LINQ to Entities - Dynamic LINQ to Entities AND,OR

Recently I had an Advanced Search requirement which had to enable Users to specify a combination of fields to search for and the operator e.g. AND, OR and the type of comparison to do e.g. LIKE, EQUALS. The DataLayer is built upon the Entity Framework and I'm using LINQ to Entities as the query mechanism. So my overriding goal was to keep this Strongly Typed but maintainable as well. Firstly I created a generic SearchInfo object of which there would be one for each field that needed to be included in the search.

    public class SearchInfo<T>
{
  public Expression<Func<T, object>> PropertySelector { get; set; }
  public QueryOperator Operator { get; set; }
  public StringComparer Comparer { get; set; }
  public string Value { get; set; }
}

public enum QueryOperator
{
  And = 0,
  Or = 1
}

public enum StringComparer
{
  Equals = 0,
  StartsWith = 1,
  EndsWith = 2,
  Contains = 3
}

Now if we just had to deal with the AND operator we could achieve this by conditionally chaining together LINQ queries although this would mean a lot of unnecessary code and would soon become hard to maintain. I knew I would have to dynamically build the predicate and to support this in LINQ to Entities (because it doesn't support Expression.Invoke) and after scouring the interweb for a while I came across this example. This approach works well but I could see that building up the expressions for all the combinations would soon turn into a nightmare. e.g.

Expression<Func<Car, bool>> theCarIsRed = p => p.Color == "Red";

Expression<Func<Car, bool>> theCarIsCheap = c => c.Price <>

Expression<Func<Car, bool>> theCarIsRedOrCheap = theCarIsRed.Or(theCarIsCheap);

var query = carQuery.Where(theCarIsRedOrCheap);

Because I would have many combination's of operators and the comparers it would soon get messy. After some exploring through the endless MSDN docs on the System.Linq.Expressions namespace and with the help of some handy Reflection I came up with a generic PredicateBuilder class.

  public class PredicateBuilder
{
   public static Expression<Func<T, bool>> BuildPredicate<T>(List<SearchInfo<T>> searchInfos)
   {

       //set the default predicate
       Expression<Func<T, bool>> predicate = t => false;

       if (searchInfos == null || searchInfos.Count == 0)
           return predicate;

       ParameterExpression e = Expression.Parameter(typeof(T), "e");

       foreach (var searchInfo in searchInfos)
       {
           if (string.IsNullOrEmpty(searchInfo.Value))
               continue;
                     
           System.Reflection.MethodInfo methodInfo = GetMethodInfo(searchInfo.Comparer);

           string propertyName = ExpressionToPropertyName<T, object>(searchInfo.PropertySelector);

           Expression property = Expression.Property(e, propertyName);

           Expression comparer = Expression.Call(property, methodInfo, Expression.Constant(searchInfo.Value));

           LambdaExpression whereArg = Expression.Lambda(comparer, e);

           var exp = whereArg as Expression<Func<T, bool>>;

           predicate = searchInfo.Operator == QueryOperator.And
                       ? predicate.And(exp)
                       : predicate.Or(exp);

   
       }

       return predicate;
   }

   private static string ExpressionToPropertyName<T, T2>(Expression<Func<T, T2>> selector)
   {
       MemberExpression me = selector.Body as MemberExpression;
       if (me == null)
           throw new ArgumentException("MemberException expected.");

       if (me.Expression.NodeType != ExpressionType.Parameter)
           throw new ArgumentException("Invalid lambda expression");

       if (selector.Parameters[0] != me.Expression)
           throw new ArgumentException("Invalid lambda parameter.");

       return me.Member.Name;
   }


   private static System.Reflection.MethodInfo GetMethodInfo(StringComparer comparer)
   {
       Type[] types = new Type[1];
       types[0] = typeof(string);

       System.Reflection.MethodInfo methodInfo = typeof(string).GetMethod(comparer.ToString(), types);
       if (methodInfo == null)
           throw new ArgumentException("No method info supported for " + comparer.ToString());

       return methodInfo;

   }
}

So the full usage would be similar to this:

public class ProductController
  {
      [AcceptVerbs(HttpVerbs.Post)]
      public ActionResult FindProduct()
      {
           SearchInfo<Product> nameInfo = new SearchInfo<Product>()
          {
              Comparer = StringComparer.Contains, //this would come from a Request["key"] value
              PropertySelector = p => p.Name,
              Operator = QueryOperator.And, //this would come from a Request["key"] value
              Value = "SQL" //this would come from a Request["key"] value
          };

          SearchInfo<Product> descriptionInfo = new SearchInfo<Product>()
          {
              Comparer = StringComparer.Contains, //this would come from a Request["key"] value
              PropertySelector = p => p.Description, //this would come from a Request["key"] value
              Operator = QueryOperator.Or, //this would come from a Request["key"] value
              Value = "database" //this would come from a textbox or some other input
          };

          List<SearchInfo<Product>> searchInfo = new List<SearchInfo<Product>>
          {
              nameInfo,
              descriptionInfo
          };

          ViewData.Model = ProductService.Find(searchInfo);
          return View();

      }

  public class ProductService
  {
      public static List<Product> Find(List<SearchInfo<Product>> searchInfo)
      {
        
          return ProductRepository.Find(searchInfo);

      }
  }

  public class ProductRepository
  {
      public static List<Product> Find(List<SearchInfo<Product>> searchInfo)
      {
          if (searchInfo == null || searchInfo.Count == 0)
              return null;

          using (ProductDemoEntities context = new ProductDemoEntities())
          {

              var query = context.Product.Where(PredicateBuilder.BuildPredicate<Product>(searchInfo));
                        
              return query.ToList();
          }

      }
  }

I've tried to keep this as simple as possible, in reality you would likely want to extend this so that you could search on all Types of fields instead of just the string as shown here. I hope this able to help someone else who has faced this. Until next time.