EnvelopeGenerator/EnvelopeGenerator.Application/Common/Extensions/MediatorExtensions.cs

using System.Collections;
using DigitalData.Core.Exceptions;
using MediatR;

namespace EnvelopeGenerator.Application.Common.Extensions;

/// <summary>
/// Extension methods for <see cref="IMediator"/> that enforce non-null and non-empty responses.
/// </summary>
public static class MediatorExtensions
{
    /// <summary>
    /// Sends a request via MediatR and throws <see cref="NotFoundException"/> when the response is <c>null</c> or an empty collection.
    /// </summary>
    /// <typeparam name="TResponse">The expected response type.</typeparam>
    /// <param name="mediator">The mediator instance.</param>
    /// <param name="request">The MediatR request whose response may be <c>null</c>.</param>
    /// <param name="exceptionMessage">Optional message for the <see cref="NotFoundException"/>.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>A guaranteed non-null <typeparamref name="TResponse"/>.</returns>
    /// <exception cref="NotFoundException">Thrown when the response is <c>null</c> or an empty collection.</exception>
    public static async Task<TResponse> SendOrNotFoundAsync<TResponse>(this IMediator mediator, IRequest<TResponse?> request, string? exceptionMessage, CancellationToken cancellationToken = default)
    {
        if(await mediator.Send(request, cancellationToken) is TResponse res)
        {
            if(res is IEnumerable enumerable && !enumerable.Cast<object>().Any())
                throw new NotFoundException(exceptionMessage);

            return res;
        }

        throw new NotFoundException(exceptionMessage);
    }

    /// <inheritdoc cref="SendOrNotFoundAsync{TResponse}(IMediator, IRequest{TResponse}, string, CancellationToken)"/>
    public static async Task<TResponse> SendOrNotFoundAsync<TResponse>(this IMediator mediator, IRequest<TResponse?> request, CancellationToken cancellationToken = default)
        => await mediator.SendOrNotFoundAsync(request, null, cancellationToken);
}