Fix BinaryReader on BE (#17893)

Fix BinaryReader on BE

The ints passed into the int array ctor needed to be endian swapped.
Interestingly, the BinaryWriter implementation currently being used
(not refsrc AFAIK) does this on write just fine.

Fixes ReadDecimal test on AIX, and adds another test to ensure that
Write(decimal) can be read back by ReadDecimal in case.

2 files changed, 25 insertions, 0 deletions

diff --git a/mcs/class/corlib/Test/System.IO/BinaryWriterTest.cs b/mcs/class/corlib/Test/System.IO/BinaryWriterTest.cs
index 8fc3d11f902..6ba707a1d7f 100644
--- a/mcs/class/corlib/Test/System.IO/BinaryWriterTest.cs
+++ b/mcs/class/corlib/Test/System.IO/BinaryWriterTest.cs
@@ -419,6 +419,24 @@ public class BinaryWriterTest {
 		Assert.AreEqual (0, bytes [45], "test#47");
 		Assert.AreEqual (0, bytes [46], "test#48");		
 	}
+
+	[Test]
+	public void WriteDecimalAndReadBack()
+	{
+		// This value is the same used in BinaryReader.ReadDecimal, which will be
+		// problematic if BinaryWriter or BinaryReader are endianness-confused.
+		Decimal writeDec = -18295873486192640;
+		using (var ms = new MemoryStream(32)) {
+			using (var bw = new BinaryWriter(ms, new UTF8Encoding(), true)) {
+				bw.Write(writeDec);
+			}
+			ms.Position = 0;
+			using (var br = new BinaryReader(ms)) {
+				Decimal readDec = br.ReadDecimal();
+				Assert.AreEqual (readDec, writeDec, "test#01");
+			}
+		}
+	}
 	
 	[Test]
 	public void WriteFloat ()
diff --git a/mcs/class/referencesource/mscorlib/system/io/binaryreader.cs b/mcs/class/referencesource/mscorlib/system/io/binaryreader.cs
index e4e1ecfe376..9f1b89de5d5 100644
--- a/mcs/class/referencesource/mscorlib/system/io/binaryreader.cs
+++ b/mcs/class/referencesource/mscorlib/system/io/binaryreader.cs
@@ -23,6 +23,7 @@ namespace System.IO {
     using System.Globalization;
     using System.Diagnostics.Contracts;
     using System.Security;
+    using System.Buffers.Binary;
 
 [System.Runtime.InteropServices.ComVisible(true)]
     public class BinaryReader : IDisposable
@@ -243,6 +244,12 @@ namespace System.IO {
             try {
                 int[] ints = new int[4];
                 Buffer.BlockCopy(m_buffer, 0, ints, 0, 16);
+                if (!BitConverter.IsLittleEndian) {
+                    // We need to reverse the ints on BE
+                    for (int i = 0; i < 4; i++) {
+                         ints[i] = BinaryPrimitives.ReverseEndianness(ints[i]);
+                    }
+                }
                 return new decimal(ints);
             }
             catch (ArgumentException e) {