Wirte new class EventExtensions.cs

Issue #297 To contain methods that help with dealing with lists of mixed event types.

src/AudioAnalysisTools/Events/EventExtentions.cs

@@ -0,0 +1,44 @@
+// <copyright file="EventExtentions.cs" company="QutEcoacoustics">
+// All code in this file and all associated files are the copyright and property of the QUT Ecoacoustics Research Group (formerly MQUTeR, and formerly QUT Bioacoustics Research Group).
+// </copyright>
+
+namespace AudioAnalysisTools.Events
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Text;
+
+    public static class EventExtentions
+    {
+        //NOTES on SYNTAX:
+        //Select is a transform - fails if it encounters anything that is not of type SpectralEvent.
+        //var spectralEvents = events.Select(x => (SpectralEvent)x).ToList();
+
+        //Where is a FILTER - only returns spectral events.
+        //var spectralEvents = events.Where(x => x is SpectralEvent).Cast<SpectralEvent>().ToList();
+        //var spectralEvents = events.Where(x => x is ChirpEvent).ToList();
+        //var chirpEvents = events.Cast<ChirpEvent>().ToList();
+
+        public static (List<T> TargetEvents, List<U> OtherEvents) FilterForEventType<T, U>(this List<U> events)
+            where U : EventCommon
+            where T : EventCommon
+        {
+            var target = new List<T>(events.Count);
+            var other = new List<U>(events.Count);
+
+            foreach (var @event in events)
+            {
+                if (@event is T t)
+                {
+                    target.Add(t);
+                }
+                else
+                {
+                    other.Add(@event);
+                }
+            }
+
+            return (target, other);
+        }
+    }
+}