Special-case Enumerable.SequenceEqual for byte[]

src/libraries/System.Linq/src/System/Linq/SequenceEqual.cs

@@ -12,11 +12,6 @@ public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnum
 
         public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource>? comparer)
         {
-            if (comparer == null)
-            {
-                comparer = EqualityComparer<TSource>.Default;
-            }
-
             if (first == null)
             {
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.first);
@@ -27,6 +22,24 @@ public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnum
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.second);
             }
 
+            if (comparer == null)
+            {
+                // It's relatively common to see code (especially in tests and testing frameworks) that ends up
+                // using Enumerable.SequenceEqual to compare two byte arrays.  Using ReadOnlySpan.SequenceEqual
+                // is significantly faster than accessing each byte via the array's IList<byte> interface
+                // implementation.  So, we special-case byte[] here.  It would be nice to be able to delegate
+                // to ReadOnlySpan.SequenceEqual for all TSource[] arrays where TSource is a value type and
+                // implements IEquatable<TSource>, but there's no good way without reflection to convince
+                // the C# compiler to let us delegate, as ReadOnlySpan.SequenceEqual requires an IEquatable<T>
+                // constraint on its type parameter, and Enumerable.SequenceEqual lacks one on its type parameter.
+                if (typeof(TSource) == typeof(byte) && first is byte[] firstArr && second is byte[] secondArr)
+                {
+                    return ((ReadOnlySpan<byte>)firstArr).SequenceEqual(secondArr);
+                }
+
+                comparer = EqualityComparer<TSource>.Default;
+            }
+
             if (first is ICollection<TSource> firstCol && second is ICollection<TSource> secondCol)
             {
                 if (firstCol.Count != secondCol.Count)

src/libraries/System.Linq/tests/SequenceEqualTests.cs

@@ -214,5 +214,37 @@ public void SecondSourceNull()
 
             AssertExtensions.Throws<ArgumentNullException>("second", () => first.SequenceEqual(second));
         }
+
+        [Fact]
+        public void ByteArrays_SpecialCasedButExpectedBehavior()
+        {
+            AssertExtensions.Throws<ArgumentNullException>("first", () => ((byte[])null).SequenceEqual(new byte[1]));
+            AssertExtensions.Throws<ArgumentNullException>("second", () => new byte[1].SequenceEqual(null));
+
+            Assert.False(new byte[1].SequenceEqual(new byte[0]));
+            Assert.False(new byte[0].SequenceEqual(new byte[1]));
+
+            var r = new Random();
+            for (int i = 0; i < 32; i++)
+            {
+                byte[] arr = new byte[i];
+                r.NextBytes(arr);
+
+                byte[] same = (byte[])arr.Clone();
+                Assert.True(arr.SequenceEqual(same));
+                Assert.True(same.SequenceEqual(arr));
+                Assert.True(same.SequenceEqual(arr.ToList()));
+                Assert.True(same.ToList().SequenceEqual(arr));
+                Assert.True(same.ToList().SequenceEqual(arr.ToList()));
+
+                if (i > 0)
+                {
+                    byte[] diff = (byte[])arr.Clone();
+                    diff[^1]++;
+                    Assert.False(arr.SequenceEqual(diff));
+                    Assert.False(diff.SequenceEqual(arr));
+                }
+            }
+        }
     }
 }