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 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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