Breaking Changes in PostSharp 6.0

PostSharp 6.0 contains the following breaking changes:

Changes causing silent changes of behavior

The internal details of the assembly loading algorithms have been changed; therefore, in some rare situations, PostSharp may fail to find or load assemblies in cases that used to be successful with PostSharp 5.0. Please report these cases to the PostSharp support team for individual resolution.
All MSBuild properties that used to be prefixed PostSharp30 are now prefixed just PostSharp. Check your projects for occurrences of the PostSharp30 substring.
The RequiredAttribute code contract now throws ArgumentOutOfRangeException instead of ArgumentNullException when a target element is assigned an empty or white-space string.

Parts of the API that were marked [Obsolete] in PostSharp 5.0 have been removed.
The way exception handlers and filters are represented in the PostSharp.Reflection.MethodBody has changed. They used to be tree nodes, but are now just properties of IBlockExpression. The MethodBodyVisitor class has been modified accordingly.
The PostSharp.Patterns.Diagnostics.RecordBuilders.ParameterDirection has been renamed ParameterKind and moved to PostSharp.dll.
The notorious QueryInterface(object) extension method has been replaced by the non-extension QueryInterface<T>(object, bool), in an effort to limit the pollution of Intellisense suggestions and documentation.

Visual Studio 2012 is no longer supported.
.NET Core SDK 1.1 is no longer supported as a build platform. You can still build .NET Core 1.1 libraries under the .NET Core SDK 2.0 or later. See Requirements and Compatibility for details.
.NET Framework 4.7.1 or later is now required on build platforms. Older versions are still supported as run-time platforms. See Requirements and Compatibility for details.
Windows 7 SP1, Windows 8.1, Windows Server 2008 R2 and early versions of Windows 10 are no longer supported as build platforms. See Requirements and Compatibility for details.
Invoking PostSharp from the command line is no longer supported.
IncrediBuild is no longer supported.

It is now mandatory to add your license key to the build server or the source repository if you are using the PostSharp.Patterns.Diagnostics namespace. See build-server for details.