{
Id = element.Id,
Description = element.JobDescription
}).ToArray();

First of all you can't create a new IQueryable from an array this will revert to pulling everything in memory and filtering there. You are working with expressions and not c# code when you do LINQ with SQL, this will only work on in memory stuff (IEnumerable).
Your query will work in SQL if you do it like this
from element in MyPredicateContext
where ids.Contains(element.Id)
select new Element
{
Id = element.Id,
Description = element.JobDescription,
}
Given that the type of IQueryable where T is an Interface or class.
The end method will look something like this
public interface IElement
{
Guid Id { get; }
string JobDescription { get; }
}
public Element[] GetByIds<T>(IQueryable<T> myPredicateContext, Guid[] ids) where T:IElement
{
return (from element in myPredicateContext
where ids.Contains(element.Id)
select new Element
{
Id = element.Id,
Description = element.JobDescription,
}).ToArray();
}
There are ways to do it with no Generics but they are a bit more advanced and will be hard to maintain.
Here is a method that will work on all T types and proper IQueryable will produce good sql just as I pointed out is a bit more advanced and you will need to lookup how expression work.
public static Element[] GetById<T, Tkey>(IQueryable<T> items,Tkey[] ids)
{
var type = typeof(T);
ParameterExpression param = Expression.Parameter(type);
var list = Expression.Constant(ids);
//The names of the properties you need to get if all models have them and are named the same and are the same type this will work
var idProp = Expression.Property(param, "Id");
var descriptionProp = Expression.Property(param, "JobDescription");
var contains = typeof(Enumerable).GetMethods().First(m => m.Name == "Contains" && m.GetParameters().Count() == 2).MakeGenericMethod(typeof(Tkey));
var where = Expression.Lambda<Func<T, bool>>(Expression.Call(contains, list, idProp), param);
return (items.
Where(where).
Select(Expression.Lambda<Func<T, Element>>(
Expression.MemberInit(
Expression.New(typeof(Element)),
Expression.Bind(typeof(Element).GetProperty("Id"), idProp),
Expression.Bind(typeof(Element).GetProperty("Description"), descriptionProp)),
param))).ToArray();
}
Call GetById(items, new Guid[] { Guid.NewGuid() })

LINQ with Querying "Memory"

Does LINQ have a way to "memorize" its previous query results while querying?
Consider the following case:
public class Foo {
public int Id { get; set; }
public ICollection<Bar> Bars { get; set; }
}
public class Bar {
public int Id { get; set; }
}
Now, if two or more Foo have same collection of Bar (no matter what the order is), they are considered as similar Foo.
Example:
foo1.Bars = new List<Bar>() { bar1, bar2 };
foo2.Bars = new List<Bar>() { bar2, bar1 };
foo3.Bars = new List<Bar>() { bar3, bar1, bar2 };
In the above case, foo1 is similar to foo2 but both foo1 and foo2 are not similar tofoo3
Given that we have a query result consisting IEnumerable or IOrderedEnumerable of Foo. From the query, we are to find the first N foo which are not similar.
This task seems to require a memory of the collection of bars which have been chosen before.
With partial LINQ we could do it like this:
private bool areBarsSimilar(ICollection<Bar> bars1, ICollection<Bar> bars2) {
return bars1.Count == bars2.Count && //have the same amount of bars
!bars1.Select(x => x.Id)
.Except(bars2.Select(y => y.Id))
.Any(); //and when excepted does not return any element mean similar bar
}
public void somewhereWithQueryResult(){
.
.
List<Foo> topNFoos = new List<Foo>(); //this serves as a memory for the previous query
int N = 50; //can be any number
foreach (var q in query) { //query is IOrderedEnumerable or IEnumerable
if (topNFoos.Count == 0 || !topNFoos.Any(foo => areBarsSimilar(foo.Bars, q.Bars)))
topNFoos.Add(q);
if (topNFoos.Count >= N) //We have had enough Foo
break;
}
}
The topNFoos List will serve as a memory of the previous query and we can skip the Foo q in the foreach loop which already have identical Bars with Any of the Foo in the topNFoos.
My question is, is there any way to do that in LINQ (fully LINQ)?
var topNFoos = from q in query
//put something
select q;
If the "memory" required is from a particular query item q or a variable outside of the query, then we could use let variable to cache it:
int index = 0;
var topNFoos = from q in query
let qc = index++ + q.Id //depends on q or variable outside like index, then it is OK
select q;
But if it must come from the previous querying of the query itself then things start to get more troublesome.
Is there any way to do that?
Edit:
(I currently am creating a test case (github link) for the answers. Still figuring out how can I test all the answers fairly)
(Most of the answers below are aimed to solve my particular question and are in themselves good (Rob's, spender's, and David B's answers which use IEqualityComparer are particularly awesome). Nevertheless, if there is anyone who can give answer to my more general question "does LINQ have a way to "memorize" its previous query results while querying", I would also be glad)
(Apart from the significant difference in performance for the particular case I presented above when using fully/partial LINQ, one answer aiming to answer my general question about LINQ memory is Ivan Stoev's. Another one with good combination is Rob's. As to make myself clearer, I look for general and efficient solution, if there is any, using LINQ)

I'm not going to answer your question directly, but rather, propose a method that will be fairly optimally efficient for filtering the first N non-similar items.
First, consider writing an IEqualityComparer<Foo> that uses the Bars collection to measure equality. Here, I'm assuming that the lists might contain duplicate entries, so have quite a strict definition of similarity:
public class FooSimilarityComparer:IEqualityComparer<Foo>
{
public bool Equals(Foo a, Foo b)
{
//called infrequently
return a.Bars.OrderBy(bar => bar.Id).SequenceEqual(b.Bars.OrderBy(bar => bar.Id));
}
public int GetHashCode(Foo foo)
{
//called frequently
unchecked
{
return foo.Bars.Sum(b => b.GetHashCode());
}
}
}
You can really efficiently get the top N non-similar items by using a HashSet with the IEqualityComparer above:
IEnumerable<Foo> someFoos; //= some list of Foo
var hs = new HashSet<Foo>(new FooSimilarityComparer());
foreach(var f in someFoos)
{
hs.Add(f); //hashsets don't add duplicates, as measured by the FooSimilarityComparer
if(hs.Count >= 50)
{
break;
}
}
#Rob s approach above is broadly similar, and shows how you can use the comparer directly in LINQ, but pay attention to the comments I made to his answer.

So, it's ... possible. But this is far from performant code.
var res = query.Select(q => new {
original = q,
matches = query.Where(innerQ => areBarsSimilar(q.Bars, innerQ.Bars))
}).Select(g => new { original = g, joinKey = string.Join(",", g.matches.Select(m => m.Id)) })
.GroupBy (g => g.joinKey)
.Select(g => g.First().original.original)
.Take(N);
This assumes that the Ids are unique for each Foo (you could also use their GetHashCode(), I suppose).
A much better solution is to either keep what you've done, or implement a custom comparer, as follows:
Note: As pointed out in the comments by #spender, the below Equals and GetHashCode will not work for collections with duplicates. Refer to their answer for a better implementation - however, the usage code would remain the same
class MyComparer : IEqualityComparer<Foo>
{
public bool Equals(Foo left, Foo right)
{
return left.Bars.Count() == right.Bars.Count() && //have the same amount of bars
left.Bars.Select(x => x.Id)
.Except(right.Bars.Select(y => y.Id))
.ToList().Count == 0; //and when excepted returns 0, mean similar bar
}
public int GetHashCode(Foo foo)
{
unchecked {
int hc = 0;
if (foo.Bars != null)
foreach (var p in foo.Bars)
hc ^= p.GetHashCode();
return hc;
}
}
}
And then your query becomes simply:
var res = query
.GroupBy (q => q, new MyComparer())
.Select(g => g.First())
.Take(N);

IEnumerable<Foo> dissimilarFoos =
from foo in query
let key = string.Join('|',
from bar in foo.Bars
order by bar.Id
select bar.Id.ToString())
group foo by key into g
select g.First();
IEnumerable<Foo> firstDissimilarFoos =
dissimilarFoos.Take(50);
Sometimes, you may not like the behavior of groupby in the above queries. At the time the query is enumerated, groupby will enumerate the entire source. If you only want partial enumeration, then you should switch to Distinct and a Comparer:
class FooComparer : IEqualityComparer<Foo>
{
private string keyGen(Foo foo)
{
return string.Join('|',
from bar in foo.Bars
order by bar.Id
select bar.Id.ToString());
}
public bool Equals(Foo left, Foo right)
{
if (left == null || right == null) return false;
return keyGen(left) == keyGen(right);
}
public bool GetHashCode(Foo foo)
{
return keyGen(foo).GetHashCode();
}
}
then write:
IEnumerable<Foo> dissimilarFoos = query.Distinct(new FooComparer());
IEnumerable<Foo> firstDissimilarFoos = dissimilarFoos.Take(50);

Idea. You might be able to hack something by devising your own fluent interface of mutators over a cache that you'd capture in "let x = ..." clauses, along the lines of,
from q in query
let qc = ... // your cache mechanism here
select ...
but I suspect you'll have to be careful to limit the updates to your cache to those "let ..." only, as I doubt the implementation of the standard Linq operators and extensions methods will be happy if you allow such side effects to happen in their back through predicates applied in the "where", or "join", "group by", etc, clauses.
'HTH,

I guess by "full LINQ" you mean standard LINQ operators/Enumerable extension methods.
I don't think this can be done with LINQ query syntax. From standard methods the only one that supports mutable processing state is Enumerable.Aggregate, but it gives you nothing more than a LINQ flavor over the plain foreach:
var result = query.Aggregate(new List<Foo>(), (list, next) =>
{
if (list.Count < 50 && !list.Any(item => areBarsSimilar(item.Bars, next.Bars)))
list.Add(next);
return list;
});
Since looks like we are allowed to use helper methods (like areBarsSimilar), the best we can do is to make it at least look more LINQ-ish by defining and using a custom extension method
var result = query.Aggregate(new List<Foo>(), (list, next) => list.Count < 50 &&
!list.Any(item => areBarsSimilar(item.Bars, next.Bars)) ? list.Concat(next) : list);
where the custom method is
public static class Utils
{
public static List<T> Concat<T>(this List<T> list, T item) { list.Add(item); return list; }
}
But note that compared to vanilla foreach, Aggregate has an additional drawback of not being able to exit earlier, thus will consume the whole input sequence (which besides the performance also means it doesn't work with infinite sequences).
Conclusion: While this should answer your original question, i.e. it's technically possible to do what you are asking for, LINQ (like the standard SQL) is not well suited for such type of processing.

How to create a query object with Func / Expressions in C#?

I want to create a reusable query object for my project that uses ElasticSearch. I am already using a similar QueryObject from the Generic Unit of Work/Repositories by LongLe for queries against the database using Entity Framework.
I can't really seem to wrap my head around exactly how to do this - I'm not sure how to "chain" the parts of the lambda expression together. I've tried using Expression/Func but this is a new area for me that I do not fully understand. I feel as though I'm just stabbing in the dark.
Since I don't even know exactly how to word my question, here's an example of what I am currently doing, what I am trying to do, and my progress so far:
What I currently have to do:
ISearchResponse<DemoIndexModel> result = client.Search<DemoIndexModel>(s => s.Query(
q => q.Term(t => t.FirstName, firstName)
&& q.Term(t => t.LastName, lastName)));
What I would like to do:
var query = new DemoIndexQuery();
query = query.ByFirstName(firstName);
query = query.ByLastName(lastName);
result = client.Search<DemoIndexModel>(s => s.Query(query.Compile()));
Code so far:
public abstract class ElasticQueryObject<T> where T : class
{
private Func<QueryDescriptor<T>, QueryContainer> _query;
// tried using Expression, still completely lost
private Expression<Func<QueryDescriptor<T>, QueryContainer>> _expression;
public Func<QueryDescriptor<T>, QueryContainer> Compile()
{
return _query;
}
public Func<QueryDescriptor<T>, QueryContainer> And(Func<QueryDescriptor<T>, QueryContainer> query)
{
if (_query == null)
{
_query = query;
}
else
{
// how do I chain the query??? I only can figure out how to set it.
}
return null;
}
}
public class DemoIndexQuery : ElasticQueryObject<DemoIndexModel>
{
public DemoIndexQuery ByFirstName(string firstName)
{
And(p => p.Term(term => term.FirstName, firstName));
return this;
}
public DemoIndexQuery ByLastName(string lastName)
{
And(p => p.Term(term => term.LastName, lastName));
return this;
}
}

You're breaking the LINQ contract. The query should be immutable - all the methods operating on the query should return a new query instead of modifying the old one. Due to the way the query is built, this is intrinsically composable, so instead of
public DemoIndexQuery ByFirstName(string firstName)
{
And(p => p.Term(term => term.FirstName, firstName));
return this;
}
you can just use this:
public DemoIndexQuery ByFirstName(string firstName)
{
return Where(p => p.Term(term => term.FirstName, firstName));
}
If this is not possible for some reason, you'll need to handle building the expression tree yourself, before you pass it forward. The simplest way would be something like this:
Expression<...> oldQuery = ...;
var newCondition = (Expression<...>)(p => p.Term(...));
return Expression.And(oldQuery, newCondition);
If your query provider doesn't support this, you'll need a bit more work - you can build the whole where predicate yourself separately, and then make sure you fix the lambdas and lambda parameters.

How would you refactor this LINQ code?

I've got a lot of ugly code that looks like this:
if (!string.IsNullOrEmpty(ddlFileName.SelectedItem.Text))
results = results.Where(x => x.FileName.Contains(ddlFileName.SelectedValue));
if (chkFileName.Checked)
results = results.Where(x => x.FileName == null);
if (!string.IsNullOrEmpty(ddlIPAddress.SelectedItem.Text))
results = results.Where(x => x.IpAddress.Contains(ddlIPAddress.SelectedValue));
if (chkIPAddress.Checked)
results = results.Where(x => x.IpAddress == null);
...etc.
results is an IQueryable<MyObject>.
The idea is that for each of these innumerable dropdowns and checkboxes, if the dropdown has something selected, the user wants to match that item. If the checkbox is checked, the user wants specifically those records where that field is null or an empty string. (The UI doesn't let both be selected at the same time.) This all adds to the LINQ Expression which gets executed at the end, after we've added all the conditions.
It seems like there ought to be some way to pull out an Expression<Func<MyObject, bool>> or two so that I can put the repeated parts in a method and just pass in what changes. I've done this in other places, but this set of code has me stymied. (Also, I'd like to avoid "Dynamic LINQ", because I want to keep things type-safe if possible.) Any ideas?

I'd convert it into a single Linq statement:
var results =
//get your inital results
from x in GetInitialResults()
//either we don't need to check, or the check passes
where string.IsNullOrEmpty(ddlFileName.SelectedItem.Text) ||
x.FileName.Contains(ddlFileName.SelectedValue)
where !chkFileName.Checked ||
string.IsNullOrEmpty(x.FileName)
where string.IsNullOrEmpty(ddlIPAddress.SelectedItem.Text) ||
x.FileName.Contains(ddlIPAddress.SelectedValue)
where !chkIPAddress.Checked ||
string.IsNullOrEmpty(x. IpAddress)
select x;
It's no shorter, but I find this logic clearer.

In that case:
//list of predicate functions to check
var conditions = new List<Predicate<MyClass>>
{
x => string.IsNullOrEmpty(ddlFileName.SelectedItem.Text) ||
x.FileName.Contains(ddlFileName.SelectedValue),
x => !chkFileName.Checked ||
string.IsNullOrEmpty(x.FileName),
x => string.IsNullOrEmpty(ddlIPAddress.SelectedItem.Text) ||
x.IpAddress.Contains(ddlIPAddress.SelectedValue),
x => !chkIPAddress.Checked ||
string.IsNullOrEmpty(x.IpAddress)
}
//now get results
var results =
from x in GetInitialResults()
//all the condition functions need checking against x
where conditions.All( cond => cond(x) )
select x;
I've just explicitly declared the predicate list, but these could be generated, something like:
ListBoxControl lbc;
CheckBoxControl cbc;
foreach( Control c in this.Controls)
if( (lbc = c as ListBoxControl ) != null )
conditions.Add( ... );
else if ( (cbc = c as CheckBoxControl ) != null )
conditions.Add( ... );
You would need some way to check the property of MyClass that you needed to check, and for that you'd have to use reflection.

Have you seen the LINQKit? The AsExpandable sounds like what you're after (though you may want to read the post Calling functions in LINQ queries at TomasP.NET for more depth).

Don't use LINQ if it's impacting readability. Factor out the individual tests into boolean methods which can be used as your where expression.
IQueryable<MyObject> results = ...;
results = results
.Where(TestFileNameText)
.Where(TestFileNameChecked)
.Where(TestIPAddressText)
.Where(TestIPAddressChecked);
So the the individual tests are simple methods on the class. They're even individually unit testable.
bool TestFileNameText(MyObject x)
{
return string.IsNullOrEmpty(ddlFileName.SelectedItem.Text) ||
x.FileName.Contains(ddlFileName.SelectedValue);
}
bool TestIPAddressChecked(MyObject x)
{
return !chkIPAddress.Checked ||
x.IpAddress == null;
}

results = results.Where(x =>
(string.IsNullOrEmpty(ddlFileName.SelectedItem.Text) || x.FileName.Contains(ddlFileName.SelectedValue))
&& (!chkFileName.Checked || string.IsNullOrEmpty(x.FileName))
&& ...);

Neither of these answers so far is quite what I'm looking for. To give an example of what I'm aiming at (I don't regard this as a complete answer either), I took the above code and created a couple of extension methods:
static public IQueryable<Activity> AddCondition(
this IQueryable<Activity> results,
DropDownList ddl,
Expression<Func<Activity, bool>> containsCondition)
{
if (!string.IsNullOrEmpty(ddl.SelectedItem.Text))
results = results.Where(containsCondition);
return results;
}
static public IQueryable<Activity> AddCondition(
this IQueryable<Activity> results,
CheckBox chk,
Expression<Func<Activity, bool>> emptyCondition)
{
if (chk.Checked)
results = results.Where(emptyCondition);
return results;
}
This allowed me to refactor the code above into this:
results = results.AddCondition(ddlFileName, x => x.FileName.Contains(ddlFileName.SelectedValue));
results = results.AddCondition(chkFileName, x => x.FileName == null || x.FileName.Equals(string.Empty));
results = results.AddCondition(ddlIPAddress, x => x.IpAddress.Contains(ddlIPAddress.SelectedValue));
results = results.AddCondition(chkIPAddress, x => x.IpAddress == null || x.IpAddress.Equals(string.Empty));
This isn't quite as ugly, but it's still longer than I'd prefer. The pairs of lambda expressions in each set are obviously very similar, but I can't figure out a way to condense them further...at least not without resorting to dynamic LINQ, which makes me sacrifice type safety.
Any other ideas?

#Kyralessa,
You can create extension method AddCondition for predicates that accepts parameter of type Control plus lambda expression and returns combined expression. Then you can combine conditions using fluent interface and reuse your predicates. To see example of how it can be implemented see my answer on this question:
How do I compose existing Linq Expressions

I'd be wary of the solutions of the form:
// from Keith
from x in GetInitialResults()
//either we don't need to check, or the check passes
where string.IsNullOrEmpty(ddlFileName.SelectedItem.Text) ||
x.FileName.Contains(ddlFileName.SelectedValue)
My reasoning is variable capture. If you're immediately execute just the once you probably won't notice a difference. However, in linq, evaluation isn't immediate but happens each time iterated occurs. Delegates can capture variables and use them outside the scope you intended.
It feels like you're querying too close to the UI. Querying is a layer down, and linq isn't the way for the UI to communicate down.
You may be better off doing the following. Decouple the searching logic from the presentation - it's more flexible and reusable - fundamentals of OO.
// my search parameters encapsulate all valid ways of searching.
public class MySearchParameter
{
public string FileName { get; private set; }
public bool FindNullFileNames { get; private set; }
public void ConditionallySearchFileName(bool getNullFileNames, string fileName)
{
FindNullFileNames = getNullFileNames;
FileName = null;
// enforce either/or and disallow empty string
if(!getNullFileNames && !string.IsNullOrEmpty(fileName) )
{
FileName = fileName;
}
}
// ...
}
// search method in a business logic layer.
public IQueryable<MyClass> Search(MySearchParameter searchParameter)
{
IQueryable<MyClass> result = ...; // something to get the initial list.
// search on Filename.
if (searchParameter.FindNullFileNames)
{
result = result.Where(o => o.FileName == null);
}
else if( searchParameter.FileName != null )
{ // intermixing a different style, just to show an alternative.
result = from o in result
where o.FileName.Contains(searchParameter.FileName)
select o;
}
// search on other stuff...
return result;
}
// code in the UI ...
MySearchParameter searchParameter = new MySearchParameter();
searchParameter.ConditionallySearchFileName(chkFileNames.Checked, drpFileNames.SelectedItem.Text);
searchParameter.ConditionallySearchIPAddress(chkIPAddress.Checked, drpIPAddress.SelectedItem.Text);
IQueryable<MyClass> result = Search(searchParameter);
// inform control to display results.
searchResults.Display( result );
Yes it's more typing, but you read code around 10x more than you write it. Your UI is clearer, the search parameters class takes care of itself and ensures mutually exclusive options don't collide, and the search code is abstracted away from any UI and doesn't even care if you use Linq at all.

Since you are wanting to repeatedly reduce the original results query with innumerable filters, you can use Aggregate(), (which corresponds to reduce() in functional languages).
The filters are of predictable form, consisting of two values for every member of MyObject - according to the information I gleaned from your post. If every member to be compared is a string, which may be null, then I recommend using an extension method, which allows for null references to be associated to an extension method of its intended type.
public static class MyObjectExtensions
{
public static bool IsMatchFor(this string property, string ddlText, bool chkValue)
{
if(ddlText!=null && ddlText!="")
{
return property!=null && property.Contains(ddlText);
}
else if(chkValue==true)
{
return property==null || property=="";
}
// no filtering selected
return true;
}
}
We now need to arrange the property filters in a collection, to allow for iterating over many. They are represented as Expressions for compatibility with IQueryable.
var filters = new List<Expression<Func<MyObject,bool>>>
{
x=>x.Filename.IsMatchFor(ddlFileName.SelectedItem.Text,chkFileName.Checked),
x=>x.IPAddress.IsMatchFor(ddlIPAddress.SelectedItem.Text,chkIPAddress.Checked),
x=>x.Other.IsMatchFor(ddlOther.SelectedItem.Text,chkOther.Checked),
// ... innumerable associations
};
Now we aggregate the innumerable filters onto the initial results query:
var filteredResults = filters.Aggregate(results, (r,f) => r.Where(f));
I ran this in a console app with simulated test values, and it worked as expected. I think this at least demonstrates the principle.

One thing you might consider is simplifying your UI by eliminating the checkboxes and using an "<empty>" or "<null>" item in your drop down list instead. This would reduce the number of controls taking up space on your window, remove the need for complex "enable X only if Y is not checked" logic, and would enable a nice one-control-per-query-field.
Moving on to your result query logic, I would start by creating a simple object to represent a filter on your domain object:
interface IDomainObjectFilter {
bool ShouldInclude( DomainObject o, string target );
}
You can associate an appropriate instance of the filter with each of your UI controls, and then retrieve that when the user initiates a query:
sealed class FileNameFilter : IDomainObjectFilter {
public bool ShouldInclude( DomainObject o, string target ) {
return string.IsNullOrEmpty( target )
|| o.FileName.Contains( target );
}
}
...
ddlFileName.Tag = new FileNameFilter( );
You can then generalize your result filtering by simply enumerating your controls and executing the associated filter (thanks to hurst for the Aggregate idea):
var finalResults = ddlControls.Aggregate( initialResults, ( c, r ) => {
var filter = c.Tag as IDomainObjectFilter;
var target = c.SelectedValue;
return r.Where( o => filter.ShouldInclude( o, target ) );
} );
Since your queries are so regular, you might be able to simplify the implementation even further by using a single filter class taking a member selector:
sealed class DomainObjectFilter {
private readonly Func<DomainObject,string> memberSelector_;
public DomainObjectFilter( Func<DomainObject,string> memberSelector ) {
this.memberSelector_ = memberSelector;
}
public bool ShouldInclude( DomainObject o, string target ) {
string member = this.memberSelector_( o );
return string.IsNullOrEmpty( target )
|| member.Contains( target );
}
}
...
ddlFileName.Tag = new DomainObjectFilter( o => o.FileName );