We have another useful class in .net- Monitor. The Monitor class provides functions through which we can implement locking mechanism in the threads. In general it works similar to the SyncLock but the advantage is that we can do lock based error handling with Monitor. For this we can use Enter() and Exit() functions with our usual error handling Try…Catch…Finally block. Let's modify the DoAction method to use Monitor as given in the class below.

To understand its implementation, just modify the existing DoAction method to use Monitor class as given below. No need to make any change in Main() for now.

Listing 9 – Thread delegate method using Monitor class

Public Class CThread
  Public Sub DoAction()
    Try
    Threading.Monitor.Enter(Me)
    Console.WriteLine("{0} : {1} : To sleep now...", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString())
    Threading.Thread.Sleep(5000)
    Console.WriteLine("{0} : {1} : is woke up...", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString())
    Catch ex As Exception
    Console.WriteLine("{0} : {1} : Error in monitor section,exiting monitor...", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString())
    ' do some job like error logging in file or database..
    Finally
    Threading.Monitor.Exit(Me)
    End Try
  End Sub
End Class

We can code Monitor.Enter and Monitor.Exit methods anywhere inside the Try…Catch…Finally block of the delegate method. In this way it gives more flexibility to implement and handle complex logic inside lock area.

Run the application (CTRL+F5), you would see console window as:

Figure 4 – Console output window of Monitor implementation

As this result same as the SyncLock example we had done before. To test how Monitor is useful with Try…Catch…Finally block, we will modify the DoAction method to read a file (say c:\abc.txt) which does not exist in the computer. The very first instance of the delegate method will be the victim of FileNotFound error. In catch block we will create that file so that next all other threads will not get that error. Here is complete code of class containing DoAction method.

Listing 10 – Thread delegate method using Monitor class and Try…Catch…Finally to handle runtime errors.

Public Class CThread
  Public Sub DoAction()
    Try
    Threading.Monitor.Enter(Me)
    Console.WriteLine("{0} : {1} : To sleep now...", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString())
    Threading.Thread.Sleep(5000)
    IO.File.ReadAllText("c:\abc.txt")
    Console.WriteLine("{0} : {1} : is woke up...", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString())
 
    Catch ioErr As IO.FileNotFoundException
    Console.WriteLine("{0} : {1} : Error- File Not Found.", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString())
    ' create the file for next all threads
    IO.File.Create("c:\abc.txt")
    Catch ex As Exception
    Console.WriteLine("{0} : {1} : Error in monitor section : {2}", _
      Threading.Thread.CurrentThread.ManagedThreadId, Now.ToString(), ex.ToString())
    ' do some job like error logging in file or database..
    Finally
    Threading.Monitor.Exit(Me)
    GC.Collect()
    End Try
  End Sub
End Class

One more thing you may have noticed that I have used GC.Collect() at the end in finally block. I used it to release any resources (a file in this example) being used by thread, so that other thread would not get any file access error.

Run the application (CTRL+F5), you would see the console output window as:

Figure 5 – Console output of the application showing error handling with Monitor uses.

You can see in the result that managed thread # 3 got a FileNotFound error and next thread # 4 executed normally since thread # 3 created the missing file in the Catch block.

Few sample project of this article can be downloaded though the download links given below.