In .NET Core 3.0 System.IO.Compression's ZipArchive does not support multiple...

In .NET Core 3.0 System.IO.Compression's ZipArchive does not support multiple ZipArchiveEntries to be open concurrently when using FileAccess.Write.  This is a requirement of the XPS serialization stack in WPF.  As such, we need to create XPS documents as FileAccess.ReadWrite in order to allow this behavior.

src/Microsoft.DotNet.Wpf/src/ReachFramework/Packaging/XpsDocument.cs

@@ -894,9 +894,13 @@ bool                                    testIsSignable
             Stream     dataStream
             )
         {
+            // In .NET Core 3.0 System.IO.Compression's ZipArchive does not allow creation of ZipArchiveEntries when
+            // a prior ZipArchiveEntry is still open.  XPS Serialization requires this as part of its implementation.
+            // To get around this, XPS creation should occur in with FileAccess.ReadWrite if the underlying stream
+            // supports it.  This allows multiple ZipArchiveEntries to be open concurrently.
             Package package = Package.Open(dataStream,
                                            FileMode.CreateNew,
-                                           FileAccess.Write);
+                                           (dataStream.CanRead) ? FileAccess.ReadWrite : FileAccess.Write);
             XpsDocument document = new XpsDocument(package);
 
             document.OpcPackage = package;

src/Microsoft.DotNet.Wpf/src/ReachFramework/Packaging/XpsManager.cs

@@ -139,9 +139,13 @@ CompressionOption           compressionOption
             {
                 if( packageAccess == FileAccess.Write )
                 {
+                    // In .NET Core 3.0 System.IO.Compression's ZipArchive does not allow creation of ZipArchiveEntries when
+                    // a prior ZipArchiveEntry is still open.  XPS Serialization requires this as part of its implementation.
+                    // To get around this, XPS creation should occur in with FileAccess.ReadWrite.  This allows multiple
+                    // ZipArchiveEntries to be open concurrently.
                     package = Package.Open(path,
                                            FileMode.Create,
-                                           packageAccess,
+                                           FileAccess.ReadWrite,
                                            FileShare.None);
                     streaming = true;
                 }