Unity Editor template where main class and editor class are both placed inside same file.

UnityEditorTemplate.cs

using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;
#endif

[System.Serializable]
public class ClassName
{
    public int Property1;
}

#if UNITY_EDITOR
[CustomEditor(typeof(ClassName))]
public class ClassNameEditor : Editor
{
    private SerializedProperty m_SerializedProperty;

    private void OnEnable()
    {
        m_SerializedProperty = serializedObject.FindProperty("m_SerializedProperty");
    }

    public override void OnInspectorGUI()
    {
        serializedObject.Update();
        
        
        
        serializedObject.ApplyModifiedProperties();
    }
}
#endif