Avoid `InvalidOperationException` when serializing `SerializableError`

- #8055
- provide unique name (`<Empty-Key>`) for XML elements that would otherwise be nameless

nits:
- remove now-useless Mono special case in updated test class
- extend updated tests to involve square brackets as well as empty keys

src/Microsoft.AspNetCore.Mvc.Formatters.Xml/SerializableErrorWrapper.cs

@@ -14,6 +14,10 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml
     [XmlRoot("Error")]
     public sealed class SerializableErrorWrapper : IXmlSerializable, IUnwrappable
     {
+        // Element name used when ModelStateEntry's Key is empty. Dash in element name should avoid collisions with
+        // other ModelState entries because the character is not legal in an expression name.
+        private static readonly string EmptyKey = "MVC-Empty";
+
         // Note: XmlSerializer requires to have default constructor
         public SerializableErrorWrapper()
         {
@@ -63,6 +67,10 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml
             {
                 var key = XmlConvert.DecodeName(reader.LocalName);
                 var value = reader.ReadInnerXml();
+                if (string.Equals(EmptyKey, key, StringComparison.Ordinal))
+                {
+                    key = string.Empty;
+                }
 
                 SerializableError.Add(key, value);
                 reader.MoveToContent();
@@ -81,6 +89,11 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml
             {
                 var key = keyValuePair.Key;
                 var value = keyValuePair.Value;
+                if (string.IsNullOrEmpty(key))
+                {
+                    key = EmptyKey;
+                }
+
                 writer.WriteStartElement(XmlConvert.EncodeLocalName(key));
                 if (value != null)
                 {
@@ -102,4 +115,4 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml
             return SerializableError;
         }
     }
-}
+}

test/Microsoft.AspNetCore.Mvc.Formatters.Xml.Test/Internal/SerializableErrorWrapperTests.cs

@@ -6,7 +6,6 @@ using System.Runtime.Serialization;
 using System.Text;
 using System.Xml;
 using Microsoft.AspNetCore.Mvc.ModelBinding;
-using Microsoft.AspNetCore.Testing;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Mvc.Formatters.Xml.Internal
@@ -28,8 +27,10 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml.Internal
         public void WrappedSerializableErrorInstance_ReturnedFromProperty()
         {
             // Arrange
-            var serializableError = new SerializableError();
+            var serializableError = new SerializableError
-            serializableError.Add("key1", "key1-error");
+            {
+                { "key1", "key1-error" }
+            };
 
             // Act
             var wrapper = new SerializableErrorWrapper(serializableError);
@@ -57,7 +58,10 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml.Internal
         {
             // Arrange
             var serializableErrorXml = "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
-                "<Error><key1>Test Error 1 Test Error 2</key1><key2>Test Error 3</key2></Error>";
+                "<Error><MVC-Empty>Test error 0</MVC-Empty>" +
+                "<key1>Test Error 1 Test Error 2</key1>" +
+                "<key2>Test Error 3</key2>" +
+                "<list_x005B_3_x005D_.key3>Test Error 4</list_x005B_3_x005D_.key3></Error>";
             var serializer = new DataContractSerializer(typeof(SerializableErrorWrapper));
 
             // Act
@@ -66,8 +70,28 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml.Internal
             var errors = wrapper.SerializableError;
 
             // Assert
-            Assert.Equal("Test Error 1 Test Error 2", errors["key1"]);
+            Assert.Collection(
-            Assert.Equal("Test Error 3", errors["key2"]);
+                errors,
+                kvp =>
+                {
+                    Assert.Equal(string.Empty, kvp.Key);
+                    Assert.Equal("Test error 0", kvp.Value);
+                },
+                kvp =>
+                {
+                    Assert.Equal("key1", kvp.Key);
+                    Assert.Equal("Test Error 1 Test Error 2", kvp.Value);
+                },
+                kvp =>
+                {
+                    Assert.Equal("key2", kvp.Key);
+                    Assert.Equal("Test Error 3", kvp.Value);
+                },
+                kvp =>
+                {
+                    Assert.Equal("list[3].key3", kvp.Key);
+                    Assert.Equal("Test Error 4", kvp.Value);
+                });
         }
 
         [Fact]
@@ -75,11 +99,18 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml.Internal
         {
             // Arrange
             var modelState = new ModelStateDictionary();
+            modelState.AddModelError(string.Empty, "Test error 0");
             modelState.AddModelError("key1", "Test Error 1");
             modelState.AddModelError("key1", "Test Error 2");
             modelState.AddModelError("key2", "Test Error 3");
+            modelState.AddModelError("list[3].key3", "Test Error 4");
             var serializableError = new SerializableError(modelState);
             var outputStream = new MemoryStream();
+            var expectedContent = "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
+                "<Error><MVC-Empty>Test error 0</MVC-Empty>" +
+                "<key1>Test Error 1 Test Error 2</key1>" +
+                "<key2>Test Error 3</key2>" +
+                "<list_x005B_3_x005D_.key3>Test Error 4</list_x005B_3_x005D_.key3></Error>";
 
             // Act
             using (var xmlWriter = XmlWriter.Create(outputStream))
@@ -91,15 +122,7 @@ namespace Microsoft.AspNetCore.Mvc.Formatters.Xml.Internal
             var res = new StreamReader(outputStream, Encoding.UTF8).ReadToEnd();
 
             // Assert
-            var expectedContent =
-                TestPlatformHelper.IsMono ?
-                    "<?xml version=\"1.0\" encoding=\"utf-8\"?><Error xmlns:i=\"" +
-                    "http://www.w3.org/2001/XMLSchema-instance\"><key1>Test Error 1 Test Error 2</key1>" +
-                    "<key2>Test Error 3</key2></Error>" :
-                    "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
-                    "<Error><key1>Test Error 1 Test Error 2</key1><key2>Test Error 3</key2></Error>";
-
             Assert.Equal(expectedContent, res);
         }
     }
-}
+}