Add more argument validation to TestScheduler.Start

Rx.NET/Source/src/Microsoft.Reactive.Testing/TestScheduler.cs

@@ -79,13 +79,24 @@ protected override long ToRelative(TimeSpan timeSpan)
         /// <param name="disposed">Virtual time at which to dispose the subscription.</param>
         /// <returns>Observer with timestamped recordings of notification messages that were received during the virtual time window when the subscription to the source sequence was active.</returns>
         /// <exception cref="ArgumentNullException"><paramref name="create"/> is null.</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="subscribed"/> is less than <paramref name="created"/>.</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="disposed"/> is less than <paramref name="created"/> or <paramref name="subscribed"/>.</exception>
         public ITestableObserver<T> Start<T>(Func<IObservable<T>> create, long created, long subscribed, long disposed)
         {
             if (create == null)
             {
                 throw new ArgumentNullException(nameof(create));
             }
 
+            if (subscribed < created)
+            {
+                throw new ArgumentOutOfRangeException(nameof(subscribed));
+            }
+            if (disposed < created || disposed < subscribed)
+            {
+                throw new ArgumentOutOfRangeException(nameof(disposed));
+            }
+
             var source = default(IObservable<T>);
             var subscription = default(IDisposable);
             var observer = CreateObserver<T>();
@@ -108,6 +119,7 @@ public ITestableObserver<T> Start<T>(Func<IObservable<T>> create, long created,
         /// <param name="disposed">Virtual time at which to dispose the subscription.</param>
         /// <returns>Observer with timestamped recordings of notification messages that were received during the virtual time window when the subscription to the source sequence was active.</returns>
         /// <exception cref="ArgumentNullException"><paramref name="create"/> is null.</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="disposed"/> is less than <see cref="ReactiveTest.Subscribed"/>.</exception>
         public ITestableObserver<T> Start<T>(Func<IObservable<T>> create, long disposed)
         {
             if (create == null)

Rx.NET/Source/tests/Tests.System.Reactive/Tests/Concurrency/TestSchedulerTest.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Reactive.Concurrency;
+using System.Reactive.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Reactive.Testing;
+using Xunit;
+
+namespace ReactiveTests.Tests
+{
+
+    public class TestSchedulerTest
+    {
+        [Fact]
+        public void Test_ArgumentChecking()
+        {
+            ReactiveAssert.Throws<ArgumentNullException>(() => new TestScheduler().Start<int>(null));
+            ReactiveAssert.Throws<ArgumentNullException>(() => new TestScheduler().Start<int>(null, 10));
+            ReactiveAssert.Throws<ArgumentOutOfRangeException>(() => new TestScheduler().Start(() => Observable.Empty<int>(), 10));
+            ReactiveAssert.Throws<ArgumentOutOfRangeException>(() => new TestScheduler().Start(() => Observable.Empty<int>(), 10, 15, 5));
+        }
+    }
+}