Clarify null name for IOptions related types

[kevincathcart-cas]

Summary

Fixes some really poor wording related to passing null to Get(string? name) methods of IOptions related types.

Was:

The name of the TOptions instance, if Microsoft.Extensions.Options.Options.DefaultName is null.

But that makes no sense. DefaultName is public static readonly string DefaultName = string.Empty; and cannot be null. The actual intention is to document that these types use the value of DefaultName if null is passed.

Now:

The name of the TOptions instance. Uses Microsoft.Extensions.Options.Options.DefaultName if null is provided.

That is accurate and contractual. Unnamed options use DefaultName for their name, as documented in many places in these docs, and passing null here is explicitly requesting that the unnamed version of the option is returned, which happens by using DefaultName. Any alternate implementation of the interfaces in question that distinguished between null and DefaultName, would be non-conforming and incompatible since apps can freely choose to pass null or DefaultName, and don't expect any difference between the two.

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[learn-build-service-prod]

Learn Build status updates of commit cdd065e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/Microsoft.Extensions.Options/IOptionsMonitor`1.xml	✅Succeeded	View
xml/Microsoft.Extensions.Options/IOptionsSnapshot`1.xml	✅Succeeded	View
xml/Microsoft.Extensions.Options/OptionsManager`1.xml	✅Succeeded	View
xml/Microsoft.Extensions.Options/OptionsMonitor`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

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[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-extensions-options

[tarekgh]

@kevincathcart-cas thanks for helping with the doc clarification. LGTM.

Should we add something telling Microsoft.Extensions.Options.Options.DefaultName is empty string too? This may help users to understand what name value they will get.

CC @gewarren

[learn-build-service-prod]

Learn Build status updates of commit 15f926f:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/Microsoft.Extensions.Options/IOptionsMonitor`1.xml	✅Succeeded	View
xml/Microsoft.Extensions.Options/IOptionsSnapshot`1.xml	✅Succeeded	View
xml/Microsoft.Extensions.Options/OptionsManager`1.xml	✅Succeeded	View
xml/Microsoft.Extensions.Options/OptionsMonitor`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel