Add dynamically predicate to expression without PredicateBuilder

Question

I use linqToEntities and would like to add OR condition dynamically.

I know that there is a great library PredicateBuilder by brother Albahari and it can solve my task, but I cannot use it.

My conditions looks like this:

IEnumerable<Condition> conditions = new List<Condition>()
{
    new Condition(){IdReference = 1, TableName = "Table1" },
    new Condition(){IdReference = 2, TableName = "Table2" },
    new Condition(){IdReference = 3, TableName = "Table3" }, 
    // and so on
};

What I have is:

var histories = db.History as IQueryable<History>;
foreach (var cond in conditions)
{
     //What code should be here to be translated into:
     /*
     histories = histories
        .Where(h => h.IdReference == 1 && h.TableName =="Table1" 
            || h.IdReference == 2 && h.TableName == "Table2"
            || h.IdReference == 3 && h.TableName == "Table3");
            // and so on
     */
}

However, I do not know in advance how many conditions would be. How is it possible to add OR conditions dynamically from IEnumerable<Condition>?

If you don't have too many combinations, just use some conditions: `if(thisIsTheCase){ histories = histories.Where(// put your conditions} else if(another Case){ histories = histories.Where(// put your conditions}` add more if else until you have covered all cases. You can also do it by creating dynamic expressions but it will not be easy because you will need to combine the expressions. — CodingYoshi, Nov 11 '18 at 14:21

score 2 · Accepted Answer · edited Nov 11 '18 at 18:39

Not sure what's the problem with using predicate builder - it doesn't have to be LINQ Kit package, the so called predicate builder is usually a single static class with 2 extension methods - like Universal Predicate Builder or my own PredicateUtils from Establish a link between two lists in linq to entities where clause and similar.

Anyway, once you want it, of course it could be built using just plain Expression class static methods.

Add the following in order to eliminate the need of writing Expression. before each call:

using static System.Linq.Expressions.Expression;

and then use something like this:

if (conditions.Any())
{
    var parameter = Parameter(typeof(History));
    var body = conditions
        .Select(condition => Expression.Constant(condition))
        .Select(condition => Expression.AndAlso(
        Expression.Equal(
            Expression.PropertyOrField(parameter, nameof(History.IdReference)),
            Expression.Convert(
                  Expression.PropertyOrField(condition, nameof(Condition.IdReference))
                , Expression.PropertyOrField(parameter, nameof(History.IdReference)).Type)
        ),
        Expression.Equal(
            Expression.PropertyOrField(parameter, nameof(History.TableName)),
            Expression.Convert(
                  Expression.PropertyOrField(condition, nameof(Condition.TableName))
                , Expression.PropertyOrField(parameter, nameof(History.TableName)).Type)
         )
        ))
        .Aggregate(Expression.OrElse);                      
    var predicate = Lambda<Func<History, bool>>(body, parameter);
    histories = histories.Where(predicate);
}

score 0 · Answer 2 · answered Nov 11 '18 at 13:58

0

As I understand you mean

var histories = db.History as IQueryable<History>;
foreach (var cond in conditions)
{
     //What code should be here to be translated into:

     histories = histories
        .Where(h => h.IdReference == cond.IdReference && 
      h.TableName ==cond.TableName );

}

answered Nov 11 '18 at 13:58

Hitham Yosri

16
2

1

He does not know the conditions until runtime. This will work if they were known. – CodingYoshi Nov 11 '18 at 14:23
As I understood he dose not know how many conditions in History list but he have fixed conditions in conditionList so looping in conditionList and comparing !! As I thought :) – Hitham Yosri Nov 11 '18 at 14:52
Chaining `Where`s is the equivilent of `and` not `or`. – David Browne - Microsoft Nov 11 '18 at 15:56

Add dynamically predicate to expression without PredicateBuilder

2 Answers2