Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Xml/System/Xml/Resolvers/XmlPreloadedResolverAsync.cs

@@ -0,0 +1,48 @@
+//------------------------------------------------------------------------------
+// <copyright file="XmlPreloadedResolver.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+// <owner current="true" primary="true">[....]</owner>
+//------------------------------------------------------------------------------
+
+using System.IO;
+using System.Xml;
+using System.Threading.Tasks;
+
+namespace System.Xml.Resolvers {
+
+    // 
+    // XmlPreloadedResolver is an XmlResolver that which can be pre-loaded with data.
+    // By default it contains well-known DTDs for XHTML 1.0 and RSS 0.91. 
+    // Custom mappings of URIs to data can be added with the Add method.
+    //
+    public partial class XmlPreloadedResolver : XmlResolver {
+
+        public override Task<Object> GetEntityAsync(Uri absoluteUri,
+                                             string role,
+                                             Type ofObjectToReturn) {
+
+            if (absoluteUri == null) {
+                throw new ArgumentNullException("absoluteUri");
+            }
+
+            PreloadedData data;
+            if (!mappings.TryGetValue(absoluteUri, out data)) {
+                if (fallbackResolver != null) {
+                    return fallbackResolver.GetEntityAsync(absoluteUri, role, ofObjectToReturn);
+                }
+                throw new XmlException(Res.GetString(Res.Xml_CannotResolveUrl, absoluteUri.ToString()));
+            }
+
+            if (ofObjectToReturn == null || ofObjectToReturn == typeof(Stream) || ofObjectToReturn == typeof(Object)) {
+                return Task.FromResult<Object>(data.AsStream());
+            }
+            else if (ofObjectToReturn == typeof(TextReader)) {
+                return Task.FromResult<Object>(data.AsTextReader());
+            }
+            else {
+                throw new XmlException(Res.GetString(Res.Xml_UnsupportedClass));
+            }
+        }
+    }
+}