如何将所有DateTime属性转换为集合中的其他TimeZone？

Question

I have a site that has users, and I have an API method to display a user's latest activity:

public List<UserActivity> GetUserActivity(Guid userId)
{
    return _dbContext.UserActivity
               .Where(ua => ua.UserId.Equals(userId))
               .OrderByDescending(ua => ua.ActivityTime)
               .Take(20)
               .ToList();
}

These are the objects I'm returning:

public class UserActivity
{        
    public Guid UserId { get; set; }
    public DateTime ActivityTime { get; set; }
    public string ActivityTitle { get; set; }
}

I'm storing all my dates for ActivityTime in UTC in my database. Each user can set which timezone they're in, so I'd like to return the ActivityTime converted to their timezone for display on the frontend when this API method is called.

Something like this:

public List<UserActivity> GetUserActivity(Guid userId)
{
    MyUser user = _dbContext.MyUsers.FirstOrDefault(u => u.Id.Equals(userId));
    TimeZoneInfo userTimeZone = TimeZoneInfo.FindSystemTimeZoneById(user.TimeZoneId);

    return _dbContext.UserActivity
               .Where(ua => ua.UserId.Equals(userId))
               .OrderByDescending(ua => ua.ActivityTime)
               .Take(20)
               // somehow call TimeZoneInfo.ConvertTimeFromUtc on ActivityTime here
               .ToList();
}

But I'm not sure how to call TimeZoneInfo.ConvertTimeFromUtc on a property in a collection to convert all of them. Is there a way to do that?

Answer 1

Once you have all your UserActivity data in memory, then you can convert it.

var activities = ...;
TimeZoneInfo userZone = TimeZoneInfo.FindSystemTimeZoneById(user.TimeZoneId);
foreach (var thisActivity in activities)
{
    DateTime localTime = 
       TimeZoneInfo.ConvertTimeFromUtc(thisActivity.ActivityTime, userZone);
    thisActivity.ActivityTime = localTime;
}

Answer 2

The important thing you need to do is to convert your DateTime entities after calling ToList() for your EntityFramework DbSet.

Here is an example using just LINQ by projecting your entities:

public List<UserActivity> GetUserActivity(Guid userId)
{
    MyUser user = _dbContext.MyUsers.FirstOrDefault(u => u.Id.Equals(userId));
    TimeZoneInfo userTimeZone = TimeZoneInfo.FindSystemTimeZoneById(user.TimeZoneId);

    var activities = _dbContext.UserActivity
                               .Where(ua => ua.UserId.Equals(userId))
                               .OrderByDescending(ua => ua.ActivityTime)
                               .Take(20)
                               .ToList();

    return activities.Select(a => new UserActivity
    {
        UserId = a.Userid,
        ActivityTitle = a.ActivityTitle,
        ActivityTime = TimeZoneInfo.ConvertTimeFromUtc(a.ActivityTime, userTimeZone)
    }).ToList();
}