异步方法上的C#实体框架错误 [英] C# Entity Framework error on async methods

问题描述

我已经看到了，但是我遇到了另一个问题.

I have already seen this, but I am experiencing another problem.

我具有用于管理ASP.NET身份角色的服务类:

I have this service class for managing ASP.NET identity roles:

public class RoleService : IRoleService
{
    private readonly RoleManager<ApplicationRole> _roleManager;

    public RoleService(RoleManager<ApplicationRole> roleManager)
    {
        this._roleManager = roleManager;
    }

    public async Task<IdentityResult> CreateAsync(ApplicationRole role)
    {
        return await this._roleManager.CreateAsync(role);
    }
}

根据此问题的建议，我使用这样的 CreateAsync 方法来避免使用LINQ <代码> foreach :

As suggested by this question, I use the CreateAsync method like this to avoid using LINQ foreach:

private async Task PopulateRoles()
{
     var roles = new[] { "A", "B", "C", "D" };

     // Used LINQ foreach previously but I coded this way instead to follow the related questions's answer
     var tasks = roles.Select(role =>
                           this._roleService.CreateAsync(new ApplicationRole(role)))
                      .ToList();

     await Task.WhenAll(tasks);
}

但是，当执行 await this.PopulateRoles()时，这会导致错误.

However, this results in an error when await this.PopulateRoles() is executed.

实体框架:已经有一个与此命令相关联的打开的DataReader，必须首先将其关闭.

Entity Framework: There is already an open DataReader associated with this Command which must be closed first.

搜索此错误仅导致我建议在我的Select LINQ中添加 ToList().我该如何解决?

Searching for this error only leads me to a suggestion of adding a ToList() in my Select LINQ. How can I fix it?

推荐答案

问题出在 RoleManager< T> 上，我们可以在内部给它一个单独的 DbContext 参见此处:

The problem lays with the RoleManager<T> which internally is given a single DbContext as we can see here:

public class RoleStore<TRole, TContext, TKey> : 
        IQueryableRoleStore<TRole>, 
        IRoleClaimStore<TRole>
        where TRole : IdentityRole<TKey>
        where TKey : IEquatable<TKey>
        where TContext : DbContext
{
    public RoleStore(TContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException("context");
        }
        Context = context;
    }
}

DbContext 本身无法处理并发调用.另一种选择是在每个 foreach 和 await 内执行每个调用:

DbContext itself can't handle concurrent calls. The alternative would be to execute each of the calls inside a foreach and await each one:

private async Task PopulateRoles()
{
     var roles = new[] { "A", "B", "C", "D" };

     foreach (var role in roles)
     {
         await _roleService.CreateAsync(new ApplicationRole(role));
     }
}

这样，尽管您没有同时应用所有角色的好处，但您仍然可以利用IO调用的异步特性，而不是阻止同步调用.

This way, though you don't have the benefit of applying all roles concurrently, you do still take advantage of the async nature of the IO call, instead of a blocking synchronous call.

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