问题
I have C# program where all DateTime objects are DateTimeKind.UTC. When saving the objects to the database it stores UTC as expected. However, when retreiving them, they are DateTimeKind.Unspecified. Is there a way to tell Entity Framework (Code First) when creating DateTime objects in C# to always use DateTimeKind.UTC?
回答1:
No, there's not. And it's actually DateTimeKind.Unspecified.
However, if you are concerned about supporting multiple timezones, you should consider using DateTimeOffset. It's like a regular DateTime, except that it does not represent a "perspective" of time, it represents an absolute view, in which 3PM (UTC - 3) equals 4PM (UTC - 2). DateTimeOffset contains both the DateTime and the time zone and it's supported by both EntityFramework and SQL Server.
回答2:
You can have your datacontext fix up all the relevant values as it goes. The following does so with a cache of properties for entity types, so as to avoid having to examine the type each time:
public class YourContext : DbContext
{
private static readonly List<PropertyInfo> EmptyPropsList = new List<PropertyInfo>();
private static readonly Hashtable PropsCache = new Hashtable(); // Spec promises safe for single-reader, multiple writer.
// Spec for Dictionary makes no such promise, and while
// it should be okay in this case, play it safe.
private static List<PropertyInfo> GetDateProperties(Type type)
{
List<PropertyInfo> list = new List<PropertyInfo>();
foreach(PropertyInfo prop in type.GetProperties())
{
Type valType = prop.PropertyType;
if(valType == typeof(DateTime) || valType == typeof(DateTime?))
list.Add(prop);
}
if(list.Count == 0)
return EmptyPropsList; // Don't waste memory on lots of empty lists.
list.TrimExcess();
return list;
}
private static void FixDates(object sender, ObjectMaterializedEventArgs evArg)
{
object entity = evArg.Entity;
if(entity != null)
{
Type eType = entity.GetType();
List<PropertyInfo> rules = (List<PropertyInfo>)PropsCache[eType];
if(rules == null)
lock(PropsCache)
PropsCache[eType] = rules = GetPropertyRules(eType); // Don't bother double-checking. Over-write is safe.
foreach(var rule in rules)
{
var info = rule.PropertyInfo;
object curVal = info.GetValue(entity);
if(curVal != null)
info.SetValue(entity, DateTime.SpecifyKind((DateTime)curVal, rule.Kind));
}
}
}
public YourContext()
{
((IObjectContextAdapter)this).ObjectContext.ObjectMaterialized += FixDates;
/* rest of constructor logic here */
}
/* rest of context class here */
}
This can also be combined with attributes so as to allow one to set the DateTimeKind each property should have, by storing a set of rules about each property, rather than just the PropertyInfo, and looking for the attribute in GetDateProperties.
回答3:
My solution, using code first: Declare the DateTime properties in this way:
private DateTime _DateTimeProperty;
public DateTime DateTimeProperty
{
get
{
return _DateTimeProperty;
}
set
{
_DateTimeProperty = value.ToKindUtc();
}
}
Also can create the property as:
private DateTime? _DateTimeProperty;
public DateTime? DateTimeProperty
{
get
{
return _DateTimeProperty;
}
set
{
_DateTimeProperty = value.ToKindUtc();
}
}
ToKindUtc() is a extension to change DateTimeKind.Unspecified to DateTimeKind.Utc or call ToUniversalTime() if kind is DateTimeKind.Local
Here the code for the extensions:
public static class DateTimeExtensions
{
public static DateTime ToKindUtc(this DateTime value)
{
return KindUtc(value);
}
public static DateTime? ToKindUtc(this DateTime? value)
{
return KindUtc(value);
}
public static DateTime ToKindLocal(this DateTime value)
{
return KindLocal(value);
}
public static DateTime? ToKindLocal(this DateTime? value)
{
return KindLocal(value);
}
public static DateTime SpecifyKind(this DateTime value, DateTimeKind kind)
{
if (value.Kind != kind)
{
return DateTime.SpecifyKind(value, kind);
}
return value;
}
public static DateTime? SpecifyKind(this DateTime? value, DateTimeKind kind)
{
if (value.HasValue)
{
return DateTime.SpecifyKind(value.Value, kind);
}
return value;
}
public static DateTime KindUtc(DateTime value)
{
if (value.Kind == DateTimeKind.Unspecified)
{
return DateTime.SpecifyKind(value, DateTimeKind.Utc);
}
else if (value.Kind == DateTimeKind.Local)
{
return value.ToUniversalTime();
}
return value;
}
public static DateTime? KindUtc(DateTime? value)
{
if (value.HasValue)
{
return KindUtc(value.Value);
}
return value;
}
public static DateTime KindLocal(DateTime value)
{
if (value.Kind == DateTimeKind.Unspecified)
{
return DateTime.SpecifyKind(value, DateTimeKind.Local);
}
else if (value.Kind == DateTimeKind.Utc)
{
return value.ToLocalTime();
}
return value;
}
public static DateTime? KindLocal(DateTime? value)
{
if (value.HasValue)
{
return KindLocal(value.Value);
}
return value;
}
}
Remember to include in the model's file.
using TheNameSpaceWhereClassIsDeclared;
The set method of property is called when reading from datatabase with EF, or when assigned in a MVC controller's edit method.
Warning, if in web forms, if you edit dates in local timezone, you MUST convert the date to UTC before send to server.
回答4:
Have a look on the michael.aird answer here: https://stackoverflow.com/a/9386364/279590 It stamp the date UTC kind during loading, with an event on ObjectMaterialized.
来源：https://stackoverflow.com/questions/29496306/how-to-specify-that-datetime-objects-retrieved-from-entityframework-should-be-da