Plugin Registration Tool: Could not load file or assembly

When registering a new assembly in the Plugin Registration Tool you might encounter the error “Could not load file or assembly”.

Unhandled Exception: System.IO.FileLoadException: Could not load file or assembly 'file://....dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
at System.Reflection.RuntimeAssembly.InternalLoadFrom(String assemblyFile, Evidence securityEvidence, Byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm, Boolean forIntrospection, Boolean suppressSecurityChecks, StackCrawlMark& stackMark)
at System.Reflection.Assembly.LoadFrom(String assemblyFile)
at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.LoadAssembly(String path)
at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.RetrievePluginsFromAssembly(String path)
at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.RetrievePluginsFromAssembly(String path)
at Microsoft.Crm.Tools.Libraries.RegistrationHelper.RetrievePluginsFromAssembly(String pathToAssembly, String appPath)
at Microsoft.Crm.Tools.AssemblyRegistration.PluginRegistrationViewModel.BtnLoadAssemblyClicked()
Inner Exception: System.NotSupportedException: An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See http://go.microsoft.com/fwlink/?LinkId=155569 for more information.

The reason you are receiving this error message is because the file is blocked. When downloading dlls from external recources, Windows might automatically block the file(s) as a security measure. When you are sure the file is trustworthy, you can unblock it by opening the properties dialog of the file (right mouse click on the file) and clicking “Unblock”.

After you have unblocked the file you should be able to register the assembly without a problem.

Tip: When you receive an archive (ZIP-file) containing the dlls, you can prevent Windows blocking the content by using 7zip as extraction tool (instead of the default archive viewer of Windows). This might save you some time!