Example where [DebuggerDisplay(...)] attribute is useful?

Question

I am looking for a good concrete example where it is clearly desirable to override ToString() with something, but to use a [DebuggerDisplay(...)] custom attribute to display something else in the debugger?

Answer 1

Suppose you have an existing application where .ToString() is expected to, say, serialize the object to a string. Not that it's a good idea, but suppose you're in that situation. You could then still use [DebuggerDisplay(...)] to make your life easier, without modifying this (admittedly bad, but I suspect not unusual) contract between the class and the rest of the application.

Answer 2

Say, for example, a Node object for a binary tree.

The ToString() would merely want to display the payload for that node, while the DebuggerDisplay would probably also show which nodes it was pointing to.

Answer 3

You can add also note that ToString() is not evaluated by the debugger in VB.NET. So if you plan on developing with several languages, getting used to this attributes in a good idea.

I prefer to use this attribute over the ToString because suppose I don't need to use the ToString() method for anything else, I don't like the idea of having a method sitting there for nothing.

And if you need another reason, I think it makes more sense to use a declarative approach because the debugger display string is just some metadata, it could also be used by some other tools.

Answer 4

Lazy in .NET4 uses it to display important properties in debug:

[Serializable, 
DebuggerDisplay("ThreadSafetyMode={Mode}, IsValueCreated={IsValueCreated}, IsValueFaulted={IsValueFaulted}, Value={ValueForDebugDisplay}"), 
DebuggerTypeProxy(typeof(System_LazyDebugView<>)), ComVisible(false), HostProtection(SecurityAction.LinkDemand, Synchronization=true, ExternalThreading=true)]
public class Lazy<T>
{
...
}

ArrayList also use:

    [Serializable, ComVisible(true), DebuggerTypeProxy(typeof(ArrayListDebugView)), 
DebuggerDisplay("Count = {Count}")]
    public class ArrayList : IList, ICollection, IEnumerable, ICloneable
    {
    ...
    }

---

Or Color structure:

    [Serializable, StructLayout(LayoutKind.Sequential), TypeConverter(typeof(ColorConverter)), 
DebuggerDisplay("{NameAndARGBValue}"), 
Editor("System.Drawing.Design.ColorEditor, System.Drawing.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
    public struct Color
    {
    ...
    }

You can see it by using .NET Reflector tool.