When you create MyAssembly.XmlSerializers.dll make sure it is same .NET version as your project. You can check that using software as dotPeek or ILSpy. I had similar problem when creating xmlserializers for my .NET 3.5 solution from visual studio, generate serialization assembly was set on for project but Sgen.exe was used from C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools and should be from C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin.
If your solution is in .NET 3.5 MyAssembly.XmlSerializers.dll must also be in .NET 3.5
