Perf: use Stream.CopyTo and zero-copy MemoryPackagePart reads

openxml4Net/OPC/Internal/MemoryPackagePart.cs

@@ -64,11 +64,7 @@ protected override Stream GetInputStreamImpl()
             {
                 return new MemoryStream();
             }
-            MemoryStream newMs = new MemoryStream((int)data.Length);
-            data.Position = 0;
-            StreamHelper.CopyStream(data, newMs);
-            newMs.Position = 0;
-            return newMs;
+            return new MemoryStream(data.GetBuffer(), 0, (int)data.Length, writable: false);
         }
 
         protected override Stream GetOutputStreamImpl()

openxml4Net/OPC/StreamHelper.cs

@@ -53,14 +53,7 @@ public static void SaveXmlInStream(XmlDocument xmlContent,
          */
         public static void CopyStream(Stream inStream, Stream outStream)
         {
-            byte[] buffer = new byte[1024];
-            int bytesRead = 0;
-            int totalRead = 0;
-            while ((bytesRead = inStream.Read(buffer, 0, buffer.Length)) > 0)
-            {
-                outStream.Write(buffer, 0, bytesRead);
-                totalRead += bytesRead;
-            }
+            inStream.CopyTo(outStream);
         }
     }