Hay una manera. Agrega estas clases:
DefaultDateTimeValueAttribute.cs
using System;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Reflection;
using System.Runtime.CompilerServices;
using Custom.Extensions;
namespace Custom.DefaultValueAttributes
{
/// <summary>
/// This class's DefaultValue attribute allows the programmer to use DateTime.Now as a default value for a property.
/// Inspired from https://code.msdn.microsoft.com/A-flexible-Default-Value-11c2db19.
/// </summary>
[AttributeUsage(AttributeTargets.Property)]
public sealed class DefaultDateTimeValueAttribute : DefaultValueAttribute
{
public string DefaultValue { get; set; }
private object _value;
public override object Value
{
get
{
if (_value == null)
return _value = GetDefaultValue();
return _value;
}
}
/// <summary>
/// Initialized a new instance of this class using the desired DateTime value. A string is expected, because the value must be generated at runtime.
/// Example of value to pass: Now. This will return the current date and time as a default value.
/// Programmer tip: Even if the parameter is passed to the base class, it is not used at all. The property Value is overridden.
/// </summary>
/// <param name="defaultValue">Default value to render from an instance of <see cref="DateTime"/></param>
public DefaultDateTimeValueAttribute(string defaultValue) : base(defaultValue)
{
DefaultValue = defaultValue;
}
public static DateTime GetDefaultValue(Type objectType, string propertyName)
{
var property = objectType.GetProperty(propertyName);
var attribute = property.GetCustomAttributes(typeof(DefaultDateTimeValueAttribute), false)
?.Cast<DefaultDateTimeValueAttribute>()
?.FirstOrDefault();
return attribute.GetDefaultValue();
}
private DateTime GetDefaultValue()
{
// Resolve a named property of DateTime, like "Now"
if (this.IsProperty)
{
return GetPropertyValue();
}
// Resolve a named extension method of DateTime, like "LastOfMonth"
if (this.IsExtensionMethod)
{
return GetExtensionMethodValue();
}
// Parse a relative date
if (this.IsRelativeValue)
{
return GetRelativeValue();
}
// Parse an absolute date
return GetAbsoluteValue();
}
private bool IsProperty
=> typeof(DateTime).GetProperties()
.Select(p => p.Name).Contains(this.DefaultValue);
private bool IsExtensionMethod
=> typeof(DefaultDateTimeValueAttribute).Assembly
.GetType(typeof(DefaultDateTimeExtensions).FullName)
.GetMethods()
.Where(m => m.IsDefined(typeof(ExtensionAttribute), false))
.Select(p => p.Name).Contains(this.DefaultValue);
private bool IsRelativeValue
=> this.DefaultValue.Contains(":");
private DateTime GetPropertyValue()
{
var instance = Activator.CreateInstance<DateTime>();
var value = (DateTime)instance.GetType()
.GetProperty(this.DefaultValue)
.GetValue(instance);
return value;
}
private DateTime GetExtensionMethodValue()
{
var instance = Activator.CreateInstance<DateTime>();
var value = (DateTime)typeof(DefaultDateTimeValueAttribute).Assembly
.GetType(typeof(DefaultDateTimeExtensions).FullName)
.GetMethod(this.DefaultValue)
.Invoke(instance, new object[] { DateTime.Now });
return value;
}
private DateTime GetRelativeValue()
{
TimeSpan timeSpan;
if (!TimeSpan.TryParse(this.DefaultValue, out timeSpan))
{
return default(DateTime);
}
return DateTime.Now.Add(timeSpan);
}
private DateTime GetAbsoluteValue()
{
DateTime value;
if (!DateTime.TryParse(this.DefaultValue, out value))
{
return default(DateTime);
}
return value;
}
}
}
DefaultDateTimeExtensions.cs
using System;
namespace Custom.Extensions
{
/// <summary>
/// Inspired from https://code.msdn.microsoft.com/A-flexible-Default-Value-11c2db19. See usage for more information.
/// </summary>
public static class DefaultDateTimeExtensions
{
public static DateTime FirstOfYear(this DateTime dateTime)
=> new DateTime(dateTime.Year, 1, 1, dateTime.Hour, dateTime.Minute, dateTime.Second, dateTime.Millisecond);
public static DateTime LastOfYear(this DateTime dateTime)
=> new DateTime(dateTime.Year, 12, 31, dateTime.Hour, dateTime.Minute, dateTime.Second, dateTime.Millisecond);
public static DateTime FirstOfMonth(this DateTime dateTime)
=> new DateTime(dateTime.Year, dateTime.Month, 1, dateTime.Hour, dateTime.Minute, dateTime.Second, dateTime.Millisecond);
public static DateTime LastOfMonth(this DateTime dateTime)
=> new DateTime(dateTime.Year, dateTime.Month, DateTime.DaysInMonth(dateTime.Year, dateTime.Month), dateTime.Hour, dateTime.Minute, dateTime.Second, dateTime.Millisecond);
}
}
Y utilice DefaultDateTimeValue como atributo de sus propiedades. El valor para ingresar a su atributo de validación son cosas como "Ahora", que se representará en tiempo de ejecución desde una instancia de DateTime creada con un Activador. El código fuente está inspirado en este hilo: https://code.msdn.microsoft.com/A-flexible-Default-Value-11c2db19 . Lo cambié para que mi clase herede con DefaultValueAttribute en lugar de ValidationAttribute.