Enumerable.Min<TSource>() documentation should mention null filtering behavior

[binki]

In regards to null, the documentation states this:

If TSource is a reference type and the source sequence is empty or contains only values that are null, this method returns null.

It also states:

If type TSource implements IComparable, the Max(IEnumerable) method uses that implementation to compare values. Otherwise, if type TSource implements IComparable, that implementation is used to compare values.

This leads me to believe that the minimum value will be calculated purely based on the output of IComparable<T>.Compare(T, T). That is, the output of .Min() would be the same as .OrderBy(x => x).First() and the output of .Max() would be the same as .OrderBy(x => x).Last(). Or that if Comparer<T>.Default.Compare(a, b) < 0, then the output of new[] { a, b, }.Min() would be a. However, I found the following behavior instead:

Console.WriteLine($"Comparer<string>.Default.Compare(null, \"a\") == {Comparer<string>.Default.Compare(null, "a")}");
Console.WriteLine($"new[]{{ null, \"a\", }}.OrderBy(x => x).First() == {new[] { null, "a", }.OrderBy(x => x).First()}");
Console.WriteLine($"new[]{{ null, \"a\", }}.Min() == {new[] { null, "a", }.Min()}");

with output:

Comparer<string>.Default.Compare(null, "a") == -1
new[]{ null, "a", }.OrderBy(x => x).First() ==
new[]{ null, "a", }.Min() == a

By experiencing this and examining the source, I know that .Min() and .Max() implicitly filter out any values equal to null as the first step (including nullable value types which box to null). However, this is not obvious from the documentation.

May you please alter the documentation to document this behavior?

Thanks!

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[binki]

I guess this does not apply to .Max() unless you use the overload which allows passing in a custom IComparer<T> implementation or you have implemented the IComparable<T> interface. The IComparable<T> interface documentation states:

By definition, any object compares greater than null, and two null references compare equal to each other.

So in .Max() the filtering would normally have no effect on the result.

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Issue Details

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In regards to null, the documentation states this:

If TSource is a reference type and the source sequence is empty or contains only values that are null, this method returns null.

It also states:

If type TSource implements IComparable, the Max(IEnumerable) method uses that implementation to compare values. Otherwise, if type TSource implements IComparable, that implementation is used to compare values.

This leads me to believe that the minimum value will be calculated purely based on the output of IComparable<T>.Compare(T, T). That is, the output of .Min() would be the same as .OrderBy(x => x).First() and the output of .Max() would be the same as .OrderBy(x => x).Last(). Or that if Comparer<T>.Default.Compare(a, b) < 0, then the output of new[] { a, b, }.Min() would be a. However, I found the following behavior instead:

Console.WriteLine($"Comparer<string>.Default.Compare(null, \"a\") == {Comparer<string>.Default.Compare(null, "a")}");
Console.WriteLine($"new[]{{ null, \"a\", }}.OrderBy(x => x).First() == {new[] { null, "a", }.OrderBy(x => x).First()}");
Console.WriteLine($"new[]{{ null, \"a\", }}.Min() == {new[] { null, "a", }.Min()}");

with output:

Comparer<string>.Default.Compare(null, "a") == -1
new[]{ null, "a", }.OrderBy(x => x).First() ==
new[]{ null, "a", }.Min() == a

By experiencing this and examining the source, I know that .Min() and .Max() implicitly filter out any values equal to null as the first step (including nullable value types which box to null). However, this is not obvious from the documentation.

May you please alter the documentation to document this behavior?

Thanks!

Author:	binki
Assignees:	-
Labels:	Pri3, area-System.Linq, :watch: Not Triaged
Milestone:	-

[krwq]

Thanks for reporting! @binki if you're interested in contributing a fix there is an Edit button (pen icon) on https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.min?view=net-6.0 which will point you to the right file to edit

[krwq]

Also if this was already fixed here: #7927 please close the issue