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Build Where Clause Dynamically in Linq

, 6 May 2014 CPOL
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Build where clause dynamically in Linq

Introduction

Imagine a scenario in which we have a collection of objects and want to allow the user to filter the collection by filtering on combination of properties. To make the scenario concrete, let's assume that our object is declared as follows:

public class Person 
{
     public string Name        { get ; set ; }
     public string Surname     { get ; set ; }
     public int    Age         { get ; set ; }
     public string City        { get ; set ; }
     public double Salary      { get ; set ; }
     public bool   IsHomeOwner { get ; set ; }
}

Now suppose we have a collection of Persons like this (this is just for explanation purposes, in real life you would get a much bigger collection from database):

 List <Person> persons = new List <Person>
 {
     new  Person  { Name = "Flamur" , Surname = "Dauti" ,    Age = 39, 
                    City = "Prishtine" , IsHomeOwner = true ,  Salary = 12000.0 },

     new  Person  { Name = "Blerta" , Surname = "Frasheri" , Age = 25, 
                    City = "Mitrovice" , IsHomeOwner = false , Salary = 9000.0 },

     new  Person  { Name = "Berat" ,  Surname = "Dajti" ,    Age = 45, 
                    City = "Peje" ,      IsHomeOwner = true ,  Salary = 10000.0 },

     new  Person  { Name = "Laura" ,  Surname = "Morina" ,   Age = 23, 
                    City = "Mitrovice" , IsHomeOwner = true ,  Salary = 25000.0 },

     new  Person  { Name = "Olti" ,   Surname = "Kodra" ,    Age = 19, 
                    City = "Prishtine" , IsHomeOwner = false , Salary = 8000.0 },

     new  Person  { Name = "Xhenis" , Surname = "Berisha" ,  Age = 26, 
                    City = "Gjakove" ,   IsHomeOwner = false , Salary = 7000.0 },

     new  Person  { Name = "Fatos" ,  Surname = "Gashi" ,    Age = 32, 
                    City = "Peje" ,      IsHomeOwner = true ,  Salary = 6000.0 },

 };

Suppose we want to allow the user to filter the collection on any property or any combination of properties (on our UI form). One way would be to have a function for each property and each combination of properties, something like:

 public IList<Person> FilterByName(string  name)
 {
     return  persons.Where(p => p.Name == name).ToList();
 }
 
 public  IList<Person> FilterBySurname(string  surname)
 {
     return  persons.Where(p => p.Surname == surname).ToList();
 }
 
 public  IList<Person> FilterByNameAndSurname(string  name, string  surname)
 {
     return  persons.Where(p => p.Name == name && p.Surname == surname).ToList();
 }
 ...

As you can see, this becomes a very tedious job since the number of functions to cover all possible combinations is quite big.

The other way to filter the collection, which is much more convenient and tidier is to build an expression tree dynamically and pass it to the where clause for filtering. The function signature that will build expression trees will look like:

public  Func <Person , bool > Build(IList <Filter > filters)

where Filter class is declared as:

 public  class  Filter 
 {
     public  string  Property { get ; set ; }
     public  object  Value { get ; set ; }
 }

And it is used to contain the name of the property and the value that we want to filter our collection on. I won’t go into the details of how to build expression trees, since there is a lot of information about it on the web, so the class for building the expression trees looks like:

 public  class  PersonExpressionBuilder 
 {
     public static Func<Person, bool> Build(IList<Filter2> filters)
     {
         ParameterExpression param = Expression.Parameter(typeof(Person), "t" );
         Expression  exp = null ;
 
         if  (filters.Count == 1)
             exp = GetExpression(param, filters[0]);
         else  if  (filters.Count == 2)
             exp = GetExpression(param, filters[0], filters[1]);
         else 
         {
             while  (filters.Count > 0)
             {
                 var  f1 = filters[0];
                 var  f2 = filters[1];
 
                 if  (exp == null )
                     exp = GetExpression(param, filters[0], filters[1]);
                 else 
                     exp = Expression.AndAlso(exp, GetExpression(param, filters[0], filters[1]));
 
                 filters.Remove(f1);
                 filters.Remove(f2);
 
                 if (filters.Count == 1)
                 {
                     exp = Expression.AndAlso(exp, GetExpression(param, filters[0]));
                     filters.RemoveAt(0);
                 }
             }
         }
 
         return Expression.Lambda<Func<Person, bool>>(exp, param).Compile();
     }
 
     private static Expression GetExpression(ParameterExpression param, Filter2 filter)
     {
         MemberExpression member = Expression.Property(param, filter.PropertyName);
         ConstantExpression constant = Expression.Constant(filter.Value);
         return Expression.Equal(member, constant);
     }
 
     private static BinaryExpression GetExpression
     (ParameterExpression param, Filter2 filter1, Filter2 filter2)
     {
         Expression bin1 = GetExpression(param, filter1);
         Expression bin2 = GetExpression(param, filter2);
 
         return  Expression.AndAlso(bin1, bin2);
     }
 }

To test our expression builder, we would use it inside a method as follows:

 List<Filter> filter = new  List<Filter>
 {
     new  Filter  { PropertyName = "City" , Value = "Mitrovice"  },
     new  Filter  { PropertyName = "IsHomeOwner" , Value = false  }
 };
 
 var  deleg = PersonExpressionBuilder .Build(filter);
 var  filteredCollection = persons.Where(deleg).ToList();

As it is, the expression builder builds expression trees that check only if the value of the property is equal to the provided value, but we can take this approach one step further and make it generic so it can be used in other places with other types as well and also extend it so it can check for other comparisons as well. So our generic expression builder class will look like:

 public static class ExpressionBuilder 
 {
     private static MethodInfo containsMethod = typeof(string).GetMethod("Contains" );
     private static MethodInfo startsWithMethod = 
     typeof(string).GetMethod("StartsWith", new Type [] {typeof(string)});
     private static MethodInfo endsWithMethod = 
     typeof(string).GetMethod("EndsWith", new Type [] { typeof(string)});
 
 
     public static Expression<Func<T, 
     bool >> GetExpression<T>(IList<Filter> filters)
     {            
         if  (filters.Count == 0)
             return null ;
 
         ParameterExpression param = Expression.Parameter(typeof (T), "t" );
         Expression exp = null ;
 
         if  (filters.Count == 1)
             exp = GetExpression<T>(param, filters[0]);
         else  if  (filters.Count == 2)
             exp = GetExpression<T>(param, filters[0], filters[1]);
         else 
         {
             while  (filters.Count > 0)
             {
                 var  f1 = filters[0];
                 var  f2 = filters[1];
 
                 if  (exp == null )
                     exp = GetExpression<T>(param, filters[0], filters[1]);
                 else 
                     exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0], filters[1]));
 
                 filters.Remove(f1);
                 filters.Remove(f2);
 
                 if  (filters.Count == 1)
                 {
                     exp = Expression .AndAlso(exp, GetExpression<T>(param, filters[0]));
                     filters.RemoveAt(0);
                 }
             }
         }
 
         return Expression.Lambda<Func<T, bool>>(exp, param);
     }

     private static Expression GetExpression<T>(ParameterExpression param, Filter filter)
     {
         MemberExpression member = Expression.Property(param, filter.PropertyName);
         ConstantExpression constant = Expression.Constant(filter.Value);
 
         switch (filter.Operation)
         {
             case  Op.Equals:
                 return Expression.Equal(member, constant);
 
             case  Op.GreaterThan:
                 return Expression.GreaterThan(member, constant);
 
             case Op.GreaterThanOrEqual:
                 return Expression.GreaterThanOrEqual(member, constant);
 
             case Op.LessThan:
                 return Expression.LessThan(member, constant);
 
             case Op.LessThanOrEqual:
                 return Expression.LessThanOrEqual(member, constant);
 
             case Op.Contains:
                 return Expression.Call(member, containsMethod, constant);
 
             case Op.StartsWith:
                 return Expression.Call(member, startsWithMethod, constant);
 
             case Op.EndsWith:
                 return Expression.Call(member, endsWithMethod, constant);
         }
 
         return null ;
     }
 
     private static BinaryExpression GetExpression<T>
     (ParameterExpression param, Filter filter1, Filter  filter2)
     {
         Expression bin1 = GetExpression<T>(param, filter1);
         Expression bin2 = GetExpression<T>(param, filter2);
 
         return  Expression.AndAlso(bin1, bin2);
     }
 }

And the filter class has been extended to take a comparison operation as well:

 public class Filter 
 {
     public string PropertyName { get ; set ; }
     public Op Operation { get ; set ; }
     public object Value { get ; set ; }
 }

And the operation is declared as enumeration:

 public enum Op 
 {
     Equals,
     GreaterThan,
     LessThan,
     GreaterThanOrEqual,
     LessThanOrEqual,
     Contains,
     StartsWith,
     EndsWith
 }

Then the new generic expression builder would be used as follows:

 List<Filter> filter = new List<Filter>()
 {
     new Filter { PropertyName = "City" , 
         Operation = Op .Equals, Value = "Mitrovice"  },
     new Filter { PropertyName = "Name" , 
         Operation = Op .StartsWith, Value = "L"  },
     new Filter { PropertyName = "Salary" , 
         Operation = Op .GreaterThan, Value = 9000.0 }
 };
 
 var deleg = ExpressionBuilder.GetExpression<Person>(filter).Compile();
 var filteredCollection = persons.Where(deleg).ToList();

The ExpressionBuilder class can be extended for other Linq operations. It can also be easily used to remotely execute Linq statements (by making the class Filter serializable, it can be passed to a WCF service and so on).

UPDATE:

There are quite a few questions about implementing other features. The most important thing to remember when adding features is that the expression generates a C# executable code. So for example if your property is Nullable<int> then you need to remember that Nullable<int> is a different type from int, and more importantly it does NOT implement all the properties/methods that int does, of if the property is string then the operations <, > >= might return unexpected results.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Fitim Skenderi
Software Developer (Senior) KOS-LIGHTS.COM
United Kingdom United Kingdom
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Comments and Discussions

 
AnswerRe: Awesome job PinmemberFitim Skenderi19-Aug-14 23:41 
Questiondynamic linq The LINQ expression node type 'Invoke' is not supported Pinmembermsanaei13-Jul-14 6:30 
AnswerRe: dynamic linq The LINQ expression node type 'Invoke' is not supported PinmemberFitim Skenderi13-Jul-14 22:58 
QuestionIf there is no filter its giving error Pinprofessionalvenkatmca00825-Jun-14 5:28 
AnswerRe: If there is no filter its giving error PinmemberFitim Skenderi25-Jun-14 9:15 
GeneralRe: If there is no filter its giving error Pinprofessionalvenkatmca00826-Jun-14 0:18 
GeneralRe: If there is no filter its giving error PinmemberFitim Skenderi26-Jun-14 12:27 
QuestionMessage Removed PinmemberKentuckyEnglishman6-Jun-14 4:27 
Question".Equals" vs. ".Equal" [modified] PinmemberKentuckyEnglishman4-Jun-14 11:46 
AnswerRe: ".Equals" vs. ".Equal" PinmemberFitim Skenderi4-Jun-14 23:40 
GeneralRe: ".Equals" vs. ".Equal" [modified] PinmemberKentuckyEnglishman5-Jun-14 5:32 
GeneralRe: ".Equals" vs. ".Equal" PinmemberFitim Skenderi5-Jun-14 7:53 
GeneralRe: ".Equals" vs. ".Equal" PinmemberKentuckyEnglishman6-Jun-14 1:14 
QuestionNice! PinprofessionalVolynsky Alex6-May-14 23:31 
AnswerRe: Nice! PinmemberFitim Skenderi4-Jun-14 23:40 
QuestionRe: Nice! PinprofessionalVolynsky Alex5-Jun-14 4:16 
GeneralGood Article... PinmemberStuart_King6-May-14 7:11 
GeneralRe: Good Article... PinmemberFitim Skenderi6-May-14 11:25 
QuestiondynamicLinq from MS Pinmemberjogibear99886-May-14 6:32 
AnswerRe: dynamicLinq from MS PinmemberFitim Skenderi6-May-14 11:27 
QuestionMejora PinmemberMember 863676627-Feb-14 5:52 
AnswerRe: Mejora PinmemberFitim Skenderi6-May-14 0:36 
GeneralRe: Mejora Pinmemberdaniel_maldonado20-May-14 11:05 
QuestionGreat Article - DateTime question... PinmemberChuck Barest27-Feb-14 3:32 
AnswerRe: Great Article - DateTime question... PinmemberFitim Skenderi6-May-14 0:30 
QuestionComplete solution PinmemberCLEVERALMEIDA24-Feb-14 5:14 
AnswerRe: Complete solution PinmemberFitim Skenderi6-May-14 0:36 
QuestionI want to ask about Equal Operators ? Pinmemberkrmfhrckmk17-Feb-14 5:16 
AnswerRe: I want to ask about Equal Operators ? PinmemberFitim Skenderi6-May-14 0:35 
BugObject reference not set to an instance of an object Pinmemberlavanya_pothu@yahoo.com13-Feb-14 20:24 
GeneralRe: Object reference not set to an instance of an object PinmemberFitim Skenderi6-May-14 0:33 
GeneralRe: Object reference not set to an instance of an object PinmemberSheetalJain1-Nov-14 8:15 
GeneralRe: Object reference not set to an instance of an object [modified] PinmemberFitim Skenderi2-Nov-14 2:58 
QuestionCase Sensitive Values PinmemberJNDIONYSIM4-Feb-14 4:04 
AnswerRe: Case Sensitive Values PinmemberFitim Skenderi4-Feb-14 10:36 
GeneralRe: Case Sensitive Values PinmemberSheetalJain1-Nov-14 8:22 
GeneralRe: Case Sensitive Values PinmemberFitim Skenderi2-Nov-14 3:19 
QuestionHow would we go about if we have a List<something> in the Model? PinmemberMember 1036055025-Oct-13 9:20 
AnswerRe: How would we go about if we have a List<something> in the Model? PinmemberFitim Skenderi6-May-14 0:31 
QuestionDynamic Linq with In Clause PinmemberJosephineMui22-Aug-13 11:49 
AnswerRe: Dynamic Linq with In Clause PinmemberFitim Skenderi6-Sep-13 23:42 
BugThe binary operator Equal is not defined for the types 'System.Boolean' and 'System.String'. PinmemberSkork12-Jul-13 6:06 
GeneralRe: The binary operator Equal is not defined for the types 'System.Boolean' and 'System.String'. PinmemberFitim Skenderi19-Aug-13 0:08 
QuestionHo to incorporate DateTime type to the Dynamic expression PinmemberRey Sanchez17-May-13 5:01 
AnswerRe: Ho to incorporate DateTime type to the Dynamic expression PinmemberFitim Skenderi17-May-13 6:41 
GeneralRe: Ho to incorporate DateTime type to the Dynamic expression PinmemberRey Sanchez20-May-13 6:15 
GeneralRe: Ho to incorporate DateTime type to the Dynamic expression PinmemberFitim Skenderi3-Jun-13 0:39 
QuestionVery nice article Pinmemberdariushsarvari14-May-13 1:10 
AnswerRe: Very nice article PinmemberFitim Skenderi14-May-13 3:08 
Generalgood post PinmemberMember 394098624-Apr-13 11:40 

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