Throw PlatformNotSupportedException in netstandard1.3 (#396)

- stop trying to use DCS in the JSON and XML formatters
- if `UseDataContractSerializer` or `!UseXmlSerializer`:
  - `CanReadType(...)` and `CanWriteType(...)` always return `false`
  - `ReadFromStreamAsync(...)` and `WriteToStreamAsync(...)` always `throw`
- change default XML formatter configuration to use `XmlSerializer`
- adjust and disable tests in reaction
- add tests of new `netstandard1.3` behavior
- nits:
  - correct namespace of `SerializerConsistencyTests`; was alone in previous namespace
  - s/DataContractFormatter/XmlSerializerFormatter/ to reflect actual `XmlMediaTypeFormatter` test actions

src/System.Net.Http.Formatting/Formatting/JsonMediaTypeFormatter.cs

@@ -6,7 +6,9 @@
 using System.Diagnostics.Contracts;
 using System.IO;
 using System.Net.Http.Headers;
+#if !NETSTANDARD1_3 // Unnecessary when targeting netstandard1.3.
 using System.Net.Http.Internal;
+#endif
 using System.Runtime.Serialization.Json;
 using System.Text;
 using System.Threading;
@@ -214,20 +216,15 @@ public override object ReadFromStream(Type type, Stream readStream, Encoding eff
             {
                 DataContractJsonSerializer dataContractSerializer = GetDataContractSerializer(type);
 
-                // JsonReaderWriterFactory is internal, CreateTextReader only supports auto-detecting the encoding
-                // and auto-detection fails in some cases for the netstandard1.3 project. In addition, DCS encodings are
-                // limited to UTF8, UTF16BE, and UTF16LE. Convert to UTF8 as we read.
-                Stream innerStream = string.Equals(effectiveEncoding.WebName, Utf8Encoding.WebName, StringComparison.OrdinalIgnoreCase) ?
+#if NETSTANDARD1_3 // Unreachable when targeting netstandard1.3. Return just to satisfy the compiler.
+                return null;
+#else
+                // DCS encodings are limited to UTF8, UTF16BE, and UTF16LE. Convert to UTF8 as we read.
+                Stream innerStream =
+                    string.Equals(effectiveEncoding.WebName, Utf8Encoding.WebName, StringComparison.OrdinalIgnoreCase) ?
                     new NonClosingDelegatingStream(readStream) :
                     new TranscodingStream(readStream, effectiveEncoding, Utf8Encoding, leaveOpen: true);
 
-#if NETSTANDARD1_3
-                using (innerStream)
-                {
-                    // Unfortunately, we're ignoring _readerQuotas.
-                    return dataContractSerializer.ReadObject(innerStream);
-                }
-#else
                 // XmlDictionaryReader will always dispose of innerStream when we dispose of the reader.
                 using XmlDictionaryReader reader =
                     JsonReaderWriterFactory.CreateJsonReader(innerStream, Utf8Encoding, _readerQuotas, onClose: null);
@@ -314,10 +311,8 @@ private void WriteObject(Stream stream, Type type, object value)
         {
             DataContractJsonSerializer dataContractSerializer = GetDataContractSerializer(type);
 
+#if !NETSTANDARD1_3 // Unreachable when targeting netstandard1.3.
             // Do not dispose of the stream. WriteToStream handles that where it's needed.
-#if NETSTANDARD1_3
-            dataContractSerializer.WriteObject(stream, value);
-#else
             using XmlWriter writer = JsonReaderWriterFactory.CreateJsonWriter(stream, Utf8Encoding, ownsStream: false);
             dataContractSerializer.WriteObject(writer, value);
 #endif
@@ -328,15 +323,26 @@ private DataContractJsonSerializer CreateDataContractSerializer(Type type, bool
         {
             Contract.Assert(type != null);
 
+#if NETSTANDARD1_3 // XsdDataContractExporter is not supported in netstandard1.3
+            if (throwOnError)
+            {
+                throw new PlatformNotSupportedException(Error.Format(
+                    Properties.Resources.JsonMediaTypeFormatter_DCS_NotSupported,
+                    nameof(UseDataContractJsonSerializer)));
+            }
+            else
+            {
+                return null;
+            }
+#else
+
             DataContractJsonSerializer serializer = null;
             Exception exception = null;
 
             try
             {
-#if !NETSTANDARD1_3 // XsdDataContractExporter is not supported in netstandard1.3
                 // Verify that type is a valid data contract by forcing the serializer to try to create a data contract
                 FormattingUtilities.XsdDataContractExporter.GetRootElementName(type);
-#endif
 
                 serializer = CreateDataContractSerializer(type);
             }
@@ -362,6 +368,7 @@ private DataContractJsonSerializer CreateDataContractSerializer(Type type, bool
             }
 
             return serializer;
+#endif
         }
 
         /// <summary>

src/System.Net.Http.Formatting/Formatting/MediaTypeFormatterCollection.cs

@@ -255,7 +255,11 @@ private static IEnumerable<MediaTypeFormatter> CreateDefaultFormatters()
             return new MediaTypeFormatter[]
             {
                 new JsonMediaTypeFormatter(),
-                new XmlMediaTypeFormatter(),
+                new XmlMediaTypeFormatter()
+#if NETSTANDARD1_3 // XsdDataContractExporter is not supported in netstandard1.3. Cannot use DCS.
+                { UseXmlSerializer = true }
+#endif
+                ,
                 new FormUrlEncodedMediaTypeFormatter()
             };
         }

src/System.Net.Http.Formatting/Formatting/XmlMediaTypeFormatter.cs

@@ -511,6 +511,23 @@ public object InvokeGetSerializer(Type type, object value, HttpContent content)
         private object CreateDefaultSerializer(Type type, bool throwOnError)
         {
             Contract.Assert(type != null, "type cannot be null.");
+
+#if NETSTANDARD1_3 // XsdDataContractExporter is not supported in netstandard1.3
+            if (!UseXmlSerializer)
+            {
+                if (throwOnError)
+                {
+                    throw new PlatformNotSupportedException(Error.Format(
+                        Properties.Resources.XmlMediaTypeFormatter_DCS_NotSupported,
+                        nameof(UseXmlSerializer)));
+                }
+                else
+                {
+                    return null;
+                }
+            }
+#endif
+
             Exception exception = null;
             object serializer = null;
 
@@ -522,13 +539,12 @@ private object CreateDefaultSerializer(Type type, bool throwOnError)
                 }
                 else
                 {
-#if !NETSTANDARD1_3 // XsdDataContractExporter is not supported in netstandard1.3
-                    // REVIEW: Is there something comparable in WinRT?
+#if !NETSTANDARD1_3 // Unreachable when targeting netstandard1.3.
                     // Verify that type is a valid data contract by forcing the serializer to try to create a data contract
                     FormattingUtilities.XsdDataContractExporter.GetRootElementName(type);
-#endif
 
                     serializer = CreateDataContractSerializer(type);
+#endif
                 }
             }
             catch (Exception caught)

src/System.Net.Http.Formatting/Properties/Resources.resx

@@ -357,4 +357,10 @@
   <data name="ObjectDisposed_StreamClosed" xml:space="preserve">
     <value>Cannot access a closed stream.</value>
   </data>
+  <data name="JsonMediaTypeFormatter_DCS_NotSupported" xml:space="preserve">
+    <value>Unable to validate types on this platform when {0} is 'true'. Please reset {0} or move to a supported platform, one where the 'netstandard2.0' assembly is usable.</value>
+  </data>
+  <data name="XmlMediaTypeFormatter_DCS_NotSupported" xml:space="preserve">
+    <value>Unable to validate types on this platform when {0} is 'false'. Please set {0} or move to a supported platform, one where the 'netstandard2.0' assembly is usable.</value>
+  </data>
 </root>