Question

I’m using LINQ to SQL to obtain data from a set of database tables. The database design is such that given a unique ID from one table (Table A) one and only one instance should be returned from an associated table (Table B).

Is there a more concise way to compose this query and ensure that only one item was returned without using the .Count() extension method like below:

var set = from itemFromA in this.dataContext.TableA
          where itemFromA.ID == inputID
          select itemFromA.ItemFromB;

if (set.Count() != 1)
{
    // Exception!
}

// Have to get individual instance using FirstOrDefault or Take(1)

FirstOrDefault helps somewhat but I want to ensure that the returned set contains only one instance and not more.

Answer 1

It sounds like you want Single:

var set = from itemFromA in this.dataContext.TableA
          where itemFromA.ID == inputID
          select itemFromA.ItemFromB;

var onlyValue = set.Single();

Documentation states:

Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.

Of course that means you don’t get to customize the message of the exception… if you need to do that, I’d use something like:

// Make sure that even if something is hideously wrong, we only transfer data
// for two elements...
var list = set.Take(2).ToList();
if (list.Count != 1)
{
    // Throw an exception
}
var item = list[0];

The benefit of this over your current code is that it will avoid evaluating the query more than once.