Why do I have both of these constructors in my class?

In my project, I see that I have two constructors. The debugger only ever attaches to the second constructor - why do I h开发者_StackOverflow中文版ave both?

public EventDialog()
{
    // Required for Windows Form Designer support
    InitializeComponent(); 

    m_timer.Interval = SystemInformation.DoubleClickTime;
    m_timer.AutoReset = false;
    m_timer.Elapsed +=new System.Timers.ElapsedEventHandler(TimerElapseCallback);
} // Constructor

public EventDialog(string[] list)
{
    // Required for Windows Form Designer support
    InitializeComponent(); 

    if(list != null)
    {
        foreach(string s in list)
        {
            if(s.Trim() != string.Empty)
                m_leafComboBox.Items.Add(s.Trim());
        }
    }

    m_timer.Interval = SystemInformation.DoubleClickTime;
    m_timer.AutoReset = false;
    m_timer.Elapsed +=new System.Timers.ElapsedEventHandler(TimerElapseCallback);
}

     //
     // Required for Windows Form Designer support
     //

Only the comment in the first constructor is accurate. You can't design the form without a default constructor. Make it look better like this:

  public EventDialog(string[] list) : this()
  {
     if (list != null)
     {
        // etc..
     }
  }

Note the added this() to call the default constructor. You now only have to add the special constructor code.

The parameter-less constructor (the first one) is required because otherwise Visual Studio designer won't be able to create an instance of this class (I suppose that's a Form class) to show in in the UI designer.

The second constructor is most probably created by a developer to pass some extra parameters when creating the dialog manually from code.

BTW I'd better write it like this:

public EventDialog(string[] list) : this() { ... }

The reason that only the second constructor is attached is that there must exist a call which passes a value to the constructor and the second constructor does not call the parameterless constructor. If you define a constructor with parameters, it will be the only constructor that is accepted. I.e., a parameterless constructor will no longer exist on the class. However, if you want to continue to allow other code to instantiate the class using a parameterless constructor and you want a constructor with parameters, you must declare both.