resolve the comments for the OpenApiLink and Expressions

Author: xuzhg

src/Microsoft.OpenApi/Expressions/RequestExpression.cs

@@ -12,26 +12,29 @@ public sealed class RequestExpression : RuntimeExpression
     {
         public const string Request = "$request.";
 
-        private SourceExpression _source;
-
         /// <summary>
         /// Gets the expression string.
         /// </summary>
         public override string Expression
         {
             get
             {
-                return Request + _source.Expression;
+                return Request + Source.Expression;
             }
         }
 
+        /// <summary>
+        /// The <see cref="SourceExpression"/> expression.
+        /// </summary>
+        public SourceExpression Source { get; }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="RequestExpression"/> class.
         /// </summary>
         /// <param name="source">The source of the request.</param>
         public RequestExpression(SourceExpression source)
         {
-            _source = source ?? throw Error.ArgumentNull(nameof(source));
+            Source = source ?? throw Error.ArgumentNull(nameof(source));
         }
 
         public RequestExpression(string expression)

src/Microsoft.OpenApi/Expressions/ResponseExpression.cs

@@ -12,26 +12,29 @@ public sealed class ResponseExpression : RuntimeExpression
     {
         public const string Response = "$response.";
 
-        private SourceExpression _source;
-
         /// <summary>
         /// Gets the expression string.
         /// </summary>
         public override string Expression
         {
             get
             {
-                return Response + _source.Expression;
+                return Response + Source.Expression;
             }
         }
 
+        /// <summary>
+        /// The <see cref="SourceExpression"/> expression.
+        /// </summary>
+        public SourceExpression Source { get; }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="ResponseExpression"/> class.
         /// </summary>
         /// <param name="source">The source of the response.</param>
         public ResponseExpression(SourceExpression source)
         {
-            _source = source ?? throw Error.ArgumentNull(nameof(source));
+            Source = source ?? throw Error.ArgumentNull(nameof(source));
         }
     }
 }

src/Microsoft.OpenApi/Models/OpenApiCallback.cs

@@ -92,7 +92,7 @@ internal override void WriteAsV3(IOpenApiWriter writer)
         /// </summary>
         internal override void WriteAsV2(IOpenApiWriter writer)
         {
-            // nothing here
+            // Callback object does not exist in V2.
         }
     }
 }

src/Microsoft.OpenApi/Models/OpenApiElementExtensions.cs

@@ -6,7 +6,6 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.OpenApi.Any;
-using Microsoft.OpenApi.Expressions;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Properties;
 

src/Microsoft.OpenApi/Models/OpenApiLink.cs

@@ -17,11 +17,13 @@ public class OpenApiLink : OpenApiElement, IOpenApiReference, IOpenApiExtension
     {
         /// <summary>
         /// A relative or absolute reference to an OAS operation.
+        /// This field is mutually exclusive of the operationId field, and MUST point to an Operation Object.
         /// </summary>
         public string OperationRef { get; set; }
 
         /// <summary>
-        /// The name of an existing, resolvable OAS operation, as defined with a unique operationId
+        /// The name of an existing, resolvable OAS operation, as defined with a unique operationId.
+        /// This field is mutually exclusive of the operationRef field.
         /// </summary>
         public string OperationId { get; set; }