Updating an object asynchronously with another one

Question

I'm trying to update an object with another object asynchronously. I'm trying to get the CustomerId value from Statues and then use it to call a specific customer and pass those values into PreviousStatuses. Then update the StatusToAdd with PreviousStatuses. If I pass Statues to StatusToAdd the values update. However, it's the wrong customer id. That's why I'm using PreviousStatuses.

This is the error I get:

crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
   at DSPDRewrite.Pages.Popups.AddStatusComponent.UpdateValues(String id)
   at DSPDRewrite.Pages.Popups.AddStatusComponent.OnInitializedAsync()
   at Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask(Task taskToHandle)




 [Parameter]
    public int CustomerId { get; set; }

[CascadingParameter]
public Task<AuthenticationState> AuthState { get; set; }

public Status Statuses;
public Status PreviousStatuses;
//IList<Status> PreviousStatuses;

public Dspd1056Status StatusToAdd = new Dspd1056Status();

public Customer customer;

public int AccountStatusId = 0;



protected override async Task OnInitializedAsync()
{
    Statuses = await dataService.Get1056StatusById(CustomerId);
    //int id = Int32.Parse(Statuses.CustomerId);
    //  Statuses = await dataService.Get1056StatusById(id);
    Console.WriteLine(Statuses.CustomerId);
    await UpdateValues(Statuses.CustomerId);

}
async Task UpdateValues(string id)
{
    PreviousStatuses = await dataService.Get1056StatusById(Int32.Parse((id)));
    StatusToAdd.AccountCurrent = PreviousStatuses.AccountCurrent;
    StatusToAdd.StartDate = PreviousStatuses.StartDate;
    StatusToAdd.EndDate = PreviousStatuses.EndDate;
    StatusToAdd.Units = PreviousStatuses.Units;
    StatusToAdd.Ppc = PreviousStatuses.Ppc;
    StatusToAdd.EndStatus = PreviousStatuses.EndStatus;
    StatusToAdd.ContinuallyFunded = PreviousStatuses.ContinuallyFunded;
    StatusToAdd.AnnualUnits = PreviousStatuses.AnnualUnits;
    StatusToAdd.Elg = PreviousStatuses.Elg;
    StatusToAdd.ReceiptDate = PreviousStatuses.ReceiptDate;
    StatusToAdd.RahTripsFunded = PreviousStatuses.RahTripsFunded;
    StatusToAdd.Rate = PreviousStatuses.Rate;
    StatusToAdd.AccountTotal = PreviousStatuses.AccountTotal;
}
protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        if (CustomerId != 0)
        {
            customer = await dataService.GetCustomerById((int)CustomerId);
            StateHasChanged();
        }
    }
}

With the code as shown, what is the value of `Statuses` on the line `Console.WriteLine(Statuses.CustomerId)`, and the value of `PreviousStatuses` in the line `StatusToAdd.AccountCurrent = PreviousStatuses.AccountCurrent;`. Also have you stepped through `UpdateValues` in debug mode to see which line errors? — MrC aka Shaun Curtis, Oct 06 '21 at 15:11
I have, the error comes from this line: int id = Int32.Parse(Statuses.CustomerId); Statuses can change depending on which customer I select. — Brsn, Oct 06 '21 at 15:37
The error you've shown - `DSPDRewrite.Pages.Popups.AddStatusComponent.UpdateValues(String id)` is not for that error - it's for something in `UpdateValues`. In the line you've shown any error is either because `dataService.Get1056StatusById` returned a null value or `Statuses.CustomerId` is null. You need to step through your code in debug mode to figure out who is the culprit. — MrC aka Shaun Curtis, Oct 06 '21 at 15:52
Does this answer your question? [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Dour High Arch, Oct 06 '21 at 19:04

score 0 · Answer 1 · answered Oct 08 '21 at 14:29

A few things to consider:

OnInitializedAsync

Can your .Get1056StatusById() method return null (e.g. if CustomerId is not found)? If so that appears to be the cause of the error: your code calls

await UpdateValues(Statuses.CustomerId);

That suggests Statuses is null. I would add a null check to this code to handle that case. Not sure why the Console.WriteLine didn't throw the null reference first, I assume the sample code didn't have it when the error was thrown.

A few other points: fields and methods in Components should normally be private or protected. If you are going to expose outside the Component they should be properties. Methods are rarely called outside a component. My suggested changes would be:

[Parameter] public int CustomerId { get; set; }

[CascadingParameter] public Task<AuthenticationState> AuthState { get; set; }

Status Statuses;
Status PreviousStatuses;

Dspd1056Status StatusToAdd = new Dspd1056Status();

Customer customer;

int AccountStatusId = 0;

protected override async Task OnInitializedAsync()
{
    Statuses = await dataService.Get1056StatusById(CustomerId);
    if(Statuses != null)
    {
       Console.WriteLine(Statuses.CustomerId);
       await UpdateValues(Statuses.CustomerId);
    }
}
async Task UpdateValues(string id)
{
    PreviousStatuses = await dataService.Get1056StatusById(Int32.Parse((id)));
    // rest...

Your component HTML should obviously check for a null Statuses before it tries to render:

@if(Statuses != null)
{
    <p>Customer is @Statuses.CustomerId</p>
}

This is such a frequently used pattern in Blazor apps I usually use a <NotNull> component: https://gist.github.com/conficient/eab57ade2587104d71ac6f26ddfd4865

Updating an object asynchronously with another one

1 Answers1