Blazor: A second operation started on this context before a previous operation completed
blazor blazor-server-side c# entity-framework entity-framework-core
Question
I'm creating a server side Blazor app. The following code is in the Startup.cs.
services.AddDbContext<MyContext>(o => o.UseSqlServer(Configuration.GetConnectionString("MyContext")), ServiceLifetime.Transient);
services.AddTransient<MyViewModel, MyViewModel>();
And in the ViewModel:
public class MyViewModel : INotifyPropertyChanged
{
public MyViewModel(MyContext myContext)
{
_myContext = myContext;
}
public async Task<IEnumerable<Dto>> GetList(string s)
{
return await _myContext.Table1.where(....)....ToListAsync();
}
And in the razor file.
@inject ViewModels.MyViewModel VM
<input id="search" type="text" @bind="search" />
<input id="search" type="button" value="Go" @onclick="SearchChanged" />
@code {
string search = "";
int currentCount = 0;
async void SearchChanged() {
currentCount++;
dtos = GetList(search);
}
}
However, sometimes the following error occur when clicking the search button?
System.InvalidOperationException: 'A second operation started on this context before a previous operation completed. This is usually caused by different threads using the same instance of DbContext. For more information on how to avoid threading issues with DbContext, see https://go.microsoft.com/fwlink/?linkid=2097913.'
Accepted Answer
You can try to create a new scope for each request:
public class MyViewModel : INotifyPropertyChanged
{
protected readonly IServiceScopeFactory _ServiceScopeFactory;
public MyViewModel(IServiceScopeFactory serviceScopeFactory)
{
_ServiceScopeFactory = serviceScopeFactory;
}
public async Task<IEnumerable<Dto>> GetList(string s)
{
using (var scope = _ServiceScopeFactory.CreateScope())
{
var referenceContext = scope.ServiceProvider.GetService<MyContext>();
return await _myContext.Table1.where(....)....ToListAsync();
}
}
Here Daniel Roth (Blazor Product Manager) talking about Using Entity Framework Core with Blazor
Popular Answer
I resolve the issue however I think I have lost the Unit Of Work because now I have more than one dbContex :
Constructor :
private AppDbContext _db;
protected override void OnInitialized()
{
_db = new AppDbContext();
var query = _db.Set<Group>().AsQueryable();
}
and later I dispose it:
public void Dispose()
{
_db?.Dispose();
}
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