InvalidOperationException - Sequence

What?

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
      System.InvalidOperationException: Sequence contains no elements
         at System.Linq.ThrowHelper.ThrowNoElementsException()
         at System.Linq.Enumerable.Average[TSource,TSelector,TAccumulator,TResult](IEnumerable`1 source, Func`2 selector)
         at CloudWeather.Report.BusinessLogic.WeatherReportAggregator.GetWeatherReportsAsync(String zip, Int32 days) in c:\dev\micro\micorservices-weather\CloudWeather.Report\BusinessLogic\WeatherReportAggregator.cs:line 43
         at Program.<>c.<<<Main>$>b__0_1>d.MoveNext() in c:\dev\micro\micorservices-weather\CloudWeather.Report\Program.cs:line 31
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Http.RequestDelegateFactory.ExecuteTaskResult[T](Task`1 task, HttpContext httpContext)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

Why?

The error System.InvalidOperationException: Sequence contains no elements in a .NET application typically occurs when you're using methods like First(), Last(), Single(), or SingleOrDefault() on an empty collection or sequence. 

These methods expect the sequence to have at least one element (except for SingleOrDefault(), which allows zero or one element but throws if there are more), and when the expectation is not met, the mentioned exception is thrown.

How?

To handle this error, we need to check if the sequence is empty before calling these methods: Use Any() to check if there are any elements in the sequence. If Any() returns true, it's safe to proceed.

var result = tempData.Any() ? tempData.Average(t => t.field1) : 0;