When using Unity, gives "System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded." error.

Hello. When I put websocketsharp.dll into my Unity project, I get this error.

Unhandled Exception: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.
at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)
at System.Reflection.Assembly.GetTypes () [0x00000] in :0
at Mono.CSharp.RootNamespace.ComputeNamespaces (System.Reflection.Assembly assembly, System.Type extensionType) [0x00000] in :0
at Mono.CSharp.RootNamespace.ComputeNamespace (Mono.CSharp.CompilerContext ctx, System.Type extensionType) [0x00000] in :0
at Mono.CSharp.GlobalRootNamespace.ComputeNamespaces (Mono.CSharp.CompilerContext ctx) [0x00000] in :0
at Mono.CSharp.Driver.LoadReferences () [0x00000] in :0
at Mono.CSharp.Driver.Compile () [0x00000] in :0
at Mono.CSharp.Driver.Main (System.String[] args) [0x00000] in :0

Missing method .ctor in assembly /media/boyned/ExtraSpace/BattleHatsCCG/Battle Hats CCG/Assets/Plugins/websocket-sharp.dll, type System.Runtime.CompilerServices.ExtensionAttribute
The class System.Runtime.CompilerServices.ExtensionAttribute could not be loaded, used in websocket-sharp
Can't find custom attr constructor image: /media/boyned/ExtraSpace/BattleHatsCCG/Battle Hats CCG/Assets/Plugins/websocket-sharp.dll mtoken: 0x0a000001

Reproduce the failure by placing websocket-sharp.dll in the Unity project's Assets/Plugins directory, then start with the ReflectionTypeLoadException and the reported ExtensionAttribute loader error. Done means Unity can load the assembly without the reported compiler and type-loading errors.