Question

In MS Exam 70-536 .Net Foundation, Chapter 7 “Threading” in Lesson 1 Creating Threads there is a text:

Be aware that because the WorkWithParameter method takes an object, Thread.Start
could be called with any object instead of the string it expects. Being careful in choosing
your starting method for a thread to deal with unknown types is crucial to good
threading code. Instead of blindly casting the method parameter into our string, it is
a better practice to test the type of the object, as shown in the following example:

' VB
Dim info As String = o as String
If info Is Nothing Then
    Throw InvalidProgramException("Parameter for thread must be a string")
End If
// C#
string info = o as string;
if (info == null)
{
    throw InvalidProgramException("Parameter for thread must be a string");
} 

So, I’ve tried this but exception is not handled properly (no console exception entry, program is terminated), what is wrong with my code (below)?

class Program
    {
        static void Main(string[] args)
        {
            Thread thread = new Thread(SomeWork);
            try
            {
                thread.Start(null);
                thread.Join();
            }
            catch (InvalidProgramException ex)
            {
                Console.WriteLine(ex.Message);
            }
            finally
            {

                Console.ReadKey();
            }
        }

        private static void SomeWork(Object o)
        {
            String value = (String)o;
            if (value == null)
            {
                throw new InvalidProgramException("Parameter for "+
                    "thread must be a string");
            }
        }
    }

Thanks for your time!

Answer 1

First start in VS in debug mode. Now to code, several issues:

    catch (InvalidProgramException ex)
    {
        Console.WriteLine(ex.Message);
    }

will never be executed because exception is local to thread. If you throw exception in spawned thread other threads will not see it.

throw new InvalidProgramException("Parameter for "+
                    "thread must be a string");

this line causes unhandled exception, because in that thread no one caught it. unhandled exception fail entire app.

    String value = (String)o;
    if (value == null)

o can be null which is valid value for String and your code will throw exceptions if o is not string. Did you meant:

    String value = o as String;
    if (value == null)