Add missing nullable ref annotation to SourceSwitch

[AArnott]

The Switch base class that this value is passed to already is marked as allowing a null value here.

[ghost]

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Issue Details

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The Switch base class that this value is passed to already is marked as allowing a null value here.

Author:	AArnott
Assignees:	-
Labels:	area-System.Diagnostics
Milestone:	-

[AArnott]

Is this the right target branch to target net6.0?

[stephentoub]

A few things:

1. Changes need to first be made into main. If approved for backporting to 6 the change can be ported to release/6.0.
2. For this change to be meaningful, it would also need to update the ref, not just the src.
   https://github.com/dotnet/runtime/blob/a7463b2a737dd9d0717e816a1644c96ce33238e5/src/libraries/System.Diagnostics.TraceSource/ref/System.Diagnostics.TraceSource.cs#L64
3. Some of the related annotation doesn't look right at quick glance, stemming from this:
   

   runtime/src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/Switch.cs

   Line 206 in 0767243

   _switchValueString = _defaultValue!;

[AArnott]

Thanks, @stephentoub. I've updated the ref source as well, and retargeted to main for now.
To your point about there being evidence that maybe null shouldn't be passed in, or at least that other annotations are wrong, I see your point. @tommcdon is looking for an owner for this area that may be able to pursue this to make any additional corrections.

[stephentoub]

Just looking at this more deeply, I'm not sure this is a valid change. For example, this fails with an argument null exception:

var s = new SourceSwitch("myswitch", null);
var a = s.Attributes;

I think this means this PR shouldn't be merged, and someone needs to look at the base annotations again.
cc: @jeffhandley, @buyaa-n

[buyaa-n]

3. Some of the related annotation doesn't look right at quick glance, stemming from this:
   

   runtime/src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/Switch.cs

   Line 206 in 0767243

   _switchValueString = _defaultValue!;

Right looks _defaultValue shouldn't be nullable

The Switch base class that this value is passed to already is marked as allowing a null value here.

@AArnott the base class annotation is wrong, sorry about that, i am gonna close this PR and will update the base annotation