Unity/Mono doesn't support dll's with System.Collections.Generic.List? Using .NET 3.5

I'm trying to import a dll that uses System.Collections.Generic.List (Iserialization) however whenever that class is called, I get the following exception:

 Exception: System.TypeLoadException: Could not load type 'System.Collections.Generic.List`1[[SD_Library.PLItem, SD Library, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]'.
   at (wrapper managed-to-native) System.Type:internal_from_name (string,bool,bool)
   at System.Type.GetType (System.String typeName, Boolean throwOnError) [0x00011] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System/Type.cs:471 
   at System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadTypeMetadata (System.IO.BinaryReader reader, Boolean isRuntimeObject, Boolean hasTypeInfo) [0x000bc] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/ObjectReader.cs:640 
   at System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadObjectInstance (System.IO.BinaryReader reader, Boolean isRuntimeObject, Boolean hasTypeInfo, System.Int64& objectId, System.Object& value, System.Runtime.Serialization.SerializationInfo& info) [0x0000a] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/ObjectReader.cs:269 
   at System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadObject (BinaryElement element, System.IO.BinaryReader reader, System.Int64& objectId, System.Object& value, System.Runtime.Serialization.SerializationInfo& info) [0x0008a] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/ObjectReader.cs:191 
   at System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadObject (BinaryElement element, System.IO.BinaryReader reader, System.Int64& objectId, System.Object& value, System.Runtime.Serialization.SerializationInfo& info) [0x00105] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/ObjectReader.cs:223 
   at System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadNextObject (BinaryElement element, System.IO.BinaryReader reader) [0x00020] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/ObjectReader.cs:130 
   at System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadObjectGraph (BinaryElement elem, System.IO.BinaryReader reader, Boolean readHeaders, System.Object& result, System.Runtime.Remoting.Messaging.Header[]& headers) [0x00004] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/ObjectReader.cs:104 
   at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.NoCheckDeserialize (System.IO.Stream serializationStream, System.Runtime.Remoting.Messaging.HeaderHandler handler) [0x00078] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/BinaryFormatter.cs:179 
   at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize (System.IO.Stream serializationStream) [0x00000] in /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Runtime.Serialization.Formatters.Binary/BinaryFormatter.cs:136 
   at GameManager.OpenStream (System.String saveLocation) [0x0003c] in C:\Users\Chris\Documents\Repositories\PL2D\Assets\Scripts\GameManager.cs:568 
 UnityEngine.Debug:LogError(Object)
 GameManager:OpenStream(String) (at Assets/Scripts/GameManager.cs:572)
 GameManager:LoadItemDatabase() (at Assets/Scripts/GameManager.cs:457)
 GameManager:Start() (at Assets/Scripts/GameManager.cs:188)
 GameManager:Awake() (at Assets/Scripts/GameManager.cs:440)
 

It is my understanding this has been a problem for well over a year and people had expected this to be resolved once Unity upgraded mono last year. With that said, I am using the latest version of Unity and I still see this problem. Has it been fixed? (clearly it hasnt) How can I get around this?

Thanks.