asp.net-core - DbContext 实例不能在 OnConfiguring 内部使用，因为此时它仍在配置中

Question

我正在尝试构建一个自定义 View 位置系统。

public class myViewLocationExpander : IViewLocationExpander
{
    private myDBContext _context;

    public myViewLocationExpander (myDBContext context)
    {
        _context = context;
    }

    public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
    {
        _context.Property.//..... //Error happened here
           Some other codes
    }

在我的启动文件中

    public void ConfigureServices(IServiceCollection services)
    {
       //other codes

        services.AddMvc();

        services.Configure<RazorViewEngineOptions>(options =>
        {
            options.ViewLocationExpanders.Add(new myViewLocationExpander (new myDBContext()));
        });

我的错误1:

An unhandled exception occurred while processing the request. InvalidOperationException: No database providers are configured. Configure a database provider by overriding OnConfiguring in your DbContext class or in the AddDbContext method when setting up services. Microsoft.Data.Entity.Internal.DatabaseProviderSelector.SelectServices(ServiceProviderSource providerSource)

错误2:

An exception of type 'System.InvalidOperationException' occurred in EntityFramework.Core.dll but was not handled in user code Additional information: An attempt was made to use the context while it is being configured. A DbContext instance cannot be used inside OnConfiguring since it is still being configured at this point.

如何在类中使用 dbcontext(我需要从数据库获取一些信息)，该类应放置在启动文件的 Configure() 方法上。

或者我可以将IViewLocation..放到另一个地方吗？

Answer 1

在 IViewLocationExpander 的代码中，您可以使用以下代码从上下文获取 IServiceProvider context.ActionContext.HttpContext.ApplicationServices.GetService( typeof( myDbContext ) ) 这将返回您的 DbContext。

public class MyViewLocationExpander : IViewLocationExpander
{
    public void PopulateValues( ViewLocationExpanderContext context )
    {
        var dbContext = context.ActionContext.HttpContext.ApplicationServices.GetService<ApplicationDbContext>();
    }

    public IEnumerable<string> ExpandViewLocations( ViewLocationExpanderContext context, IEnumerable<string> viewLocations )
    {
        var dbContext = context.ActionContext.HttpContext.ApplicationServices.GetService<ApplicationDbContext>();
        return viewLocations;
    }
}

我注意到它在第二次调用 PopulateValues 时起作用，而不是第一次我没有时间进一步研究这个