dotnet/Documentation/compatibility/wpf-dispatchersynchronizationcontext_createcopy-now-returns-a-new-copy-instead-of-the-current-instance.md

## WPF DispatcherSynchronizationContext.CreateCopy now returns a new copy instead of the current instance

### Scope
Minor

### Version Introduced
4.5

### Source Analyzer Status
Available

### Change Description

In the .NET Framework 4,
<xref:System.Windows.Threading.DispatcherSynchronizationContext.CreateCopy>
returned a reference to the current instance, primarily as a performance
optimization. In the .NET Framework 4.5, it returns a new instance which makes
it possible for the first time to conclude that equal references indicate the
executing thread is in the correct synchronization context.  It is unlikely that
code that checks the identity of these references will be affected, but because
of the change, code that calls
<xref:System.Windows.Threading.DispatcherSynchronizationContext.CreateCopy>
should be tested as part of migration to the .NET Framework 4.5 or newer.

- [ ] Quirked
- [ ] Build-time break

### Recommended Action

Be aware that
<xref:System.Windows.Threading.DispatcherSynchronizationContext.CreateCopy>
will now return a new
<xref:System.Threading.SynchronizationContext?displayProperty=name> object.
Previously, code that used equivalence of references generated this way was not
actually checking whether it was in the proper context, but does when built
against .NET Framework 4.5 or later.  While unlikely to cause issues, exercising the
affected code paths should be enough to determine if this poses any problem.

### Affected APIs
* `M:System.Windows.Threading.DispatcherSynchronizationContext.CreateCopy`

### Category
Windows Presentation Foundation (WPF)

<!-- breaking change id: 38 -->