How to prevent Debug.Assert(...) to show a modal dialog

Codekas answer is correct, if you want to hit things with a really big hammer. You can use the <assert> element in your application configuration file to set assertuienabled property to false and optionally give a log file for Asserts to be written to. Then you won't have to write your own listener.

You can read more about the assert element at its MSDN page.

I wouldn't recommend it. The problem is that Debug.Assert is only supposed to be fired when you have bugs in your code. If you just ignore them or don't fix them, then you are doing your users a disservice. If, on the other hand, you're firing Debug.Assert for things that aren't bugs, then you're also doing your users a disservice (by reducing the impact of Debug.Assert).

Having said that, you can disable it. The first thing you need to do is remove the default listener from the Debug.Listeners collection:

Debug.Listeners.Clear();

Then, add your own instead:

Debug.Listeners.Add(new MyTraceListener());

You need to create a class that inherits from TraceListener:

class MyTraceListener : TraceListener
{
    // ...

    public override void Fail(string msg, string detailedMsg)
    {
        // log the message (don't display a MessageBox)
    }
}

The important method is the TraceListener.Fail method, which in the implementation of DefaultTraceListener is what displays the message box.

There is no need for Debug.Listeners.Clear()

Just add to your .config:

<system.diagnostics>
    <assert assertuienabled="false"/>
</system.diagnostics>