## TargetFrameworkName for default app domain no longer defaults to null if not set

### Scope
Edge

### Version Introduced
4.6

### Source Analyzer Status
Planned

### Change Description

The <xref:System.AppDomainSetup.TargetFrameworkName?displayProperty=name> was
previously null in the default app domain, unless it was explicitly set.
Beginning in 4.6, the
<xref:System.AppDomainSetup.TargetFrameworkName?displayProperty=name> property
for the default app domain will have a default value derived from the
TargetFrameworkAttribute (if one is present). Non-default app domains will
continue to inherit their
<xref:System.AppDomainSetup.TargetFrameworkName?displayProperty=name> from the
default app domain (which will not default to null in 4.6) unless it is
explicitly overridden.

- [ ] Quirked
- [ ] Build-time break

### Recommended Action

Code should be updated to not depend on
<xref:System.AppDomainSetup.TargetFrameworkName>
defaulting to null. If it is required that this property continue to evaluate to
null, it can be explicitly set to that value.

### Affected APIs
* `M:System.AppDomainSetup.get_TargetFrameworkName`

### Category
Core

<!-- breaking change id: 75 -->