What exception should I throw when expecting a null value?

Question

Is there a standard Exception to invoke when excepting null instead?

Commonly, exceptions are thrown for null references and arguments such as NullReferenceException and ArgumentNullException exceptions.

• The NullReferenceException is thrown when there is an attempt to dereference a null object reference.

• The ArgumentNullException is thrown when a null reference is passed to a method that does not accept it as a valid argument.

But what about in the event we expect the result to be null? For example, I am trying to validate if an entity exists in the database, if it does then an exception should be thrown and then I can log the error. The purpose of this validation is simply to ensure a duplicate entity is not created based on Clean Architecture in ASP.NET Core.

The code is as follows:

private async Task ValidateEntityIfExist(EntityModel entityModel, CancellationToken cancellationToken)
{
    Entity entity = await this._repository.GetEntityByIdAsync(entityModel.Id, cancellationToken);
    if (entity != null)
    {
        throw new NotNullReferenceException($"{entityModel} with this id already exists.");
    }
}

I have tried to create a custom NotNullReferenceException class that inherits from Exception class which I use for logging the event:

The code is as follows:

public class NotNullReferenceException : Exception
{
    internal NotNullReferenceException(string message)
        : base(message)
    {
    }
    internal NotNullReferenceException(string message, Exception exception)
        : base(message, exception)
    {
    }
}

Is there a standard Exception to invoke when excepting null instead? If not, what would be the best solution or workaround to such a scenario?

---

Update

Please note that this is based on Clean Architecture principles. For more information, please visit this link.

Answer 1

No one wants to be informed that something is not null and therefore exceptional. Someone wants to be informed if the entity with the given id already exists. So either throw a custom exception like EntityExistsException or don't use exceptions at all. Your method validates an entity, so why it does not return a bool?

Answer 2

In Clean Architecture, exceptions can be handled in a centralized spot to return consistent responses.

Once the business-level entities are defined, common exceptions and business-specific exceptions should be defined so that they can be shared among projects like API, Function App, Console App, etc..

Once the exceptions are defined, we need a centralized spot to handle all the pre-defined exceptions as well as the unknown exceptions (most likely a 500 error), log the errors, and then return a consistent API response so that any client that calls the API knows what to expect.

For example, in your case, we define an EntityAlreadyExistsException at the Core Layer as follows:

using System;
using System.Collections.Generic;
using System.Text;

namespace MyApp.Core.Exceptions
{
    public class EntityAlreadyExistsException : Exception
    {
        public EntityAlreadyExistsException()
        { }

        public EntityAlreadyExistsException(string message) : base(message)
        { }

        public EntityAlreadyExistsException(string message, Exception inner) : base(message, inner)
        { }
    }
}

In order to catch all the exceptions, we can define a filter that runs asynchronously after an action has thrown an exception. For example:

using MyApp.Core.Exceptions;
using MyApp.WebAPI.Infrastructure.ApiExceptions;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;
using Serilog;
using System;
using System.Collections.Generic;

namespace MyApp.WebAPI.Infrastructure.Filters
{
    public class ApiExceptionFilterAttribute : ExceptionFilterAttribute
    {
        private readonly IDictionary<Type, Action<ExceptionContext>> _exceptionHandlers;

        public ApiExceptionFilterAttribute()
        {
            // Register known exception types and handlers.
            _exceptionHandlers = new Dictionary<Type, Action<ExceptionContext>>
            {
                { typeof(EntityAlreadyExistsException), HandleAlreadyExistsException }
            };
        }

        public override void OnException(ExceptionContext context)
        {
            HandleException(context);
            base.OnException(context);
        }

        private void HandleException(ExceptionContext context)
        {
            Log.Error(context.Exception, "Handling exception:");
            Type type = context.Exception.GetType();
            if (_exceptionHandlers.ContainsKey(type))
            {
                _exceptionHandlers[type].Invoke(context);
                return;
            }
            if (!context.ModelState.IsValid)
            {
                HandleInvalidModelStateException(context);
                return;
            }
            HandleUnknownException(context);
        }

        private void HandleAlreadyExistsException(ExceptionContext context)
        {
            EntityAlreadyExistsException exception = context.Exception as EntityAlreadyExistsException;
            ProblemDetails details = new ProblemDetails()
            {
                Type = "https://tools.ietf.org/html/rfc7231#section-6.5.4",
                Title = "The specified resource already exists.",
                Detail = exception.Message
            };
            context.Result = new NotFoundObjectResult(details);
            context.ExceptionHandled = true;
        }
    }
}

Then register the Filter:

services.AddControllers(options =>
  // handle exceptions thrown by an action
  options.Filters.Add(new ApiExceptionFilterAttribute())
);

You can find a Template to setup centralized exception handling and logging below:

https://github.com/ShawnShiSS/clean-architecture-azure-cosmos-db