Improve consistency of StreamReader and StreamWriter

src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs

@@ -25,7 +25,6 @@ public class StreamReader : TextReader
         // saves construction time.  This does break adaptive buffering,
         // but this is slightly faster.
         private const int DefaultBufferSize = 1024;  // Byte buffer size
-        private const int DefaultFileStreamBufferSize = 4096;
         private const int MinBufferSize = 128;
 
         private readonly Stream _stream;
@@ -123,7 +122,7 @@ public StreamReader(Stream stream, Encoding? encoding, bool detectEncodingFromBy
 
         // Creates a new StreamReader for the given stream.  The
         // character encoding is set by encoding and the buffer size,
-        // in number of 16-bit characters, is set by bufferSize.
+        // in bytes, is set by bufferSize.
         //
         // Note that detectEncodingFromByteOrderMarks is a very
         // loose attempt at detecting the encoding by looking at the first
@@ -227,9 +226,13 @@ private static FileStream ValidateArgsAndOpenPath(string path, FileStreamOptions
         private static FileStream ValidateArgsAndOpenPath(string path, int bufferSize)
         {
             ArgumentException.ThrowIfNullOrEmpty(path);
-            ArgumentOutOfRangeException.ThrowIfNegativeOrZero(bufferSize);
 
-            return new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultFileStreamBufferSize);
+            if (bufferSize != -1)
+            {
+                ArgumentOutOfRangeException.ThrowIfNegativeOrZero(bufferSize);
+            }
+
+            return new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, FileStream.DefaultBufferSize);
         }
 
         public override void Close()

src/libraries/System.Private.CoreLib/src/System/IO/StreamWriter.cs

@@ -24,7 +24,6 @@ public class StreamWriter : TextWriter
         // which means we take advantage of adaptive buffering code.
         // The performance using UnicodeEncoding is acceptable.
         private const int DefaultBufferSize = 1024;   // char[]
-        private const int DefaultFileStreamBufferSize = 4096;
         private const int MinBufferSize = 128;
 
         // Bit bucket - Null has no backing store. Non closable.
@@ -156,7 +155,7 @@ public StreamWriter(string path, FileStreamOptions options)
         }
 
         public StreamWriter(string path, Encoding? encoding, FileStreamOptions options)
-            : this(ValidateArgsAndOpenPath(path, options), encoding, DefaultFileStreamBufferSize)
+            : this(ValidateArgsAndOpenPath(path, options), encoding, DefaultBufferSize)
         {
         }
 
@@ -184,9 +183,13 @@ private static FileStream ValidateArgsAndOpenPath(string path, FileStreamOptions
         private static FileStream ValidateArgsAndOpenPath(string path, bool append, int bufferSize)
         {
             ArgumentException.ThrowIfNullOrEmpty(path);
-            ArgumentOutOfRangeException.ThrowIfNegativeOrZero(bufferSize);
 
-            return new FileStream(path, append ? FileMode.Append : FileMode.Create, FileAccess.Write, FileShare.Read, DefaultFileStreamBufferSize);
+            if (bufferSize != -1)
+            {
+                ArgumentOutOfRangeException.ThrowIfNegativeOrZero(bufferSize);
+            }
+
+            return new FileStream(path, append ? FileMode.Append : FileMode.Create, FileAccess.Write, FileShare.Read, FileStream.DefaultBufferSize);
         }
 
         public override void Close()

src/libraries/System.Runtime/tests/System.IO.Tests/StreamReader/StreamReader.CtorTests.cs

@@ -68,6 +68,34 @@ byte[] GetEncodedDataWithPreamble(byte[] data, Encoding encoding)
                 => encoding.GetPreamble().Concat(data).ToArray();
         }
 
+        [Fact]
+        public static void NegativeBufferSize_ThrowsArgumentOutOfRangeException()
+        {
+            var ms2 = new MemoryStream();
+
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamReader(ms2, bufferSize: -2));
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamReader(ms2, bufferSize: 0));
+
+            ms2.Dispose();
+        }
+
+        [Fact]
+        public static void NegativeOneBufferSize_ShouldNotThrowException()
+        {
+            var ms2 = new MemoryStream();
+            try
+            {
+                using (var sr = new StreamReader(ms2,  bufferSize: -1))
+                {
+                    Assert.NotNull(sr);
+                }
+            }
+            finally
+            {
+                ms2.Dispose();
+            }
+        }
+
         [Theory]
         [MemberData(nameof(EncodingsWithEncodedDataWithPreamble))]
         public static void CreationFromMemoryStreamWithEncodingTrueDetectsCorrectEncodingWhenPreambleAvailable(Encoding expectedEncoding, byte[] encodedData, string expectedOutput)

src/libraries/System.Runtime/tests/System.IO.Tests/StreamReader/StreamReader.StringCtorTests.cs

@@ -50,10 +50,27 @@ public static void NullEncodingParamInCtor_ShouldNotThrowException()
         [Fact]
         public static void NegativeBufferSize_ThrowsArgumentOutOfRangeException()
         {
-            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamReader("path", Encoding.UTF8, true, -1));
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamReader("path", Encoding.UTF8, true, -2));
             AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamReader("path", Encoding.UTF8, true, 0));
         }
 
+        [Fact]
+        public static void NegativeOneBufferSize_ShouldNotThrowException()
+        {
+            string testfile = Path.GetTempFileName();
+            try
+            {
+                using (var sr = new StreamReader(testfile, Encoding.UTF8, true, -1))
+                {
+                    Assert.NotNull(sr);
+                }
+            }
+            finally
+            {
+                File.Delete(testfile);
+            }
+        }
+
         [Fact]
         public static void LackOfReadAccess_ThrowsArgumentException()
         {

src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.CtorTests.cs

@@ -70,5 +70,33 @@ private static void TestEncoding(System.Text.Encoding? encoding, string testStri
             str2 = sr2.ReadToEnd();
             Assert.Equal(testString, str2);
         }
+
+        [Fact]
+        public static void NegativeBufferSize_ThrowsArgumentOutOfRangeException()
+        {
+            var ms2 = new MemoryStream();
+
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamWriter(ms2, bufferSize: -2));
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamWriter(ms2, bufferSize: 0));
+
+            ms2.Dispose();
+        }
+
+        [Fact]
+        public static void NegativeOneBufferSize_ShouldNotThrowException()
+        {
+            var ms2 = new MemoryStream();
+            try
+            {
+                using (var sw = new StreamWriter(ms2, bufferSize: -1))
+                {
+                    Assert.NotNull(sw);
+                }
+            }
+            finally
+            {
+                ms2.Dispose();
+            }
+        }
     }
 }

src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.StringCtorTests.cs

@@ -37,10 +37,27 @@ public static void EmptyPath_ThrowsArgumentException()
         [Fact]
         public static void NegativeBufferSize_ThrowsArgumentOutOfRangeException()
         {
-            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamWriter("path", false, Encoding.UTF8, -1));
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamWriter("path", false, Encoding.UTF8, -2));
             AssertExtensions.Throws<ArgumentOutOfRangeException>("bufferSize", () => new StreamWriter("path", true, Encoding.UTF8, 0));
         }
 
+        [Fact]
+        public static void NegativeOneBufferSize_ShouldNotThrowException()
+        {
+            string testfile = Path.GetTempFileName();
+            try
+            {
+                using (StreamWriter sw = new StreamWriter(testfile, false, Encoding.UTF8, -1))
+                {
+                    Assert.NotNull(sw);
+                }
+            }
+            finally
+            {
+                File.Delete(testfile);
+            }
+        }
+
         [Fact]
         public static void LackOfWriteAccess_ThrowsArgumentException()
         {