How to Typecast JS Variables as C# Classes?

If I want to typecast a variable in js as defined in cs. How do I do that?

For example:

C# Code

 namespace SocketIO
 {
     public class SocketIOEvent
     {
         public string name { get; set; }
 
         public JSONObject data { get; set; }
 
         public SocketIOEvent(string name) : this(name, null) { }
         
         public SocketIOEvent(string name, JSONObject data)
         {
             this.name = name;
             this.data = data;
         }
         
         public override string ToString()
         {
             return string.Format("[SocketIOEvent: name={0}, data={1}]", name, data);
         }
     }
 }

JS Code

 // custom callback function which is passed a SocketIOEvent object
 function OnMessageReceived (event: SocketIOEvent) {
 
 }

How do I correctly typecast event on the OnMessageReceived function?

I am (unsurprisingly) receiving the error: The name 'SocketIOEvent' does not denote a valid type ('not found').