fixes #619

Harmony/Tools/AccessTools.cs

@@ -1994,7 +1994,8 @@ public static object MakeDeepCopy(object source, Type resultType, Func<string, T
 			{
 				var path = pathRoot.Length > 0 ? pathRoot + "." + name : name;
 				var value = processor is not null ? processor(path, src, dst) : src.GetValue();
-				_ = dst.SetValue(MakeDeepCopy(value, dst.GetValueType(), processor, path));
+				if (dst.IsWriteable)
+					_ = dst.SetValue(MakeDeepCopy(value, dst.GetValueType(), processor, path));
 			});
 			return result;
 		}

Harmony/Tools/Traverse.cs

@@ -183,6 +183,35 @@ public Type GetValueType()
 			return null;
 		}
 
+		/// <summary>Checks if the current traverse instance is for a field</summary>
+		/// <returns>True if its a field</returns>
+		///
+		public bool IsField => _info is FieldInfo;
+
+		/// <summary>Checks if the current traverse instance is for a property</summary>
+		/// <returns>True if its a property</returns>
+		///
+		public bool IsProperty => _info is PropertyInfo;
+
+		/// <summary>Checks if the current field or property is writeable</summary>
+		/// <returns>True if writing is possible</returns>
+		///
+		public bool IsWriteable
+		{
+			get
+			{
+				if (_info is FieldInfo fi)
+				{
+					var isConst = fi.IsLiteral && fi.IsInitOnly == false && fi.IsStatic;
+					var isStaticReadonly = fi.IsLiteral == false && fi.IsInitOnly && fi.IsStatic;
+					return isConst == false && isStaticReadonly == false;
+				}
+				if (_info is PropertyInfo pi)
+					return pi.CanWrite;
+				return false;
+			}
+		}
+
 		Traverse Resolve()
 		{
 			if (_root is null)
@@ -407,7 +436,11 @@ public static void IterateProperties(object source, object target, Action<string
 
 		/// <summary>A default field action that copies fields to fields</summary>
 		/// 
-		public static Action<Traverse, Traverse> CopyFields = (from, to) => { _ = to.SetValue(from.GetValue()); };
+		public static Action<Traverse, Traverse> CopyFields = (from, to) =>
+		{
+			if (to.IsWriteable)
+				_ = to.SetValue(from.GetValue());
+		};
 
 		/// <summary>Returns a string that represents the current traverse</summary>
 		/// <returns>A string representation</returns>