Question

I have a nullable boolean property of an object inside a list, I want to check if all are equal and return its value, and return null if there is a value that is not equal.

e.g.

Object1
    string Name
    bool? NullBool

Object1 is in list e.g. List<Object1> and I just need to check and return NullBool using Linq

I want to do something like

bool? returnVal = <my Linq / Function to get what I want>

Answer 1

Not pretty, but:

IEnumerable<Object1> objects = ...

var firstTwoDistinctValues = objects.Select(o => o.NullBool)
                                    .Distinct()
                                    .Take(2) // Early exit
                                    .ToList();

var result = firstTwoDistinctValues.Count == 2
             ? null : firstTwoDistinctValues.SingleOrDefault();

Now that’s pretty efficient because:

• It works without enumerating the source-collection more than once. This isn’t such a big deal when the source is a list, but could be important if the source is an expensive / non-deterministic query.
• It exits early if there’s more than unique value.

Additionally, it will work fine when the source-collection is empty.

For a list, you can also efficiently do:

IList<Object1> objects = ...

var firstNullBool = objects.Select(o => o.NullBool).FirstOrDefault();
var result = objects.All(o => o.NullBool == firstNullBool) ? firstNullBool : null;