Error **code** is an optional and unique string value for the exception. Thrown `Exception` should implement the `IHasErrorCode` interface to fill this field. Example JSON value:
````json
{
"error": {
"code": "App:010042",
"message": "This topic is locked and can not add a new message"
}
}
````
Error code can also be used to localize the exception and customize the HTTP status code (see the related sections below).
##### Error Details
Error **details** in an optional field of the JSON error message. Thrown `Exception` should implement the `IHasErrorDetails` interface to fill this field. Example JSON value:
```json
{
"error": {
"code": "App:010042",
"message": "This topic is locked and can not add a new message",
"details": "A more detailed info about the error..."
}
}
```
##### Validation Errors
**validationErrors** is a standard field that is filled if the thrown exception implements the `IHasValidationErrors` interface.
````json
{
"error": {
"code": "App:010046",
"message": "Your request is not valid, please correct and try again!",
`AbpValidationException` implements the `IHasValidationErrors` interface and it is automatically thrown by the framework when a request input is not valid. So, usually you don't need to deal with validation errors unless you have higly customised validation logic.
Exceptions are logged with the `Error` level by default. The Log level can be determined by the exception if it implements the `IHasLogLevel` interface. Example:
`BusinessException` implements the `IBusinessException` interface in addition to the `IHasErrorCode`, `IHasErrorDetails` and `IHasLogLevel` interfaces. The default log level is `Warning`.
If an exception implements the `IUserFriendlyException` interface, then ABP does not change it's `Message` and `Details` properties and directly send it to the client.
In this way, there is **no need for localization** at all. If you want to localize the message, you can inject and use the standard **string localizer** (see the [localization document](Localization.md)). Example:
* The `IUserFriendlyException` interface is derived from the `IBusinessException` and the `UserFriendlyException` class is derived from the `BusinessException` class.
Then any of the exceptions with `Volo.Qa` namespace will be localized using their given localization resource. The localization resource should always have an entry with the error code key. Example:
* Throwing any exception implementing the `IHasErrorCode` interface behaves the same. So, the error code localization approach is not unique to the `BusinessException` class.
* Defining localized string is not required for an error message. If it's not defined, ABP sends the default error message to the client. It does not use the `Message` property of the exception! if you want that, use the `UserFriendlyException` (or use an exception type that implements the `IUserFriendlyException` interface).
The `IHttpExceptionStatusCodeFinder` is used to automatically determine the HTTP status code. The default implementation is the `DefaultHttpExceptionStatusCodeFinder` class. It can be replaced or extended as needed.
-`AbpAuthorizationException` is thrown if the current user has no permission to perform the requested operation. See authorization document (TODO: link) for more.
-`AbpValidationException` is thrown if the input of the current request is not valid. See validation document (TODO: link) for more.
-`EntityNotFoundException` is thrown if the requested entity is not available. This is mostly thrown by [repositories](Repositories.md).