为什么引用和值类型的集合之间的Enumerable Min或Max不一致？

Question

When dealing with empty sequences, I was surprised to find out that the behavior for min or max is different depending on whether the source collection elements are of value type or of reference type :

var refCollection = new object[0];
var valCollection = new int[0];
var nullableCollection = new int?[0];

var refMin = refCollection.Min(x => x); // null
var valMin = valCollection.Min(); // InvalidOperationException
var nullableMin = nullableCollection.Min(); // null

This difference of behaviors is nicely seen on .NET Core implementation of Enumerable extensions .

This, however, is not the case when looking at Jon Skeet's MinBy extension , for example, which throws on either case as I would have expected.

Doesn't the difference in behaviors just cause confusions? Is there any benefit to returning null for collections of ref types?

Answer 1

It's worth bearing in mind that nullable types (both nullable value types and reference types) behave differently to non-nullable value types in general when it comes to Min : a null value is treated as "missing" in general, so it's not unreasonable for the result of "the minimum of only missing values" to be "the missing value".

For example:

int?[] nullableInts = new int?[5]; // All values null
int? min = nullableInts.Min(); // No exception, min is null
nullableInts[3] = 2;
min = nullableInts.Min(); // No exception, min is 2

For non-nullable value types, there really isn't the option of indicating a "missing value" (unless the return type were changed to always be a nullable type), hence the exception... but this is reasonably easy to spot anyway, as for a non-nullable value type, the only situation where there isn't a minimum is when the source is empty.

(It's possible that MinBy should actually behave the same way :)

This is also consistent with the conversions in LINQ to XML:

XElement element = null;
int? x = (int?) element; // null
int y = (int) element; // Bang

Basically, it does make a certain amount of sense - no option would be consistent with everything.