IFields fields = new FieldsClass();
IFieldsEdit fieldsEdit = (IFieldsEdit)fields;
fieldsEdit.FieldCount_2 = 10;

IField field = new Field();
IFieldEdit fieldEdit = (IFieldEdit)field;
fieldEdit.Name_2 = "RULEID";
fieldEdit.AliasName_2 = "RULEID";
fieldEdit.Type_2 = esriFieldType.esriFieldTypeOID;
fieldsEdit.set_Field(0, field);

// Add other fields...

// use IFieldChecker to create validated fields collection
IFieldChecker fieldChecker = new FieldChecker();
IEnumFieldError enumFieldError = null;
IFields validatedFields = null;
fieldChecker.ValidateWorkspace = (IWorkspace)fws;
fieldChecker.Validate(fields, out enumFieldError, out validatedFields);