Add validation rule (+ test) to create an error when a path parameter does not appear in the path.

Author: DCollart

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

@@ -96,6 +96,22 @@ public static class OpenApiParameterRules
                     context.Exit();
                 });
 
+        /// <summary>
+        /// Validate that a path parameter should always appear in the path 
+        /// </summary>
+        public static ValidationRule<OpenApiParameter> PathParameterShouldBeInThePath =>
+            new ValidationRule<OpenApiParameter>(
+                (context, parameter) =>
+                {
+                    if (parameter.In == ParameterLocation.Path && !context.PathString.Contains("{" + parameter.Name + "}"))
+                    {
+                        context.Enter("in");
+                        context.CreateError(
+                            nameof(PathParameterShouldBeInThePath),
+                            $"Declared path parameter \"{parameter.Name}\" needs to be defined as a path parameter at either the path or operation level");
+                        context.Exit();
+                    }
+                });
         // add more rule.
     }
 }

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

@@ -173,5 +173,79 @@ public void ValidateExamplesShouldNotHaveDataTypeMismatchForSimpleSchema()
                 "#/examples/example2/value"
             });
         }
+
+        [Fact]
+        public void PathParameterNotInThePathShouldReturnAnError()
+        {
+            // Arrange
+            IEnumerable<OpenApiError> errors;
+
+            var parameter = new OpenApiParameter()
+            {
+                Name = "parameter1",
+                In = ParameterLocation.Path,
+                Required = true,
+                Schema = new OpenApiSchema()
+                {
+                    Type = "string",
+                }
+            };
+
+            // Act
+            var validator = new OpenApiValidator(ValidationRuleSet.GetDefaultRuleSet());
+            validator.Enter("1");
+
+            var walker = new OpenApiWalker(validator);
+            walker.Walk(parameter);
+
+            errors = validator.Errors;
+            bool result = errors.Any();
+
+            // Assert
+            result.Should().BeTrue();
+            errors.OfType<OpenApiValidatorError>().Select(e => e.RuleName).Should().BeEquivalentTo(new[]
+            {
+                "PathParameterShouldBeInThePath"
+            });
+            errors.Select(e => e.Pointer).Should().BeEquivalentTo(new[]
+            {
+                "#/1/in"
+            });
+        }
+
+        [Fact]
+        public void PathParameterInThePastShouldBeOk()
+        {
+            // Arrange
+            IEnumerable<OpenApiError> errors;
+
+            var parameter = new OpenApiParameter()
+            {
+                Name = "parameter1",
+                In = ParameterLocation.Path,
+                Required = true,
+                Schema = new OpenApiSchema()
+                {
+                    Type = "string",
+                }
+            };
+
+            // Act
+            var validator = new OpenApiValidator(ValidationRuleSet.GetDefaultRuleSet());
+            validator.Enter("paths");
+            validator.Enter("/{parameter1}");
+            validator.Enter("get");
+            validator.Enter("parameters");
+            validator.Enter("1");
+
+            var walker = new OpenApiWalker(validator);
+            walker.Walk(parameter);
+
+            errors = validator.Errors;
+            bool result = errors.Any();
+
+            // Assert
+            result.Should().BeFalse();
+        }
     }
 }