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Assets/Mirror/Editor/Weaver/Helpers.cs

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+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))
+            );
+        }
+    }
+}