Adding System.Web.Script reference in class library
The ScriptIgnoreAttribute class is in the System.Web.Extensions.dll assembly (Located under Assemblies > Framework in the VS Reference Manager). You have to add a reference to that assembly in your class library project.
You can find this information at top of the MSDN page for the ScriptIgnoreAttribute class.
You need to add a reference to System.Web.Extensions.dll in project for System.Web.Script.Serialization error.