fix: add debugger display to ManualTimer

Author: egil

src/TimeProviderExtensions/ManualTimeProvider.ManualTimer.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
 namespace TimeProviderExtensions;
@@ -9,10 +10,13 @@ namespace TimeProviderExtensions;
 /// Learn more at <see href="https://github.com/egil/TimeProviderExtensions"/>.
 /// </remarks>
 public partial class ManualTimeProvider : TimeProvider
-{    
+{
+    [DebuggerDisplay("ManualTimer: {scheduledCallback.ToString(),nq}, due time = {dueTime}, period = {period}.")]
     private sealed class ManualTimer : ITimer
     {
         private ManualTimerScheduler? scheduledCallback;
+        private TimeSpan dueTime;
+        private TimeSpan period;
 
         public ManualTimer(TimerCallback callback, object? state, ManualTimeProvider timeProvider)
         {
@@ -30,6 +34,9 @@ public bool Change(TimeSpan dueTime, TimeSpan period)
                 return false;
             }
 
+            this.dueTime = dueTime;
+            this.period = period;
+
             scheduledCallback.Change(dueTime, period);
 
             return true;

src/TimeProviderExtensions/ManualTimeProvider.ManualTimerScheduledCallback.cs

@@ -1,3 +1,5 @@
+using System.Globalization;
+
 namespace TimeProviderExtensions;
 
 /// <summary>
@@ -34,6 +36,11 @@ public int CompareTo(ManualTimerScheduler? other)
             ? Comparer<DateTimeOffset>.Default.Compare(CallbackTime, other.CallbackTime)
             : -1;
 
+        public override string ToString()
+            => running
+            ? $"Next callback invocation {CallbackTime.ToString("yyyy-MM-ddTHH:mm:ss.fff", CultureInfo.InvariantCulture)}"
+            : "Idle";
+
         internal void Cancel()
         {
             if (running)