Fixes based on PR review

Author: darrelmiller

src/Microsoft.OpenApi.Readers/OpenApiReaderSettings.cs

@@ -52,6 +52,6 @@ public class OpenApiReaderSettings
         /// <summary>
         /// URL where relative references should be resolved from if the description does not contain Server definitions
         /// </summary>
-        public Uri BaseUrl { get; set; } = new Uri("https://example.org/");
+        public Uri BaseUrl { get; set; } 
     }
 }

src/Microsoft.OpenApi.Readers/V2/OpenApiDocumentDeserializer.cs

@@ -132,35 +132,63 @@ private static void MakeServers(IList<OpenApiServer> servers, ParsingContext con
             }
 
             // Fill in missing information based on the defaultUrl
-            host = host ?? defaultUrl.GetComponents(UriComponents.NormalizedHost, UriFormat.SafeUnescaped);
-            basePath = basePath ?? defaultUrl.GetComponents(UriComponents.Path, UriFormat.SafeUnescaped);
-            schemes = schemes ?? new List<string> { defaultUrl.GetComponents(UriComponents.Scheme, UriFormat.SafeUnescaped) };
-
+            if (defaultUrl != null)
+            {
+                host = host ?? defaultUrl.GetComponents(UriComponents.NormalizedHost, UriFormat.SafeUnescaped);
+                basePath = basePath ?? defaultUrl.GetComponents(UriComponents.Path, UriFormat.SafeUnescaped);
+                schemes = schemes ?? new List<string> { defaultUrl.GetComponents(UriComponents.Scheme, UriFormat.SafeUnescaped) };
+            } else if (String.IsNullOrEmpty(host) && String.IsNullOrEmpty(basePath))
+            {
+                return;  // Can't make a server object out of just a Scheme
+            }
 
             // Create the Server objects
             if (schemes != null)
             {
                 foreach (var scheme in schemes)
                 {
-                    var ub = new UriBuilder(scheme, host)
+                    var uriBuilder = new UriBuilder(scheme, host)
                     {
                         Path = basePath
                     };
 
                     var server = new OpenApiServer
                     {
-                        Url = ub.ToString()
+                        Url = uriBuilder.ToString()
                     };
 
-                    // Server Urls are always appended to Paths and Paths must start with /
-                    // so removing the slash prevents a double slash.
-                    if (server.Url.EndsWith("/"))
-                    {
-                        server.Url = server.Url.Substring(0, server.Url.Length - 1);
-                    }
                     servers.Add(server);
                 }
-            } 
+            }
+            else
+            {
+                if (!String.IsNullOrEmpty(host))
+                {
+                    host = "//" + host;
+                }
+                var uriBuilder = new UriBuilder()
+                {
+                    Scheme = null,
+                    Host = host,
+                    Path = basePath
+                };
+                var server = new OpenApiServer
+                {
+                    Url = uriBuilder.ToString()
+                };
+
+                servers.Add(server);
+            }
+
+            foreach (var server in servers)
+            {
+                // Server Urls are always appended to Paths and Paths must start with /
+                // so removing the slash prevents a double slash.
+                if (server.Url.EndsWith("/"))
+                {
+                    server.Url = server.Url.Substring(0, server.Url.Length - 1);
+                }
+            }
         }
 
         public static OpenApiDocument LoadOpenApi(RootNode rootNode)

src/Microsoft.OpenApi/Extensions/OpenApiSerializableExtensions.cs

@@ -114,6 +114,42 @@ public static void Serialize<T>(this T element, IOpenApiWriter writer, OpenApiSp
             writer.Flush();
         }
 
+        /// <summary>
+        /// Serializes the <see cref="IOpenApiSerializable"/> to Open API document using the given specification version and writer.
+        /// </summary>
+        /// <typeparam name="T">the <see cref="IOpenApiSerializable"/></typeparam>
+        /// <param name="element">The Open API element.</param>
+        /// <param name="writer">The output writer.</param>
+        public static void Serialize<T>(this T element, IOpenApiWriter writer)
+            where T : IOpenApiSerializable
+        {
+            if (element == null)
+            {
+                throw Error.ArgumentNull(nameof(element));
+            }
+
+            if (writer == null)
+            {
+                throw Error.ArgumentNull(nameof(writer));
+            }
+
+            switch (writer.Settings.SpecVersion)
+            {
+                case OpenApiSpecVersion.OpenApi3_0:
+                    element.SerializeAsV3(writer);
+                    break;
+
+                case OpenApiSpecVersion.OpenApi2_0:
+                    element.SerializeAsV2(writer);
+                    break;
+
+                default:
+                    throw new OpenApiException(string.Format(SRResource.OpenApiSpecVersionNotSupported, writer.Settings.SpecVersion));
+            }
+
+            writer.Flush();
+        }
+
         /// <summary>
         /// Serializes the <see cref="IOpenApiSerializable"/> to the Open API document as a string in JSON format.
         /// </summary>

src/Microsoft.OpenApi/Writers/IOpenApiWriter.cs

@@ -8,6 +8,11 @@ namespace Microsoft.OpenApi.Writers
     /// </summary>
     public interface IOpenApiWriter
     {
+        /// <summary>
+        /// Returns the settings being used the Writer for controlling serialization
+        /// </summary>
+        OpenApiSerializerSettings Settings { get; }
+
         /// <summary>
         /// Write the start object.
         /// </summary>

src/Microsoft.OpenApi/Writers/OpenApiWriterBase.cs

@@ -48,6 +48,11 @@ public OpenApiWriterBase(TextWriter textWriter, OpenApiSerializerSettings settin
             this._settings = settings;
         }
 
+        /// <summary>
+        /// Returns the settings being used the Writer for controlling serialization
+        /// </summary>
+        public OpenApiSerializerSettings Settings { get; }
+
         /// <summary>
         /// Base Indentation Level.
         /// This denotes how many indentations are needed for the property in the base object.

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiServerTests.cs

@@ -28,7 +28,7 @@ public void NoServer()
         }
 
         [Fact]
-        public void JustScheme()
+        public void JustSchemeNoDefault()
         {
             var input = @"
 swagger: 2.0
@@ -45,9 +45,51 @@ public void JustScheme()
 
             var doc = reader.Read(input, out var diagnostic);
 
+            Assert.Equal(0, doc.Servers.Count);
+        }
+
+        [Fact]
+        public void JustHostNoDefault()
+        {
+            var input = @"
+swagger: 2.0
+info: 
+  title: test
+  version: 1.0.0
+host: www.foo.com
+paths: {}
+";
+            var reader = new OpenApiStringReader(new OpenApiReaderSettings()
+            {
+            });
+
+            var doc = reader.Read(input, out var diagnostic);
+
             var server = doc.Servers.First();
-            Assert.Equal(1,doc.Servers.Count);
-            Assert.Equal("http://example.org", server.Url);
+            Assert.Equal(1, doc.Servers.Count);
+            Assert.Equal("//www.foo.com", server.Url);
+        }
+
+        [Fact]
+        public void JustBasePathNoDefault()
+        {
+            var input = @"
+swagger: 2.0
+info: 
+  title: test
+  version: 1.0.0
+basePath: /baz
+paths: {}
+";
+            var reader = new OpenApiStringReader(new OpenApiReaderSettings()
+            {
+            });
+
+            var doc = reader.Read(input, out var diagnostic);
+
+            var server = doc.Servers.First();
+            Assert.Equal(1, doc.Servers.Count);
+            Assert.Equal("/baz", server.Url);
         }
 
         [Fact]