PracticeCalendar/PracticeCalendar.Api/Controllers/ApiControllerBase.cs

using MediatR;
using Microsoft.AspNetCore.Mvc;

namespace PracticeCalendar.API.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class ApiControllerBase : ControllerBase
    {
        private ISender _mediator = null!;

        protected ISender Mediator => _mediator ??= HttpContext.RequestServices.GetRequiredService<ISender>();
    }
}