the-biergarten-app/web/backend/API/API.Core/GlobalException.cs

// API.Core/Filters/GlobalExceptionFilter.cs

using Domain.Exceptions;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;
using Microsoft.Data.SqlClient;
using Shared.Contracts;
using ValidationException = FluentValidation.ValidationException;

namespace API.Core;

/// <summary>
///     MVC exception filter that converts unhandled exceptions raised by controller actions into
///     consistent JSON error responses with appropriate HTTP status codes.
/// </summary>
/// <param name="logger">Logger used to record unhandled exceptions before they are translated into a response.</param>
public class GlobalExceptionFilter(ILogger<GlobalExceptionFilter> logger)
    : IExceptionFilter
{
    /// <summary>
    ///     Logs the exception and sets <see cref="ExceptionContext.Result" /> to an appropriate error response,
    ///     marking the exception as handled.
    /// </summary>
    /// <remarks>
    ///     Maps exception types to responses as follows:
    ///     <list type="bullet">
    ///         <item>
    ///             <description>
    ///                 <see cref="FluentValidation.ValidationException" /> → <c>400 Bad Request</c> with per-property
    ///                 validation error messages.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>
    ///                 <see cref="ConflictException" /> → <c>409 Conflict</c> with a <see cref="ResponseBody" />
    ///                 message.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>
    ///                 <see cref="NotFoundException" /> → <c>404 Not Found</c> with a <see cref="ResponseBody" />
    ///                 message.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>
    ///                 <see cref="UnauthorizedException" /> → <c>401 Unauthorized</c> with a
    ///                 <see cref="ResponseBody" /> message.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>
    ///                 <see cref="ForbiddenException" /> → <c>403 Forbidden</c> with a <see cref="ResponseBody" />
    ///                 message.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>
    ///                 <see cref="SqlException" /> → <c>503 Service Unavailable</c> with a generic database error
    ///                 message.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>
    ///                 <see cref="Domain.Exceptions.ValidationException" /> → <c>400 Bad Request</c> with a
    ///                 <see cref="ResponseBody" /> message.
    ///             </description>
    ///         </item>
    ///         <item>
    ///             <description>Any other exception → <c>500 Internal Server Error</c> with a generic error message.</description>
    ///         </item>
    ///     </list>
    /// </remarks>
    /// <param name="context">
    ///     The context for the unhandled exception, including the exception itself and the result to
    ///     populate.
    /// </param>
    public void OnException(ExceptionContext context)
    {
        logger.LogError(context.Exception, "Unhandled exception occurred");

        switch (context.Exception)
        {
            case ValidationException fluentValidationException:
                Dictionary<string, string[]> errors = fluentValidationException
                    .Errors.GroupBy(e => e.PropertyName)
                    .ToDictionary(
                        g => g.Key,
                        g => g.Select(e => e.ErrorMessage).ToArray()
                    );

                context.Result = new BadRequestObjectResult(
                    new { message = "Validation failed", errors }
                );
                context.ExceptionHandled = true;
                break;

            case ConflictException ex:
                context.Result = new ObjectResult(
                    new ResponseBody { Message = ex.Message }
                )
                {
                    StatusCode = 409
                };
                context.ExceptionHandled = true;
                break;

            case NotFoundException ex:
                context.Result = new ObjectResult(
                    new ResponseBody { Message = ex.Message }
                )
                {
                    StatusCode = 404
                };
                context.ExceptionHandled = true;
                break;

            case UnauthorizedException ex:
                context.Result = new ObjectResult(
                    new ResponseBody { Message = ex.Message }
                )
                {
                    StatusCode = 401
                };
                context.ExceptionHandled = true;
                break;

            case ForbiddenException ex:
                context.Result = new ObjectResult(
                    new ResponseBody { Message = ex.Message }
                )
                {
                    StatusCode = 403
                };
                context.ExceptionHandled = true;
                break;

            case SqlException ex:
                context.Result = new ObjectResult(
                    new ResponseBody { Message = "A database error occurred." }
                )
                {
                    StatusCode = 503
                };
                context.ExceptionHandled = true;
                break;

            case Domain.Exceptions.ValidationException ex:
                context.Result = new ObjectResult(
                    new ResponseBody { Message = ex.Message }
                )
                {
                    StatusCode = 400
                };
                context.ExceptionHandled = true;
                break;

            default:
                context.Result = new ObjectResult(
                    new ResponseBody
                    {
                        Message = "An unexpected error occurred"
                    }
                )
                {
                    StatusCode = 500
                };
                context.ExceptionHandled = true;
                break;
        }
    }
}