Importing scripts through Asset bundles

I am trying to load new scripts through asset bundles. By reading the manual on this topic this is what i did:

I created a .dll file for test purposes(pre-compiled assembly means .dll file, right?) This is the content of that .dll file

 using System;
 namespace TestLibrary{
     public class MyClass{
         public string str = "If u see me, DLL thing works";
         public MyClass (){}
     }
 }

I copied this file in my unity assets folder and tried to export this as an asset bundle but this warning came up

Unrecognized assets cannot be included in AssetBundle: "Assets/TestLibrary.dll".

I changed the file extension from .dll to .txt and built it again(Could this mess things up?) Here is my code for building the assetbundle (I kind of copied this from a source I am not exactly sure how each of this parts work):

 static void ExportResource2(){
     string path = "AssetBundles/scriptbundle01.unity3d";
     Object[] selection = Selection.GetFiltered(typeof(Object), SelectionMode.DeepAssets);
     BuildPipeline.BuildAssetBundle(Selection.activeObject, selection, path,
                                        BuildAssetBundleOptions.CollectDependencies
                                        | BuildAssetBundleOptions.CompleteAssets);
 }

I loaded the asset bundle and tried to load a class from it (even though the class does nothing right now) I followed the code from the manual:

 IEnumerator Start () {
             // Start a download of the given URL
             Caching.CleanCache();
     
             WWW www = WWW.LoadFromCacheOrDownload (url, 0);
             while(!www.isDone){
                 yield return null;
             }
             // Wait for download to complete
             Debug.Log("asda");
             
             // Load and retrieve the AssetBundle
             AssetBundle bundle = www.assetBundle;
             
             // Load the TextAsset object
             TextAsset txt = bundle.LoadAsset("TestLibrary.txt", typeof(TextAsset)) as TextAsset;
             
             // Load the assembly and get a type (class) from it
             var assembly = System.Reflection.Assembly.Load(txt.bytes);
             Type type = assembly.GetType("MyClass");
             Activator.CreateInstance(type);
             Debug.Log(type.Name);
         }
 

I have replaced bundle.Load() with bundle.LoadAsset() as the prior is deprecated. It generates the following error:

BadImageFormatException

I am not sure whet exactly are the mistakes i am making. Any kind of help will be very much appreciated. Also, How am I supposed to get the reference to that string in the "MyClass" dll from my unity code if the class would ever instantiate...

One last thing if i want to include Monobehaviour classes in my dll should reference to UnityEngine be enough?