Why it's happening me this error: Object synchronization?

I'm developing a Smart Array (it's a request I cannot use a List of int that I know it's easier because I made both codes). I have done this class and below is the example of how I use it. The error is often in this line (153 from class):

// Ensure that the lock is released.
Monitor.Exit(array);

If I use a List nothing wrong happens just when I translate to an array. Thanks for your help.

SmartArray3.cs

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading; using System.Threading.Tasks;  namespace SmartArray {     class SmartArray3     {         private int[] array;         private int size = 0;         private int count = 0;          public SmartArray3()         {             Resize(1);         }          public SmartArray3(int size)         {             this.size = size;             array = new int[this.size];         }          public bool Resize(int size)         {             try             {                 if (array == null)                     array = new int[size];                 else                     Array.Resize(ref array, size);                  this.size++;                 return true;             }             catch             {                 return false;             }         }          private void add(int value)         {             try             {                 if (array == null)                 {                     this.size = 1;                     Resize(this.size);                      array[0] = value;                     this.count++;                 }                 else                 {                     if (this.count == (this.size - 1))                     {                         this.size *= 2;                         this.Resize(this.size);                     }                      if ((this.count - 1) < 0)                         array[0] = value;                     else                         array[this.count - 1] = value;                     this.count++;                 }             }              catch (Exception ex)             {                 Console.Write(ex.ToString());                 throw new System.IndexOutOfRangeException("Index out of Range.");             }          }          // Lock the array and add an element.          public void Add(int value)         {             // Request the lock, and block until it is obtained.             Monitor.Enter(array);             try             {                 if (array == null)                 {                     this.size = 1;                     Resize(this.size);                      array[0] = value;                     this.count++;                 }                 else                 {                     if (this.count == (this.size - 1))                     {                         this.size *= 2;                         this.Resize(this.size);                     }                      if ((this.count - 1) < 0)                         array[0] = value;                     else                         array[this.count - 1] = value;                     this.count++;                 }             }             finally             {                 // Ensure that the lock is released.                 Monitor.Exit(array);             }         }          // Try to add an element to the List: Add the element to the List           // only if the lock is immediately available.          public bool TryAdd(int value)         {             // Request the lock.              if (Monitor.TryEnter(array))             {                 try                 {                     if (array == null)                     {                         this.size = 1;                         Resize(this.size);                          array[0] = value;                         this.count++;                     }                     else                     {                         if (this.count == (this.size - 1))                         {                             this.size *= 2;                             this.Resize(this.size);                         }                          if ((this.count - 1) < 0)                             array[0] = value;                         else                             array[this.count - 1] = value;                         this.count++;                     }                 }                 finally                 {                     // Ensure that the lock is released.                     Monitor.Exit(array);                 }                 return true;             }             else             {                 return false;             }         }          public int Get(int index)         {             try             {                 return array[index];             }             catch (IndexOutOfRangeException ex)             {                 throw new System.IndexOutOfRangeException("Index out of range");             }         }     } }

Code for called the Class:

        private static int threadsRunning = 0;         private SmartArray3 sa = new SmartArray3();         private List times;          private static string[] titles ={                                 "Add                            ", "Add failed                     ", "TryAdd succeeded               ", "TryAdd failed                  "};         private static int[][] results = new int[3][];          //Event to Create Threads         private void newTest()         {             for (int i = 0; i < 3; i++)             {                 Thread t = new Thread(ThreadProc);                 t.Start(i);                 Interlocked.Increment(ref threadsRunning);             }         }          private void ThreadProc(object state)         {             times = new List();             DateTime finish = DateTime.Now.AddSeconds(10);             Random rand = new Random();             int[] result = { 0, 0, 0, 0};             int threadNum = (int)state;              while (DateTime.Now < finish)             {                 Stopwatch sw = Stopwatch.StartNew();                 int what = rand.Next(250);                 int how = rand.Next(25);                  if (how < 16)                 {                     try                     {                         sa.Add(what);                         result[(int)ThreadResultIndex.AddCt] += 1;                         times.Add(sw.Elapsed.TotalMilliseconds);                     }                     catch                     {                         result[(int)ThreadResultIndex.AddFailCt] += 1;                     }                 }                 else                 {                     if (sa.TryAdd(what))                     {                         result[(int)ThreadResultIndex.TryAddSucceedCt] += 1;                     }                     else                     {                         result[(int)ThreadResultIndex.TryAddFailCt] += 1;                     }                 }                 sw.Stop();             }             results[threadNum] = result;              if (0 == Interlocked.Decrement(ref threadsRunning))             {                 StringBuilder sb = new StringBuilder(                    "                               Thread 1 Thread 2 Thread 3 Total\n");                  for (int row = 0; row < 4; row++)                 {                     int total = 0;                     sb.Append(titles[row]);                      for (int col = 0; col < 3; col++)                     {                         sb.Append(String.Format("{0,4} ", results[col][row]));                         total += results[col][row];                     }                      sb.AppendLine(String.Format("{0,4} ", total));                 }                  Console.WriteLine(sb.ToString());             }         }          private enum ThreadResultIndex         {             AddCt,             AddFailCt,             TryAddSucceedCt,             TryAddFailCt         }