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Exception-Handling.md

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Exception Handling

ABP provides a built-in infrastructure and offers a standard model to exception handling for a web application.

  • Automatically handles all exceptions and sends a standard formatted error message to the client for an API/AJAX request.
  • Automatically hides internal infrastructure errors and returns a standard error message.
  • Provides configuration option to localize exception messages.
  • Automatically maps standard exceptions to HTTP status codes and provides configuration option to map custom exceptions.

Automatic Exception Handling

AbpExceptionFilter handles an exception if any of the following conditions meets;

  • Exception is thrown by a controller action which returns an object result (not a view result).
  • The request is an AJAX request (X-Requested-With HTTP header value is XMLHttpRequest).
  • Client explicitly accepts the application/json content type (via accept HTTP header).

If the exception is handled it's automatically logged and a formatted JSON message is returned to the client.

Error Message Format

Error Message is an instance of the RemoteServiceErrorResponse class. The simplest error JSON has a message property as shown below:

{
  "error": {
    "message": "This topic is locked and can not add a new message"
  }
}

There are optional fields those can be filled based on the occurred exception.

Error Code

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:

{
  "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:

{
  "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.

{
  "error": {
    "code": "App:010046",
    "message": "Your request is not valid, please correct and try again!",
    "validationErrors": [{
      "message": "Username should be minimum lenght of 3.",
      "members": ["userName"]
    },
    {
      "message": "Password is required",
      "members": ["password"]
    }]
  }
}

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 deal with validation errors except you have a very custom validation logic.

Logging

Caught exceptions are automatically logged.

Log Level

Exceptions are logged with the Error level by default. Log level can be determined by the exception if it implements the IHasLogLevel interface. Example:

public class MyException : Exception, IHasLogLevel
{
    public LogLevel LogLevel { get; set; } = LogLevel.Warning;

    //...
}
Self Logging Exceptions

Some exception types may need to write additional logs. They can implement the IExceptionWithSelfLogging if needed. Example:

public class MyException : Exception, IExceptionWithSelfLogging
{
    public void Log(ILogger logger)
    {
        //...log additional info
    }
}

ILogger.LogException extension methods is used to write exception logs. You can use the same extension method when needed.

Business Exceptions

Most of your own exceptions will be business exceptions. The IBusinessException interface is used to mark an exception as a business exception.

BusinessException implements the IBusinessException interface in addition to the IHasErrorCode, IHasErrorDetails and IHasLogLevel interfaces. Default log level is Warning.

Usually you have an error code related to a particular business exception. Example:

throw new BusinessException(QaErrorCodes.CanNotVoteYourOwnAnswer);

QaErrorCodes.CanNotVoteYourOwnAnswer is just a const string. Such an error code format is suggested:

<code-namespace>:<error-code>

code-namespace is a unique value specific to your module/application. Example:

Volo.Qa:010002

Volo.Qa is the code-namespace here. code-namespace is then will be used while localizing exception messages.

  • You can directly throw a BusinessException or derive your own exception types from it when needed.
  • All properties are optional for the BusinessException class. But you generally set either ErrorCode or Message property.

Exception Localization

One problem with throwing exception is how to localize error messages while sending to clients. ABP offers two models and their variants.

User Friendly Exception

If an exception implements the IUserFriendlyException interface, then ABP does not change it's Message and Details properties and directly send to the client.

UserFriendlyException class is the built-in implementation of the IUserFriendlyException interface. Example usage:

throw new UserFriendlyException(
    "Username should be unique!"
);

In this way, no need to localization at all. If you want to localize the message, you can inject and use the standard string localizer (see the localization document). Example:

throw new UserFriendlyException(_stringLocalizer["UserNameShouldBeUniqueMessage"]);

Then define it in the localization resource for each language. Example:

{
  "culture": "en",
  "texts": {
    "UserNameShouldBeUniqueMessage": "Username should be unique!"
  }
}

String localizer already supports parameterized messages. Example:

throw new UserFriendlyException(_stringLocalizer["UserNameShouldBeUniqueMessage", "john"]);

Then the localization text can be:

"UserNameShouldBeUniqueMessage": "Username should be unique! '{0}' is already taken!"
  • The IUserFriendlyException interface is derived from the IBusinessException and the UserFriendlyException class is derived from the BusinessException class.

Using Error Codes

UserFriendlyException is fine, but has a few problems in advanced usages:

  • It requires to inject string localizer everywhere and always use it while throwing exceptions.
  • Some of the cases, it may not possible to inject the string localizer (in a static context or in an entity method).

Instead of localizing the message while throwing the exception, you can separate the process using error codes.

First, define the code-namespace to localization resource mapping in the module configuration:

services.Configure<ExceptionLocalizationOptions>(options =>
{
    options.MapCodeNamespace("Volo.Qa", typeof(QaResource));
});

Then any of the exceptions with Volo.Qa namespace will be localized using the given localization resource. The localization resource should have an entry with the error code key. Example:

{
  "culture": "en",
  "texts": {
    "Volo.Qa:010002": "You can not vote your own answer!"
  }
}

Then a business exception can be thrown with the error code:

throw new BusinessException(QaDomainErrorCodes.CanNotVoteYourOwnAnswer);
  • 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 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).
Using Message Parameters

If you have a parameterized error message, then you can set it with the exception's Data property. Example:

throw new BusinessException("App:010046")
{
    Data =
    {
        {"UserName", "john"}
    }
};

Fortunately there is a shortcut for it:

throw new BusinessException("App:010046")
    .WithData("UserName", "john");

Then the localized text can contain theUserName parameter:

{
  "culture": "en",
  "texts": {
    "App:010046": "Username should be unique. '{UserName}' is already taken!"
  }
}
  • WithData can be chained for more than one parameter (like .WithData(...).WithData(...)).

HTTP Status Code Mapping

ABP tries to automatically determine the most suitable HTTP status code for common exception types by following these rules:

  • For the AbpAuthorizationException:
    • Returns 401 (unauthorized) if user has not logged in.
    • Returns 403 (forbidden) if user has logged in.
  • Returns 400 (bad request) for the AbpValidationException.
  • Returns 404 (not found) for the EntityNotFoundException.
  • Returns 403 (forbidden) for the IBusinessException (and IUserFriendlyException since it extends the IBusinessException).
  • Returns 501 (not implemented) for the NotImplementedException.
  • Returns 500 (internal server error) for other exceptions (those are assumed as infrastructure exceptions).

The IHttpExceptionStatusCodeFinder is used to automatically determine the HTTP status code. Default implementation is the DefaultHttpExceptionStatusCodeFinder class. It can be replaced or extended if needed.

Custom Mappings

Automatic HTTP status code determination can be overrided by custom mappings. Example:

services.Configure<ExceptionHttpStatusCodeOptions>(options =>
{
    options.Map("Volo.Qa:010002", HttpStatusCode.Conflict);
});

Built-In Exceptions

Some exception types are automatically thrown by the framework.

  • 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.

You can also throw these type of exceptions in your code (while it's rarely needed).