Referencing a DLL in C#

I have created a .dll file that sets up printing capabilities and now I want to be able to reference the function in my C# code (on a button press)

My .dll is as follows:

 using Android.Print;
 using System;
 using Android.App;
 using Android.Content;
 using Android.Runtime;
 using Android.Views;
 using Android.Widget;
 using Android.OS;
 using Android.Graphics;
 using Android.Print.Pdf;
 using System.IO;
 
 namespace PrintIMPORTteste
 {
     
     public class PrintActivity : Activity
     {
         protected override void OnCreate(Bundle bundle)
         {
             base.OnCreate(bundle);
             SetContentView(Resource.Layout.Print);
             var txtview = FindViewById<TextView>(Resource.Id.textview1);
             txtview.Text = "Hello";
             var button = FindViewById<Button>(Resource.Id.button1);
             button.Click += (object sender, EventArgs e) = > {
                 var printManager = (PrintManager)GetSystemService(Context.PrintService);
                 var content = FindViewById<LinearLayout>(Resource.Id.linearLayout1);
                 var printAdapter = new GenericPrintAdapter(this, content);
                 printManager.Print("MyPrintJob", printAdapter, null);
             };
         }
     }
 }
 float add(float x, float y)
 {
     return x + y;
 }

I then call the .dll in my C# code by using:

     [DllImport("native")]

From there I don't know how to reference the 'OnCreate' void.

I understand you can access the float add (at the bottom of the .dll) using:

 private static extern float add(float x, float y);

Any help would be appreciated