How to combine Linq query results

Question

I need to be able to add an unkown number of where clauses to a Linq query. Currently I have been trying to combine the queries using Concat(). For example:

var r1 =
    from field in db.fields
    where ID == 1
    select field;

var r2 = 
    from field in db.fields
    where ID == 2
    select field

var r3 = r1.Concat(r2);

This is giving me odd results, so I know that there must be a better way to do this. Is there any way to do something along the lines of an "accumulating result"; such as:

r1 = r1 + r2

Where r1 gets all of the results it had before plus all of the results from r2. This would allow me to iterate through a list of "where" filters and combine them as an OR statement would. Thanks to anyone who can help!

Answer 1

It sounds like you are looking for the Union operator:

var r1 = Context.Fields.Where<FieldObject>(f => f.ID == 1).Select<FieldObject, Field>(f => f.Field);
var r2 = Context.Fields.Where<FieldObject>(f => f.ID == 2).Select<FieldObject, Field>(f => f.Field);
var r3 = r1.Union<Field>(r2);

Answer 2

In case you are trying to do r1 = r1 + r2, then you should use some immediate query execution behavior, like ToList. You see, In LINQ the query is executed only if needed, and not at the place it was defined. So the query should look like

r1 = r1.Union(r2).ToList()

Also, notice the use of Union instead of Concat. "Concat method returns all the original elements in the input sequences. The Union method returns only unique elements." (from msdn)

EDIT: Or you could use PredicateBuilder to build the OR-ed predicate, somthing like

var predicate = PredicateBuilder.False<Field>();
foreach(Expression<Func<Field,Boolean>> filt in filters){
  predicate = predicate.Or(filt);
}
var result = db.fields.Where(predicate);