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Elegantly dealing with TimeZones in MVC Core / WebApi

In any new application handling TimeZone/DateTime is mostly least priority and generally, if someone is concerned then it would be handled by using DateTime.UtcNow on codes while creating current dates and converting incoming Date to UTC to save on servers.

Basically, the process is followed by saving DateTime to UTC format in a database and keep converting data to native format based on user region or single region in the application's presentation layer.

The above is tedious work and have to be followed religiously. If any developer misses out the manual conversion, then that area of code/view would not work.

With newer frameworks, there are flexible ways to deal/intercept incoming or outgoing calls to simplify conversion of TimeZones.
These are steps/process to achieve it.
1. Central code for storing user's state about TimeZone. Also, central code for conversion logic based on TimeZones.
2. Dependency injection for the above class to be able to use globally.
3. Creating Model Binder to convert all incoming DateTime to UTC format automatically.
4. Modifying JSON serializer to parse UTC to User's local time zone.
5. Register ModelBinder and JSON Converter on Startup
6. MVC Views using @inject keyword to get the class and convert to Native timezone.
7. A client side TimeZone detection technique to prompt the user to change TimeZone and save data into the persistent layer. [Not covered]  

1. Class to save current TimeZone for user and conversion logic. 

This class is responsible for holding the value of User's timezone, and date format. Also at this place codes are written to convert Universal time to local or vice-versa to be consistent. For any DateTime conversion related to user this class would be used, for the same reason Dependency Injection has to be implemented.

 using System;  
 /// <summary>  
 /// User's culture information.  
 /// </summary>  
 public class UserCultureInfo  
 {  
   /// <summary>  
   /// Initializes a new instance of the <see cref="UserCultureInfo"/> class.  
   /// </summary>  
   public UserCultureInfo()  
   {  
     // TODO: Need to through DB Context.  
     TimeZone = TimeZoneInfo.FindSystemTimeZoneById("India Standard Time");  
     DateTimeFormat = "M/d/yyyy h:m:ss tt"; // Default format.  
   }  
   /// <summary>  
   /// Gets or sets the date time format.  
   /// </summary>  
   /// <value>  
   /// The date time format.  
   /// </value>  
   public string DateTimeFormat { get; set; }  
   /// <summary>  
   /// Gets or sets the time zone.  
   /// </summary>  
   /// <value>  
   /// The time zone.  
   /// </value>  
   public TimeZoneInfo TimeZone { get; set; }  
   /// <summary>  
   /// Gets the user local time.  
   /// </summary>  
   /// <returns></returns>  
   public DateTime GetUserLocalTime()  
   {  
     return TimeZoneInfo.ConvertTime(DateTime.UtcNow, TimeZone);  
   }  
   /// <summary>  
   /// Gets the UTC time.  
   /// </summary>  
   /// <param name="datetime">The datetime.</param>  
   /// <returns>Get universal date time based on User's Timezone</returns>  
   public DateTime GetUtcTime(DateTime datetime)  
   {  
     return TimeZoneInfo.ConvertTime(datetime, TimeZone).ToUniversalTime();  
   }  
 }  

The class is plain and self-explanatory to understand. On this place, we should get user's TimeZone population through Database or any other persistent medium like Cookie/Session.

2. Dependency Injection for UserCultureInfo on Startup.cs

This would allow us to get the class throughout the application and same would be used on View to convert date time.
 services.AddScoped<UserCultureInfo>();  

3. Model Binder for DateTime

This would be divided into several classed/tasks to create and register model binder for DateTime. This would be responsible for collecting all incoming DateTime data and converting to UTC format. With this approach, developers do not have to manually convert DateTime data to UTC format while saving into the database.

DateTime binder 

It tries to get DateTime and convert to UTC format through UserCultureInfo class if unsuccessful generates model state error. I have not spent much time on ModelState error generation part which can be improved.

 using Microsoft.AspNetCore.Mvc.Internal;  
 using Microsoft.AspNetCore.Mvc.ModelBinding;  
 using System;  
 using System.Globalization;  
 using System.Threading.Tasks;  
 /// <summary>  
 /// Date time model binder.  
 /// </summary>  
 /// <seealso cref="IModelBinder" />  
 public class DateTimeBinder  
     : IModelBinder  
 {  
   /// <summary>  
   /// The user culture  
   /// </summary>  
   protected readonly UserCultureInfo UserCulture;  
   /// <summary>  
   /// Initializes a new instance of the <see cref="DateTimeBinder"/> class.  
   /// </summary>  
   /// <param name="userCulture">The user culture.</param>  
   public DateTimeBinder(UserCultureInfo userCulture)  
   {  
     UserCulture = userCulture;  
   }  
   public Task BindModelAsync(ModelBindingContext bindingContext)  
   {  
     if (bindingContext == null)  
     {  
       throw new ArgumentNullException(nameof(bindingContext));  
     }  
     var valueProviderResult = bindingContext.ValueProvider  
       .GetValue(bindingContext.ModelName);  
     if (string.IsNullOrEmpty(valueProviderResult.FirstValue))  
     {  
       return null;  
     }  
     DateTime datetime;  
     if (DateTime.TryParse(valueProviderResult.FirstValue, null, DateTimeStyles.AdjustToUniversal, out datetime))  
     {  
       bindingContext.Result =  
           ModelBindingResult.Success(UserCulture.GetUtcTime(datetime));  
     }  
     else  
     {  
       // TODO: [Enhancement] Could be implemented in better way.  
       bindingContext.ModelState.TryAddModelError(  
           bindingContext.ModelName,  
           bindingContext.ModelMetadata  
           .ModelBindingMessageProvider.AttemptedValueIsInvalidAccessor(  
             valueProviderResult.ToString(), nameof(DateTime)));  
     }  
     return TaskCache.CompletedTask;  
   }  
 }  

DateTime Binder Provider

Registers model binder against DateTime data type.

 using Microsoft.AspNetCore.Mvc.ModelBinding;  
 using ProjecTales.Web.Infrastructure.Globalization;  
 using System;  
 /// <summary>  
 /// <see cref="DateTimeBinder"/> provider.  
 /// </summary>  
 /// <seealso cref="IModelBinderProvider" />  
 public class DateTimeBinderProvider  
     : IModelBinderProvider  
 {  
   /// <summary>  
   /// The user culture  
   /// </summary>  
   protected readonly Func<UserCultureInfo> UserCulture;  
   /// <summary>  
   /// Initializes a new instance of the <see cref="DateTimeBinderProvider"/> class.  
   /// </summary>  
   /// <param name="userCulture">The user culture.</param>  
   public DateTimeBinderProvider(Func<UserCultureInfo> userCulture)  
   {  
     UserCulture = userCulture;  
   }  
   /// <summary>  
   /// Creates a <see cref="T:Microsoft.AspNetCore.Mvc.ModelBinding.IModelBinder" /> based on <see cref="T:Microsoft.AspNetCore.Mvc.ModelBinding.ModelBinderProviderContext" />.  
   /// </summary>  
   /// <param name="context">The <see cref="T:Microsoft.AspNetCore.Mvc.ModelBinding.ModelBinderProviderContext" />.</param>  
   /// <returns>  
   /// An <see cref="T:Microsoft.AspNetCore.Mvc.ModelBinding.IModelBinder" />.  
   /// </returns>  
   /// <exception cref="System.ArgumentNullException">context</exception>  
   public IModelBinder GetBinder(ModelBinderProviderContext context)  
   {  
     if (context == null)  
     {  
       throw new ArgumentNullException(nameof(context));  
     }  
     if (context.Metadata.UnderlyingOrModelType == typeof(DateTime))  
     {  
       return new DateTimeBinder(UserCulture());  
     }  
     return null; // TODO: Find alternate.  
   }  
 }  

Initialize Binder through extension method

DateTimeBinderProviderExtension.cs

 using Microsoft.AspNetCore.Mvc;  
 using Microsoft.Extensions.DependencyInjection;  
 using System;  
 /// <summary>  
 /// <see cref="DateTimeBinderProvider"/> initializer  
 /// </summary>  
 public static class DateTimeBinderProviderExtension  
 {  
   /// <summary>  
   /// Registers the date time provider.  
   /// </summary>  
   /// <param name="option">The option.</param>  
   /// <param name="serviceCollection">The service collection.</param>  
   /// <returns></returns>  
   /// <exception cref="System.ArgumentNullException">option</exception>  
   public static MvcOptions RegisterDateTimeProvider(this MvcOptions option,  
     IServiceCollection serviceCollection)  
   {  
     if (option == null)  
     {  
       throw new ArgumentNullException(nameof(option));  
     }  
     // TODO: BuildServiceProvider could be optimized
     option.ModelBinderProviders.Insert(0, new DateTimeBinderProvider(  
       () => serviceCollection.BuildServiceProvider().GetService<UserCultureInfo>()  
       ));  
     return option;  
   }  
 }  


The registration of this extension method would be shown later on Startup.cs.

4. JSON Converter to help while doing serialization

More of all this acts like Model Binder but just to return values from web server to client, through WebApi JSON serialization settings.

DateTimeConverter .cs

 using Newtonsoft.Json;  
 using Newtonsoft.Json.Converters;  
 using ProjecTales.Web.Infrastructure.Globalization;  
 using System;  
 /// <summary>  
 /// JSON Date time converter.  
 /// </summary>  
 /// <seealso cref="DateTimeConverterBase" />  
 public class DateTimeConverter  
 : DateTimeConverterBase  
 {  
   /// <summary>  
   /// The user culture  
   /// </summary>  
   protected readonly Func<UserCultureInfo> UserCulture;  
   /// <summary>  
   /// Initializes a new instance of the <see cref="DateTimeConverter"/> class.  
   /// </summary>  
   /// <param name="userCulture">The user culture.</param>  
   public DateTimeConverter(Func<UserCultureInfo> userCulture)  
   {  
     UserCulture = userCulture;  
   }  
   /// <summary>  
   /// Determines whether this instance can convert the specified object type.  
   /// </summary>  
   /// <param name="objectType">Type of the object.</param>  
   /// <returns>  
   /// <c>true</c> if this instance can convert the specified object type; otherwise, <c>false</c>.  
   /// </returns>  
   public override bool CanConvert(Type objectType)  
   {  
     return objectType == typeof(DateTime) || objectType == typeof(DateTime?);  
   }  
   /// <summary>  
   /// Reads the JSON representation of the object.  
   /// </summary>  
   /// <param name="reader">The <see cref="T:Newtonsoft.Json.JsonReader" /> to read from.</param>  
   /// <param name="objectType">Type of the object.</param>  
   /// <param name="existingValue">The existing value of object being read.</param>  
   /// <param name="serializer">The calling serializer.</param>  
   /// <returns>  
   /// The object value.  
   /// </returns>  
   /// <exception cref="System.NotImplementedException"></exception>  
   public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)  
   {  
     throw new NotImplementedException();  
   }  
   /// <summary>  
   /// Writes the JSON representation of the object.  
   /// </summary>  
   /// <param name="writer">The <see cref="T:Newtonsoft.Json.JsonWriter" /> to write to.</param>  
   /// <param name="value">The value.</param>  
   /// <param name="serializer">The calling serializer.</param>  
   public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)  
   {  
     // Null are already filtered  
     var userCulture = UserCulture();  
     writer.WriteValue(TimeZoneInfo.ConvertTime(Convert.ToDateTime(value), userCulture.TimeZone)  
     .ToString(userCulture.DateTimeFormat));  
     writer.Flush();  
   }  
 }  

DateTimeConverterExtension.cs
 using Microsoft.AspNetCore.Mvc;  
 using Microsoft.Extensions.DependencyInjection;  
   /// <summary>  
   /// <see cref="DateTimeConverter"/> initializer.  
   /// </summary>  
   public static class DateTimeConverterExtension  
   {  
     /// <summary>  
     /// Registers the date time converter.  
     /// </summary>  
     /// <param name="option">The option.</param>  
     /// <param name="serviceCollection">The service collection.</param>  
     /// <returns></returns>  
     public static MvcJsonOptions RegisterDateTimeConverter(this MvcJsonOptions option,  
       IServiceCollection serviceCollection)  
     {  
       // TODO: BuildServiceProvider could be optimized  
       option.SerializerSettings.Converters.Add(  
           new DateTimeConverter(() => serviceCollection.BuildServiceProvider().GetService<UserCultureInfo>())  
           );  
       return option;  
     }  
   }  

5. Register Model binder and JSON converter on Startup.cs

Through both extension classes, JSON Converter and model binder are registered in a much simpler manner.

 services.AddMvc(option =>  
         option.RegisterDateTimeProvider(services))  
       .AddJsonOptions(jsonOption =>  
         jsonOption.RegisterDateTimeConverter(services));  

6. Controller and view to test Approach

TestController.cs

   public class TestModel  
   {  
     public int Id { get; set; }  
     public DateTime Date { get; set; }  
     public DateTime? NullableDate { get; set; }  
   }  
   public class TestController : Controller  
   {  
     // GET: /<controller>/  
     public IActionResult Index()  
     {  
       return View();  
     }  
     [HttpPost]  
     public IActionResult Index(TestModel model)  
     {  
       return View(model);  
     }  
     public IActionResult Json()  
     {  
       return Json(new TestModel  
       {  
         Id = 1,  
         Date = DateTime.UtcNow,  
         NullableDate = null,  
       });  
     }  
   }  

Index and Json actions can be directly tested. 


Views/Test.cshtml

 @model MyProject.Web.Controllers.TestModel  
 @inject MyProject.Web.Infrastructure.Globalization.UserCultureInfo culture;  
 @{  
   Layout = null;  
 }  
 <form asp-action="Index" asp-controller="Test">  
   <label asp-for="@Model.Id" /><br />  
   <input asp-for="@Model.Id" value="1276"/>  
   <label asp-for="@Model.Date" /><br />  
   <input asp-for="@Model.Date" value="@culture.GetUserLocalTime()" />  
   <label asp-for="@Model.NullableDate" /><br />  
   <input asp-for="@Model.NullableDate" value="@culture.GetUserLocalTime()" /><br />  
   <input type="submit"/>  
 </form>  


GetUserLocalTime has to be on Views whenever accessing anything related to DateTime to convert based on user's local TimeZone. Since there are not many ways to handle this it seems to be a clean approach.

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