Linq All on empty collection

Question

I need to check if all definitions contains some specific data. It works fine except the case when GroupBy returns empty collection.

var exist = dbContext.De         


        

Answer 1

Here is another trick:

var exist = dbContext.Definitions
    .Where(x => propertyTypeIds.Contains(x.PropertyTypeId) && x.CountryId == countryId)
    .GroupBy(x => x.PropertyTypeId)
    .Min(some_condition ? (int?)1 : 0) == 1;

It utilizes the fact that the above Min<int?> method returns:

(A) null when the set is empty
(B) 0 if the condition is not satisfied for some element
(C) 1 if the condition is satisfied for all elements

so we simple check the result for (C) using the nullable value comparison rules.

Answer 2

What about writing your own extension method? (I am pretty sure you will name it better)

public static bool NotEmptyAll<T>(
    this IEnumerable<T> collection, 
    Func<T, bool> predicate)
{
    return collection != null
        && collection.Any()
        && collection.All(predicate);
}

Then call it instead of All

var exist = definitions.Where(
        x => propertyTypeIds.Contains(x.PropertyTypeId) && x.CountryId == countryId)
         .GroupBy(x => x.PropertyTypeId)
         .NotEmptyAll(
             ...some condition...));

Answer 3

Edit: first answer wouldn't have worked.

If you rearrange your query somewhat, you can use DefaultIfEmpty without needing to change your condition:

var exist = dbContext.Definitions
                     .Where(x => propertyTypeIds.Contains(x.PropertyTypeId) 
                                  && x.CountryId == countryId)
                     .GroupBy(x => x.PropertyTypeId);

           // apply the condition to all entries, 
           // resulting in sequence of bools (or empty), 
           // to permit the next step
                     .Select(...some condition...) 

           //if seq is empty, add `false`
                     .DefaultIfEmpty(false)

           //All with identity function to apply the query and calculate result
                     .All(b => b)
         );

Answer 4

Here's the alternative to All that returns false if collection is empty:

var collection = Enumerable.Range(0, 0); //empty collection

collection
  .Select(IsValid)
  .DefaultIfEmpty(false)
  .All(b => b);

Or as an extension method:

public static bool AnyAndAll<T>(IEnumerable<T> collection, Func<T, bool> predicate) =>
  collection
    .Select(predicate)
    .DefaultIfEmpty(false)
    .All(b => b);

Answer 5

If you're using LINQ to Objects, I'd just write my own extension method. My Edulinq project has sample code for All, and adapting that is pretty simple:

public static bool AnyAndAll<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw new ArgumentNullException(nameof(source));
    }
    if (predicate == null)
    {
        throw new ArgumentNullException(nameof(predicate));
    }

    bool any = false;
    foreach (TSource item in source)
    {
        any = true;
        if (!predicate(item))
        {
            return false;
        }
    }
    return any;
}

This avoids evaluating the input more than once.

Answer 6

You may be able to do this using an Aggregate, along the lines of:

.Aggregate(new {exists = 0, matches = 0}, (a, g) =>
        new {exists = a.exists + 1, matches = a.matches + g > 10 ? 1 : 0})

(Here, g > 10 is my test)

And then simple logic that exists is greater than zero and that exists and matches have the same value.

This avoids running the whole query twice.