C# – Intercepting and cancelling the Click event of a WinForms Button

ccompact-frameworkeventswinforms

Is there a way to capture the Click event of a Button from the parent Control and prevent it occuring if a variable within the parent Control is true?

For example:

private void AssignEventOverrides()
{
    foreach (Button button in Buttons) 
    {
        // Get the event assigned at design time
        var someUnknownEventHandler = GetTheClickHandlerSomehow(button);

        // Unsubscribe the unknown event
        button.Click -= SomeUnknownEventHandler;

        // Assign the intercepting event
        button.Click += button_Click;
    }
}

private void button_Click(object sender, EventArgs e)
{
    if (!preventClick)
    {
        // Fire the intercepted event that was previously unsubscribed
    }
}

Hopefully there's a nicer way to do this than the above example. Just to note, I don't have any control over the events assigned to the afforementioned buttons. They're subscribed elsewhere and I just need to prevent them happening if a variable is true on the consuming parent Control.

My real scenario:

I'm using Windows Mobile 6.5 (legacy application rewrite) I've created a panel control that can contain N number of child controls. The panel control allows for iPhone style vertical scrolling. I subscribe to the MouseMove event of the child Control and set a variable _isScrolling to true. In the MouseUp event I set _isScrolling to false.

As Click occurs before MouseUp I can check within the Click event to ensure a scroll hadn't occured when the button was pressed. So basically, if a scroll of my panel occurs, I need to prevent the Click event (subscribed at design time) from firing.

Best Answer

This might provide some useful insight

How to remove all event handlers from a control

Specifically, look at

private void RemoveClickEvent(Button b)
{
    FieldInfo f1 = typeof(Control).GetField("EventClick", 
        BindingFlags.Static | BindingFlags.NonPublic);
    object obj = f1.GetValue(b);
    PropertyInfo pi = b.GetType().GetProperty("Events",  
        BindingFlags.NonPublic | BindingFlags.Instance);
    EventHandlerList list = (EventHandlerList)pi.GetValue(b, null);
    list.RemoveHandler(obj, list[obj]);
}

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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