defrain-shooter-unity/Assets/Mirror/Editor/Weaver/Helpers.cs

using System.IO;
using System.Linq;
using System.Reflection;
using Mono.CecilX;

namespace Mirror.Weaver
{
    static class Helpers
    {
        // This code is taken from SerializationWeaver
        public static string UnityEngineDllDirectoryName()
        {
            string directoryName = Path.GetDirectoryName(Assembly.GetExecutingAssembly().CodeBase);
            return directoryName?.Replace(@"file:\", "");
        }

        public static bool IsEditorAssembly(AssemblyDefinition currentAssembly)
        {
            // we want to add the [InitializeOnLoad] attribute if it's available
            // -> usually either 'UnityEditor' or 'UnityEditor.CoreModule'
            return currentAssembly.MainModule.AssemblyReferences.Any(assemblyReference =>
                assemblyReference.Name.StartsWith(nameof(UnityEditor))
            );
        }
    }
}