Possible to accessing child "DebuggerDisplay" attribute of property?

Copied possible solution from OP

Probably, my requirement is not possible as per this SO answer. Maybe a good solution would be to override ToString in class B and do some if..else and use the Debugger.IsAttached property to behave different only inside the debugger.

Something like:

[DebuggerDisplay(@"Three = {Three}")]
public class B
{
    public int Three { get; set; }

    public override string ToString()
    {
        if (Debugger.IsAttached)
        {
            return string.Format(@"Three = {0}", Three);
        }
        else
        {
            return base.ToString();
        }
    }
}

Piecing together a few things I've come up with this solution. It has the caveat it expects you to follow https://blogs.msdn.microsoft.com/jaredpar/2011/03/18/debuggerdisplay-attribute-best-practices/

[DebuggerDisplay("{DebuggerDisplay,nq}")]
public class B
{
    public int Three { get; set; }

    private string DebuggerDisplay => $"Three = {Three}";
}

[DebuggerDisplay("{DebuggerDisplay,nq}")]
public class A
{
    public int One { get; set; }
    public B Two { get; set; }

    private string DebuggerDisplay => $"One = {One}, two = {Two.ReadDebuggerDisplay()}";
}

You'll need to make sure you have proper imports for where ever you stick this helper in relation to the code that needs to read child debugger displays.

public static class ReflectionHelper
{
    // https://stackoverflow.com/a/13650728/37055
    public static object ReadProperty(
        this object target,
        string propertyName)
    {
        var args = new[] {CSharpArgumentInfo.Create(0, null)};
        var binder = Binder.GetMember(0, propertyName, target.GetType(), args);
        var site = CallSite<Func<CallSite, object, object>>.Create(binder);
        return site.Target(site, target);
    }

    public static string ReadDebuggerDisplay(
        this object target, 
        string propertyName = "DebuggerDisplay")
    {
        string debuggerDisplay = null;
        try
        {
            var value = ReadProperty(target, propertyName) ?? "<null object>";

            debuggerDisplay = value as string ?? value.ToString();
        }
        catch (Exception)
        {
            // ignored
        }
        return debuggerDisplay ?? 
              $"<ReadDebuggerDisplay failed on {target.GetType()}[{propertyName}]>";
    }
}

I feel like this a pretty fair balance of purity and pragmatism for lowering the friction on achieving this. If you are less concerned about purity you could just make DebuggerDisplay public. I prefer that the ReadDebuggerDisplay operates in a "type-less" manner (avoids generic constraints and interfaces that would be needed to access DebuggerDisplay publicly).

[Disclaimer I'm affiliated with OzCode]

You can use OzCode's Reveal feature which supports nested debug information.
The plus side is that you do not need to change your production code and once you define it for an instance it would be used automatically for all instances of that type.