aspnetcore: Blazor concurrency problem using Entity Framework Core

My goal:

I want to create a Blazor page with a list of users that it is populated through an IQueryable<IdentityUser> using UserManager<IdentityUser>'s property and a button that create a new user when is clicked.

What did I expect

1. When the page is loaded I want to see users previously created.
2. When I click on “click me” button I want to see a new row inside the users’ list

Problem:

When I set my dbContext to SQLServer provider and I click on “click me” button I willl get the following error:

InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread-safe.

What I tried:

• someone on Stackoverflow suggests changing dbContext scope to transient. I did it but the problem still persists. I think because Create method and IQueryable<IdentityUser> Users are called from the same UserManager’s instance so, same dbContext instance.

Problem sample:

https://github.com/Blackleones/BlazorConcurrencyProblem or create a new ASP.NET Blazor Server Side project with Authentication and change Index.razor page in this way `

@page "/"

<h1>Hello, world!</h1>

number of users: @Users.Count()
<button @onclick="@(async () => await Add())">click me. I work if you use Sqlite</button>
<ul>
@foreach(var user in Users) 
{
    <li>@user.UserName</li>
}
</ul>

@code {
    [Inject] UserManager<IdentityUser> UserManager { get; set; }

    IQueryable<IdentityUser> Users;

    protected override void OnInitialized()
    {
        Users = UserManager.Users;
    }

    public async Task Add()
    {
        await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
    }
}

`

Extra info about the problem:

• if I use Sqlite provider then the error will never show.
• this is a reproduction of a problem that I have inside my project. I’ve seen that there are a few workarounds to avoid concurrency problems, like:
  • set dbContext scope to Transient: I don’t know if it is correct doing it.
  • create a dbContext instance per CRUD request. I would like to avoid this because it is against dependency injection concepts.
• I’ve already seen this post: Using Entity Framework Core with Blazor #10448 but I didn’t find a solution.
• someone suggests to not expose IQueryable<T> but return an IList<T> to the user’s interface. I am exposing IQueryable<T> because I am using DevExpress components that they can manage IQueryable<T>. These components can apply filters directly on SQL query if the data source is an IQueryable<T>.

System info:

• ASP.NET Core 3.1.0 Blazor server side
• Entity Framework Core 3.1.0