Is there a better way of calling LINQ Any + NOT All?

Question

I need to check if a sequence has any items satisfying some condition but at the same time NOT all items satisfying the same condition.

For example, for a sequence of 10

Answer 1

If you wanted to define this as a method, you could take then approach Linq takes in defining both IEnumerable and IQueryable extension methods. This allows for the optimal approach to be taken automatically:

public static bool SomeButNotAll(this IQueryable source, Expression> predicate)
{
  if(source == null)
    throw new ArgumentNullException("source");
  if(predicate == null)
    throw new ArgumentNullException("predicate");
  return source.
    Select(predicate)
    .Distinct()
    .Take(2)
    .Count() == 2;
}
public static bool SomeButNotAll(this IEnumerable source, Func predicate)
{
  if(source == null)
    throw new ArgumentNullException("source");
  if(predicate == null)
    throw new ArgumentNullException("predicate");
  using(var en = source.GetEnumerator())
    if(en.MoveNext())
    {
      bool first = predicate(en.Current);
      while(en.MoveNext())
        if(predicate(en.Current) != first)
          return true;
    }
  return false;
}

If you're using EntityFramework (or another provider that provides a CountAsync you can also provide an asynchronous version easily:

public static async Task SomeButNotAllAsync(this IQueryable source, Expression> predicate, CancellationToken cancel)
{
  if(source == null)
    throw new ArgumentNullException("source");
  if(predicate == null)
    throw new ArgumentNullException("predicate");
  cancel.ThrowIfCancellationRequested();
  return await source.
    Select(predicate)
    .Distinct()
    .Take(2)
    .CountAsync(cancel)
    .ConfigureAwait(false) == 2;
}
public static Task SomeButNotAllAsync(this IQueryable source, Expression> predicate)
{
  return source.SomeButNotAllAsync(predicate, CancellationToken.None);
}