From b144f717560b173d3a26c03cccc89443d6cc42f3 Mon Sep 17 00:00:00 2001
From: Sergio Pedri <sergio0694@live.com>
Date: Thu, 22 Oct 2020 22:06:59 +0200
Subject: [PATCH] Fixed MemoryStream seek to end validation

---
 .../Streams/MemoryStream.Validate.cs          |  4 +--
 .../Streams/Test_MemoryStream.cs              | 36 +++++++++++++++++++
 2 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/Microsoft.Toolkit.HighPerformance/Streams/MemoryStream.Validate.cs b/Microsoft.Toolkit.HighPerformance/Streams/MemoryStream.Validate.cs
index 9cd0ff62936..2f35380918f 100644
--- a/Microsoft.Toolkit.HighPerformance/Streams/MemoryStream.Validate.cs
+++ b/Microsoft.Toolkit.HighPerformance/Streams/MemoryStream.Validate.cs
@@ -13,14 +13,14 @@ namespace Microsoft.Toolkit.HighPerformance.Streams
     internal static partial class MemoryStream
     {
         /// <summary>
-        /// Validates the <see cref="Stream.Position"/> argument.
+        /// Validates the <see cref="Stream.Position"/> argument (it needs to be in the [0, length]) range.
         /// </summary>
         /// <param name="position">The new <see cref="Stream.Position"/> value being set.</param>
         /// <param name="length">The maximum length of the target <see cref="Stream"/>.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static void ValidatePosition(long position, int length)
         {
-            if ((ulong)position >= (ulong)length)
+            if ((ulong)position > (ulong)length)
             {
                 ThrowArgumentOutOfRangeExceptionForPosition();
             }
diff --git a/UnitTests/UnitTests.HighPerformance.Shared/Streams/Test_MemoryStream.cs b/UnitTests/UnitTests.HighPerformance.Shared/Streams/Test_MemoryStream.cs
index 8a9e19111cb..f2c63182c63 100644
--- a/UnitTests/UnitTests.HighPerformance.Shared/Streams/Test_MemoryStream.cs
+++ b/UnitTests/UnitTests.HighPerformance.Shared/Streams/Test_MemoryStream.cs
@@ -78,6 +78,42 @@ public void Test_MemoryStream_Seek()
             Assert.AreEqual(stream.Position, 32);
         }
 
+        // See https://github.com/windows-toolkit/WindowsCommunityToolkit/issues/3536
+        [TestCategory("MemoryStream")]
+        [TestMethod]
+        public void Test_MemoryStream_WriteToEndAndRefreshPosition()
+        {
+            byte[]
+                array = new byte[10],
+                temp = new byte[1];
+            ReadOnlyMemory<byte> memory = array;
+
+            using var stream = memory.AsStream();
+
+            for (int i = 0; i < array.Length; i++)
+            {
+                int read = stream.Read(temp, 0, 1);
+
+                Assert.AreEqual(read, 1);
+                Assert.AreEqual(stream.Position, i + 1);
+            }
+
+            Assert.AreEqual(stream.Position, array.Length);
+
+            // These should not throw, seeking to the end is valid
+            stream.Position = stream.Position;
+            Assert.AreEqual(stream.Position, array.Length);
+
+            stream.Seek(array.Length, SeekOrigin.Begin);
+            Assert.AreEqual(stream.Position, array.Length);
+
+            stream.Seek(0, SeekOrigin.Current);
+            Assert.AreEqual(stream.Position, array.Length);
+
+            stream.Seek(0, SeekOrigin.End);
+            Assert.AreEqual(stream.Position, array.Length);
+        }
+
         [TestCategory("MemoryStream")]
         [TestMethod]
         public void Test_MemoryStream_ReadWrite_Array()