Add async xmlreader methods

Directory.Build.props

@@ -70,6 +70,7 @@
         <LatestTargetFramework>net8.0</LatestTargetFramework>
         <SamplesFrameworks>net8.0</SamplesFrameworks>
         <SamplesFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(SamplesFrameworks);net472</SamplesFrameworks>
+        <DefineConstants Condition=" '$(TargetFramework)' != 'net35' And '$(TargetFramework)' != 'net40' And '$(TargetFramework)' != 'net46' And '$(TargetFramework)' != 'net472' ">$(DefineConstants);FEATURE_ASYNC_SAX_XML</DefineConstants>
       </PropertyGroup>
     </Otherwise>
   </Choose>

src/DocumentFormat.OpenXml.Framework/OpenXmlPartReader.cs

@@ -10,6 +10,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Text;
 using System.Xml;
 
 namespace DocumentFormat.OpenXml
@@ -26,14 +27,10 @@ public class OpenXmlPartReader : OpenXmlReader
         private readonly List<KeyValuePair<string, string>> _nsDecls = new List<KeyValuePair<string, string>>();
         private readonly Stack<OpenXmlElement> _elementStack = new Stack<OpenXmlElement>();
 
-        [DebuggerBrowsable(DebuggerBrowsableState.Never)]
-        private readonly string? _encoding;
-
-        [DebuggerBrowsable(DebuggerBrowsableState.Never)]
-        private readonly bool? _standalone;
-
         private ElementState _elementState;
 
+        private OpenXmlPartReaderState? _openXmlPartReaderState;
+
         /// <summary>
         /// Initializes a new instance of the OpenXmlPartReader class using the supplied OpenXmlPart class.
         /// </summary>
@@ -100,7 +97,8 @@ public OpenXmlPartReader(Stream partStream, IFeatureCollection features, OpenXml
 
             _resolver = features.GetRequired<IOpenXmlNamespaceResolver>();
             _rootElements = features.GetRequired<IRootElementFeature>();
-            _xmlReader = CreateReader(partStream, options.CloseStream, options.MaxCharactersInPart, ignoreWhitespace: options.IgnoreWhitespace, out _standalone, out _encoding);
+            _xmlReader = CreateReader(partStream, options.CloseStream, options.MaxCharactersInPart, ignoreWhitespace: options.IgnoreWhitespace, out bool? standalone, out string? encoding);
+            _openXmlPartReaderState = new OpenXmlPartReaderState(standalone, encoding);
         }
 
         /// <summary>
@@ -114,7 +112,8 @@ public override string? Encoding
             get
             {
                 ThrowIfObjectDisposed();
-                return _encoding;
+
+                return _openXmlPartReaderState?.Encoding;
             }
         }
 
@@ -129,7 +128,7 @@ public override bool? StandaloneXml
 
                 // default is true for standalone
                 // <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
-                return _standalone;
+                return _openXmlPartReaderState?.StandaloneXml;
             }
         }
 

src/DocumentFormat.OpenXml.Framework/OpenXmlPartReaderState.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace DocumentFormat.OpenXml;
+
+internal readonly struct OpenXmlPartReaderState
+{
+    internal OpenXmlPartReaderState(bool? standalone, string? encoding)
+    {
+        StandaloneXml = standalone ?? true;
+        Encoding = encoding;
+    }
+
+    internal string? Encoding { get; }
+
+    internal bool? StandaloneXml { get; }
+}