add a validation rule for the OAuthFlow object

Author: xuzhg

src/Microsoft.OpenApi/Validations/Rules/OpenApiOAuthFlowRules.cs

@@ -0,0 +1,55 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. 
+
+using System;
+using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Properties;
+
+namespace Microsoft.OpenApi.Validations.Rules
+{
+    /// <summary>
+    /// The validation rules for <see cref="OpenApiOAuthFlow"/>.
+    /// </summary>
+    internal static class OpenApiOAuthFlowRules
+    {
+        /// <summary>
+        /// Validate the field is required.
+        /// </summary>
+        public static readonly ValidationRule<OpenApiOAuthFlow> FieldIsRequired =
+            new ValidationRule<OpenApiOAuthFlow>(
+                (context, flow) =>
+                {
+                    // authorizationUrl
+                    context.Push("authorizationUrl");
+                    if (flow.AuthorizationUrl == null)
+                    {
+                        ValidationError error = new ValidationError(ErrorReason.Required, context.PathString,
+                            String.Format(SRResource.Validation_FieldIsRequired, "authorizationUrl", "OAuth Flow"));
+                        context.AddError(error);
+                    }
+                    context.Pop();
+
+                    // tokenUrl
+                    context.Push("tokenUrl");
+                    if (flow.TokenUrl == null)
+                    {
+                        ValidationError error = new ValidationError(ErrorReason.Required, context.PathString,
+                            String.Format(SRResource.Validation_FieldIsRequired, "tokenUrl", "OAuth Flow"));
+                        context.AddError(error);
+                    }
+                    context.Pop();
+
+                    // scopes
+                    context.Push("scopes");
+                    if (flow.Scopes == null)
+                    {
+                        ValidationError error = new ValidationError(ErrorReason.Required, context.PathString,
+                            String.Format(SRResource.Validation_FieldIsRequired, "scopes", "OAuth Flow"));
+                        context.AddError(error);
+                    }
+                    context.Pop();
+                });
+
+        // add more rule.
+    }
+}

src/Microsoft.OpenApi/Validations/ValidationRuleSet.cs

@@ -40,6 +40,8 @@ public sealed class ValidationRuleSet : IEnumerable<ValidationRule>
 
             OpenApiResponseRules.FieldIsRequired,
 
+            OpenApiOAuthFlowRules.FieldIsRequired,
+
             OpenApiExtensibleRules.ExtensionNameMustStartWithXDash,
             // add more default rules.
         };

test/Microsoft.OpenApi.Tests/Validations/OpenApiOAuthFlowValidationTests.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. 
+
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Validations;
+using Xunit;
+
+namespace Microsoft.OpenApi.Tests.Models
+{
+    public class OpenApiOAuthFlowValidationTests
+    {
+        [Fact]
+        public void ValidateFixedFieldsIsRequiredInResponse()
+        {
+            // Arrange
+            IEnumerable<ValidationError> errors;
+            OpenApiOAuthFlow oAuthFlow = new OpenApiOAuthFlow();
+
+            // Act
+            bool result = oAuthFlow.Validate(out errors);
+
+            // Assert
+            Assert.False(result);
+            Assert.NotNull(errors);
+            Assert.Equal(2, errors.Count());
+            Assert.Equal(new[] { "The field 'authorizationUrl' in 'OAuth Flow' object is REQUIRED.",
+                "The field 'tokenUrl' in 'OAuth Flow' object is REQUIRED."}, errors.Select(e => e.ErrorMessage));
+        }
+    }
+}