// You may need to install the Microsoft.AspNetCore.Http.Abstractions package into your project
public class CustomExceptionMiddleware
{
   private readonly RequestDelegate _next;
   private readonly ILogger<CustomExceptionMiddleware> _logger;
   public CustomExceptionMiddleware(RequestDelegate next, ILogger<CustomExceptionMiddleware> logger)
   {
       _next = next;
       _logger = logger;
   }

   public async Task Invoke(HttpContext httpContext)
   {
       try
       {
           await _next(httpContext);
       }
       catch (Exception ex)
       {
           _logger.LogError("Unhandled exception....", ex);
           await HandleExceptionAsync(httpContext, ex);
       }
   }

   private Task HandleExceptionAsync(HttpContext httpContext, Exception ex)
   {
       //todo
       return Task.CompletedTask;
   }
}

// Extension method used to add the middleware to the HTTP request pipeline.
public static class CustomExceptionMiddlewareExtensions
{
   public static IApplicationBuilder UseCustomExceptionMiddleware(this IApplicationBuilder builder)
   {
       return builder.UseMiddleware<CustomExceptionMiddleware>();
   }
}