How do you declare a by-reference parameter variable for use in a Linq query?

Question

I have a class MyClass with a method:

public bool MyMethod(out DateTime? MyDate) {
  ...
}

I'd like to call this method in the following way:

var q = from mc in MyClasses.AsEnumerable()
        from output in mc.MyMethod(out dt) // how to declare dt?
        select new { mc, output, dt };

Obviously this doesn't compile, coz I haven't declared dt. I can declare it outside the query, but that doesn't give me a warm fuzzy feeling: (a) I don't like declaring variables at a level greater than the necessary scope, and (b) it isn't immediately and intuitively obvious that the value would be calculated correctly for each row inside the query.

Is there some syntax that will allow me to declare DateTime? dt inside the query?

Answer 1

My first thought is "ew" (thats my code smell noise). I feel you're really trying to force using that method where it doesn't belong.

My next thought would be to simply create the behavior you want, maybe something such as (although I still don't recommend this as a good solution):

public DateTime? GetMyDate() 
{
   DateTime? dt;
   MyMethod(out dt);
   return dt;
}

Then...

var q = from mc in MyClasses.AsEnumerable()
        select new { mc, output, Date = GetMyDate() };

I'm making the assumption that the bool returned from MyDate isn't important in this context, which really breaks intent. I only assume that because, given your desired result, you don't seem to care about the bool return either.

In either case, I would recommend rethinking this instead of making due with what you have.

Answer 2

Its not just the scope of dt that is the problem here, your MyMethod() would need to return an enumeration for the SelectMany to work (the second from clause)

Not really sure what you are trying to achieve here but I would be inclined to have your function return a struct or class that contains the bool and the date and assign it via a let statement...something like...

var qry = from mc in MyClasses.FunctThatReturnsAnIEnumerable()
          let result = mc.MyMethod() //this returns the struct
          select new {mc, result.BoolVal, result.DateVal};