Dynamic Linq using Or

Question

I have the following dynamic linq

var results=(from a in alist where a.id==id select a)
if(...something)
{
results=(from a in results where a.amount>input1 && a.typeId==1 select a)
}
if(...something else)
{
results=(from a in results where a.amount>input2 && a.typeId==2 select a)
}
if(...something else again)
{
results=(from a in results where a.amount>input3 && a.typeId==3 select a)
}

However this produces an AND statement which means all the statements need to be true for anything to be returned.

I need the last 3 statements to be ORed together.

eg I want

Where (a.id==id) && ((a.amount>input1 && a.typeId==1) || (a.amount>input2 && a.typeId==2) || (a.amount>input3 && a.typeId==3))

How do I do this?

Answer 1

Check the PredicateBuilder class. This is a famous implementation of extensions methods for Linq to easily perform dynamic logic operations with OR and AND.

Given your list is of a TypeA for sample, you coul try this:

Expression<Func<TypeA, bool>> filter = a => a.id == id;

if(...something)
{
   filter = filter.Or(a => a...);
}

if(...something)
{
   filter = filter.Or(a => a...);
}

if(...something)
{
   filter = filter.Or(a => a...);
}


var results = alist.Where(filter).ToList();

Answer 2

Use .Concat()

I am not absolutely sure if I understood you question correctly, but this code will create a resultset that is appended to if your if conditions are true, rather than replacing the original resultset.

var results=(from a in alist where a.id==id select a)
if(...something)
{
    results = results.Concat((from a in alist where a.amount>input1 && a.typeId==1 select a))
}
if(...something else)
{
    results = results.Concat((from a in alist where a.amount>input2 && a.typeId==2 select a))
}
//....

Edited as per Peter B's comment.

If multiple lists may contain the same element and you only wish to have every element at most once, use .Union instead of .Concat. This has some performance penalty of course (having to compare the elements).

After your edit

Your edit clarified things a bit. You have two options:

1. Move your a.id == id check into the inner queries:

   var results=Enumerable.Empty<typeofa>()
   if(...something)
   {
       results = results.Concat((from a in alist where a.id == id && a.amount>input1 && a.typeId==1 select a))
   }
   if(...something else)
   {
       results = results.Concat((from a in alist where a.id == id && a.amount>input2 && a.typeId==2 select a))
   }
   //....
   

2. First filter the set using the id, materialize that, then further narrow that down using the method I showed above.

   var results=Enumerable.Empty<typeofa>();
   var fileterdList = (from a in alist where a.id==id select a).ToList();
   if(...something)
   {
       results = results.Concat((from a in fileterdList where a.amount>input1 && a.typeId==1 select a))
   }
   if(...something else)
   {
       results = results.Concat((from a in fileterdList where a.amount>input2 && a.typeId==2 select a))
   }
   //....
   

   Whichever works better depends on your situation. General advice is that prefiltering is more efficient if it narrows down the list considerably and/or the original source is relatively expensive to query (sql for example), but as always, you should profile your concrete example yourself.