Added missing collection instantiations

Author: darrelmiller

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -41,7 +41,7 @@ public class OpenApiDocument : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// An element to hold various schemas for the specification.
         /// </summary>
-        public OpenApiComponents Components { get; set; }
+        public OpenApiComponents Components { get; set; } = new OpenApiComponents();
 
         /// <summary>
         /// A declaration of which security mechanisms can be used across the API.

src/Microsoft.OpenApi/Models/OpenApiPathItem.cs

@@ -46,7 +46,7 @@ public class OpenApiPathItem : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiAny> Extensions { get; set; }
+        public IDictionary<string, IOpenApiAny> Extensions { get; set; } = new Dictionary<string, IOpenApiAny>();
 
         /// <summary>
         /// Add one operation into this path item.

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -7,6 +7,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.OpenApi.Interfaces;
+using System.Collections;
 
 namespace Microsoft.OpenApi.Writers
 {
@@ -123,6 +124,11 @@ public static void WriteOptionalObject<T>(
         {
             if (value != null)
             {
+                IEnumerable values = value as IEnumerable;
+                if (values != null && !values.GetEnumerator().MoveNext() )
+                {
+                    return;  // Don't render optional empty collections
+                }
                 writer.WriteRequiredObject(name, value, action);
             }
         }

test/Microsoft.OpenApi.Readers.Tests/Microsoft.OpenApi.Readers.Tests.csproj

@@ -47,12 +47,18 @@
     <None Update="V2Tests\Samples\basic.v3.yaml">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="V2Tests\Samples\definitions.v3.yaml">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
     <None Update="V2Tests\Samples\host.v2.yaml">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
     <None Update="V2Tests\Samples\host.v3.yaml">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="V2Tests\Samples\definitions.v2.yaml">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
     <None Update="V2Tests\Samples\minimal.v3.yaml">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>

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

@@ -18,6 +18,7 @@ public class ComparisonTests
         [Theory]
         [InlineData("minimal")]
         [InlineData("basic")]
+      //  [InlineData("definitions")]  Currently broken due to V3 references not behaving the same as V2
         public void EquivalentV2AndV3DocumentsShouldProductEquivalentObjects(string fileName)
         {
             using (var streamV2 = File.OpenRead(Path.Combine(SampleFolderPath, $"{fileName}.v2.yaml")))

test/Microsoft.OpenApi.Readers.Tests/V2Tests/Samples/definitions.v2.yaml

@@ -0,0 +1,17 @@
+swagger: 2.0
+info: 
+  title: This is a simple example
+  version: 1.0.0
+paths:
+  '/':
+    get:
+      produces:
+        - application/json
+      200: 
+        description: Ok
+        schema:
+          $ref: '#/definitions/simple'
+definitions:
+  simple:
+    type: string
+        

test/Microsoft.OpenApi.Readers.Tests/V2Tests/Samples/definitions.v3.yaml

@@ -0,0 +1,18 @@
+openapi: 3.0
+info: 
+  title: This is a simple example
+  version: 1.0.0
+paths:
+  '/':
+    get:
+      200: 
+        description: Ok
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/simple'
+components:
+  schemas:
+    simple:
+      type: string
+