Implicit operator throws when checking ArraySegment != null or ArraySegment == null

[MindyStivers]

Symptom

Using != or == operator to compare an ArraySegment to null throws an ArgumentNullException. For example:
if (new ArraySegment<byte>() == null)

Cause

The != and == operator trigger implicit conversion from an array to ArraySegment on null. The resulting call to the ArraySegment constructor with a null array parameter throws an ArgumentNullException:

public static implicit operator ArraySegment<T>(T[] array) => new ArraySegment<T>(array);
results in:
new ArraySegment<T>(null);
which throws ArgumentNullException.

dotnet/corefx#10528
dotnet/coreclr#9926

[benaadams]

ArraySegment is a struct so can never actually be null; do you want to check if its array is null?

if (new ArraySegment<byte>().Array == null)

[MindyStivers]

D'oh! This just started throwing out of the depths of an older test when we moved to .net core 2.0. Fair 'nuf that the answer is we have bad test code that was previously sneaking through.

I know the implicit operator docs say it should not throw so I wanted to make sure to follow up.

[benaadams]

I do wonder what it will do for struct tests in generics e.g.

if (default(T) == null)
if (default(T) != null)

Where T is ArraySegment

[MindyStivers]

The generic tests you suggested work fine when T is an ArraySegment. Perhaps worth noting that (although it's rather pointless) we can compare other value types == or != null without an exception.

[serkantkaraca]

I think we are talking here about breaking the contract on null check on struct types. It doesn't really matter whether it makes sense or not to do a null check. Null check on struct compiles but throws an undescriptive NullRef which breaks the former behavior. We need to fix this.

[ghost]

Thanks @stephentoub and prioritizing it. Redirected from https://github.com/dotnet/corefx/issues/26987.

[jkotas]

We should add a test to corefx (if there is not one already).

[stephentoub]

We should add a test to corefx (if there is not one already).

I'll do so.