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Authoring OpenAPI Validation Rules (in C#)


In the AutoRest pipeline, OpenAPI files get deserialized into intermediate classes, which are then used to create the language-independent client model. Our validation logic is performed on these deserialization classes to allow logic written in C# to be used to check the object representation of the OpenAPI spec.

The root OpenAPI spec is deserialized into a ServiceDefinition object. The validation step recursively traverses this object tree and applies the validation rules that apply to each property and consolidate the messages from all rules. Validation rules are associated with a property by decorating it with a RuleAttribute. This RuleAttribute will be passed the value for that property and determines if that value satisfies the rule or not. Multiple RuleAttribute attributes can be applied to the same property, and any rules that fail will be part of the output.

Steps for writing a rule (see instructions below)

  1. Define a canonical name that represents the rule and a message that should be shown to the user explaining the validation failure
  2. Determine if your rule is an Info, a Warning or an Error
  3. Determine if your rule is applicable to ARM, DataPlane or Default OpenAPI specs
  4. For advanced usage, determine if rule is applicable in the Individual or Composed merge state
  5. Implement the logic that validates this rule against a given object
  6. Define where this validation rule gets applied in the object tree
  7. Write a test that verifies that this rule correctly validates OpenAPI specs


1. Add the rule name and message

2. Add a Rule subclass that implements your validation rule logic

  • Create a subclass of the Rule class, and override the bool IsValid(object entity) method.
  • For more complex rules (including getting type information in IsValid(), see the Complex rules section below.

3. Decorate the appropriate Swagger model property that your rule applies to

  • Add a [Rule(typeof({MyRule})] attribute above the property that should satisfy the rule. Replace {MyRule} with the subclass that you implemented.
  • The typeof() is necessary because C# doesn't support generics in attributes.

4. Add a test to OpenAPIModelerValidationTests.cs that validates your validation rule

5. Ensure your validation rule applies to clean-complex-spec.json

  • clean-complex-spec.json is a complex json that passes each validation rule specified in OpenAPIModelerValidationTests.cs
  • Upon adding your validation rule, ensure this json passes the rule by running the CleanFileValidation test

Complex rules

Typed rules

The IsValid() method of the Rule class only passes an object with no type information. You can have your rule subclass work on a typed model class by inheriting from the TypedRule<T> class instead. By replacing T with a model class, your override of IsValid() will use T as the type for the entity parameter.

Message interpolation (e.g. "'foo' is not a valid MIME type for consumes")

Simple rules can simply override the bool IsValid(object entity) method when subclassing Rule and return true or false, depending on if the object satisfies the rule. However, some messages are more useful if they provide the incorrect value as part of the message.

Rules can override a different IsValid overload (IsValid(object enitity, out object[] formatParameters). Any objects returned in formatParameters will be passed on to string.Format() along with the message associated with the rule. When writing the message, use standard string.Format() conventions to define where replacements go (e.g. "'{0}' is not a valid MIME type for consumes").

Collection rules

Sometimes, a rule should apply to every item in a list or dictionary, but it cannot be applied to the class definition (since the same class can be used in multiple places in the ServiceDefinition tree).

An example of this is the AnonymousTypesDiscouraged rule. The purpose of this rule is to have schemas defined in the definitions section of the OpenAPI file instead of in the parameter that it will be used for. It validates the Schema class, but it cannot be applied to all instances of this class, because the definitions section also uses the Schema class.

Since we want to apply this rule to parameters in an operation, we can decorate the Parameters property of the OperationResponse class with the CollectionRule attribute. When the object tree is traversed to apply validation rules, each item in the collection will be validated against the AnonymousParameterTypes logic.