[release/6.0] Manually backporting #74599 to 6.0 (#78247)

* Manually backporting #74599 to 7.0 for RC2. (#75366)

* Manually backporting #74599 to 7.0 for RC2.

* Fix a couple mis-copied lines of code and a couple nits.

* Missed some file updates when manually porting.

* Advance when reading decimals regardless of endian-ness.

* Backport the actual 7.0 implementation for decimal on big-endian platforms.

Author: StephenMolloy

src/libraries/System.Private.DataContractSerialization/src/System/Xml/XmlBinaryReader.cs

@@ -1346,14 +1346,14 @@ private unsafe int ReadArray(float[] array, int offset, int count)
 
         public override int ReadArray(string localName, string namespaceUri, float[] array, int offset, int count)
         {
-            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.FloatTextWithEndElement))
+            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.FloatTextWithEndElement) && BitConverter.IsLittleEndian)
                 return ReadArray(array, offset, count);
             return base.ReadArray(localName, namespaceUri, array, offset, count);
         }
 
         public override int ReadArray(XmlDictionaryString localName, XmlDictionaryString namespaceUri, float[] array, int offset, int count)
         {
-            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.FloatTextWithEndElement))
+            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.FloatTextWithEndElement) && BitConverter.IsLittleEndian)
                 return ReadArray(array, offset, count);
             return base.ReadArray(localName, namespaceUri, array, offset, count);
         }
@@ -1372,14 +1372,14 @@ private unsafe int ReadArray(double[] array, int offset, int count)
 
         public override int ReadArray(string localName, string namespaceUri, double[] array, int offset, int count)
         {
-            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DoubleTextWithEndElement))
+            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DoubleTextWithEndElement) && BitConverter.IsLittleEndian)
                 return ReadArray(array, offset, count);
             return base.ReadArray(localName, namespaceUri, array, offset, count);
         }
 
         public override int ReadArray(XmlDictionaryString localName, XmlDictionaryString namespaceUri, double[] array, int offset, int count)
         {
-            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DoubleTextWithEndElement))
+            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DoubleTextWithEndElement) && BitConverter.IsLittleEndian)
                 return ReadArray(array, offset, count);
             return base.ReadArray(localName, namespaceUri, array, offset, count);
         }
@@ -1398,14 +1398,14 @@ private unsafe int ReadArray(decimal[] array, int offset, int count)
 
         public override int ReadArray(string localName, string namespaceUri, decimal[] array, int offset, int count)
         {
-            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DecimalTextWithEndElement))
+            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DecimalTextWithEndElement) && BitConverter.IsLittleEndian)
                 return ReadArray(array, offset, count);
             return base.ReadArray(localName, namespaceUri, array, offset, count);
         }
 
         public override int ReadArray(XmlDictionaryString localName, XmlDictionaryString namespaceUri, decimal[] array, int offset, int count)
         {
-            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DecimalTextWithEndElement))
+            if (IsStartArray(localName, namespaceUri, XmlBinaryNodeType.DecimalTextWithEndElement) && BitConverter.IsLittleEndian)
                 return ReadArray(array, offset, count);
             return base.ReadArray(localName, namespaceUri, array, offset, count);
         }