"Cannot modify the return value" error when modifying content of List<T>

I am currently working on a C# project and I need add data to a custom List array. I then later on need to enumerate through the list, finding a particular record and then modify the data. However, I am getting an error within Visual Studio ``` Cannot modify the return value of System.Collections.Generic.List<EmailServer.ThreadCheck>.this[int] because it is not a variable. ``` Below is the code that initialises the List Array ``` static List<ThreadCheck> threadKick = new List<ThreadCheck>(); ``` Below is how I am adding data to the List array ``` public static void addThread(ILibraryInterface library, int threadID, string threadName) { if (Watchdog.library == null) { Watchdog.library = library; } long currentEpochTime = library.convertDateTimeToEpoch(DateTime.Now); threadKick.Add(new ThreadCheck() { threadName = threadName, threadID = threadID, epochTimeStamp = currentEpochTime }); library.logging(classDetails + MethodInfo.GetCurrentMethod().Name, string.Format("Thread ID: {0} has been added to watchdog for {1}", threadID, threadName)); } ``` Below is the code where I am trying to modify the data within the list ``` public static void kickThread(int threadId) { lock (threadKick) { for (int i = 0; i < threadKick.Count; i++) { if (threadKick[i].threadID == threadId) { threadKick[i].epochTimeStamp = library.convertDateTimeToEpoch(DateTime.Now); break; } } } } ``` How can I modify the content of the list array without getting the error as above?