Create a new Script on a GameObject and start editing the file right away

ScriptHelper.cs

// ===============================
// AUTHOR          : J. Giezendanner
// CREATE DATE     : 19.02.2020
// MODIFIED DATE   : 
// PURPOSE         : Adds a context menu to create a new script on the selected GameObject, and opens the editor to
//                   edit the script
// SPECIAL NOTES   : Inspired by http://answers.unity.com/answers/1007397/view.html
// ===============================
// Change History:
//==================================

using System;
using System.IO;
using UnityEngine;
using UnityEditor;

public static class ScriptHelper
{
    [MenuItem("GameObject/Giezi's tools/Add Script and Edit", false, 0)]
    static void AddRequiredScript()
    {
        if (Selection.activeGameObject != null)
        {
            GameObject go = Selection.activeGameObject;
            
            // Ask for script creation location, set default location to "Assets/Scripts"
            string assetPath = Application.dataPath + Path.DirectorySeparatorChar + "Scripts" + Path.DirectorySeparatorChar;
            string path = EditorUtility.SaveFilePanel("Save new script", assetPath, "New Script","cs");
            if (path.Length == 0)
                return;

            path = path.Replace(Application.dataPath + Path.AltDirectorySeparatorChar, "");
            path = "Assets" + Path.AltDirectorySeparatorChar + path;
        
            string name = Path.GetFileNameWithoutExtension(path);
            string dir = Path.GetDirectoryName(path);
            
            name = name.Replace(" ","_");
            name = name.Replace("-","_");
            name = name.Replace(".","_");
            
            // Check if the class already exists
            try
            {
                Activator.CreateInstance("Assembly-CSharp", name);
                Debug.Log("A class with this name does already exist");
                return;
            }
            catch (Exception e)
            {}
            
            // Create the class
            string copyPath = dir + Path.DirectorySeparatorChar + name + ".cs";
            Debug.Log("Creating Classfile: " + copyPath);
            if( File.Exists(copyPath) == false ){ // do not overwrite
                using (StreamWriter outfile = 
                    new StreamWriter(copyPath))
                {
                    outfile.WriteLine("using UnityEngine;");
                    outfile.WriteLine("using System.Collections;");
                    outfile.WriteLine("");
                    outfile.WriteLine("public class "+name+" : MonoBehaviour {");
                    outfile.WriteLine(" ");
                    outfile.WriteLine("}");
                }//File written
            }
            
            // Open the file in the default editor to start coding right away
            UnityEditorInternal.InternalEditorUtility.OpenFileAtLineExternal(copyPath, 0);
            
            // Keep a reference for later
            EditorPrefs.SetString ("New Class Name", name);
            EditorPrefs.SetInt("Selected GameObject Name", go.GetInstanceID());
            
            
            // refresh so that the script gets compiled
            AssetDatabase.Refresh();
        }
    }
        
    [UnityEditor.Callbacks.DidReloadScripts]
    private static void ScriptReloaded() 
    {
        // If the key doesn’t exist, don’t bother, as we’re not generating stuff.
        if (!EditorPrefs.HasKey ("New Class Name") || !EditorPrefs.HasKey ("Selected GameObject Name"))
        {
            return;
        }
             
        // If they key does exist and the object doesn’t, it’s just a left over key.
        string name = EditorPrefs.GetString ("New Class Name");

        int goName = EditorPrefs.GetInt("Selected GameObject Name");
        GameObject go = EditorUtility.InstanceIDToObject(goName) as GameObject;
        
        if (go == null)
        {
            Debug.Log("Object not found");
            return;
        }
        
        Type type = Activator.CreateInstance("Assembly-CSharp", name).Unwrap().GetType();
        go.AddComponent(type);
        
        
        
        //Delete the key because we don’t need it anymore!
        EditorPrefs.DeleteKey("New Class Name");
        EditorPrefs.DeleteKey("Selected GameObject Name");
    }
}