Can someone explain this exception to me? I have this method in User controller

[HttpGet("{username}/plays")]
public async Task<ActionResult<int>> GetUserPlays(string username)
{
    return Ok(await _unitOfWork.TrackRepository.CountPlays(username));
} 

And without going into the TrackRepository method, it throws an exception:

2022-05-31 14:37:02.8644|0|ERROR|gspark.Middleware.ExceptionMiddleware|Object reference not set to an instance of an object. System.NullReferenceException: Object reference not set to an instance of an object.
   at gspark.Controllers.UsersController.GetUserPlays(String username) in C:\Users\GSpark\source\repos\gspark\gspark\Controllers\UsersController.cs:line 119
   at lambda_method202(Closure , Object )
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|20_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.Invoke(HttpContext context)

TrackRepository method:

public async Task<int> CountPlays(string username)
{
    return await _context.Tracks
       .Include(t => t.User)
       .Where(t => t.User.UserName == username)
       .SumAsync(t => t.Plays);
}

Is it something about mapping? Or if it's Repository method causing a problem and if so why debug don't get into this method? Where and why this exception is thrown?

CodePudding user response：

If you don't reach CountPlays then that would suggest that either _unitOfWork or TrackRepository is null.

To clarify this, drop in a breakpoint and inspect the values in the debugger. Or, if you cannot do this then I would recommend an if statement similar to:

if (_unitOfWork == null || _unitOfWork.TrackRepository == null) 
{
    return new Exception("Ooops");
}

If you still have no joy, then share the code that initialises the _unitOfWork object