ASP.NET 核心实体框架。 如何使用子查询编写异步查询？

Question

This is code which I try to async.

public void MarkAsRead(Guid id)
{
   var notificationMessages = DbContext.NotificationMessages
        .Where(nm => !nm.IsRead && nm.CreatedDate <  DbContext.NotificationMessages.FirstOrDefault(x => x.Id == id).CreatedDate)
        .ToList();

        notificationMessages.ForEach(nm => nm.IsRead = true);
        DbContext.SaveChanges();
}

I have alrady tried this, but it didn't work

public async Task MarkAsRead(Guid id)
{
   var notificationMessages = await DbContext.NotificationMessages
        .Where( async nm => !nm.IsRead && nm.CreatedDate < await DbContext.NotificationMessages.FirstOrDefaultAsync(x => x.Id == id).Result.CreatedDate)
        .ToListAsync();

        notificationMessages.ForEach(nm => nm.IsRead = true);
        await DbContext.SaveChangesAsync();
}

The main problem that I can't get subquery field CreatedDate .

The error message says:

'DateTime' does not contain a definition for 'GetAwaiter' and no accessible extension method 'GetAwaiter' accepting a first argument of type 'DateTime' could be found (are you missing a using directive or an assembly reference?)

Answer 1

The expression that is passed to Where is simply converted to SQL by EF, so there is no need to try and make it async.

The ToListAsync method executes the query on the database asynchronously, so should be awaited:

public async Task MaskAsRead(Guid id)
{
   var notificationMessages = await DbContext.NotificationMessages
        .Where(nm => !nm.IsRead && nm.CreatedDate < DbContext.NotificationMessages.FirstOrDefault(x => x.Id == id).Result.CreatedDate)
        .ToListAsync();

    notificationMessages.ForEach(nm => nm.IsRead = true);
    await DbContext.SaveChangesAsync();
}