如何从代码中检索数据注释？(编程)

Question

我正在使用System.ComponentModel.DataAnnotations我的Entity Framework 4.1项目进行验证.

例如:

public class Player
{
    [Required]
    [MaxLength(30)]
    [Display(Name = "Player Name")]
    public string PlayerName { get; set; }

    [MaxLength(100)]
    [Display(Name = "Player Description")]
    public string PlayerDescription{ get; set; }
}

我需要检索Display.Name注释值以在消息中显示它,例如所选的"玩家名称"是Frank.

================================================== ===============================

我需要检索注释的另一个例子:

var playerNameTextBox = new TextBox();
playerNameTextBox.MaxLength = GetAnnotation(myPlayer.PlayerName, MaxLength);

我怎样才能做到这一点？

Answer 1

扩展方法:

public static T GetAttributeFrom<T>(this object instance, string propertyName) where T : Attribute
{
    var attrType = typeof(T);
    var property = instance.GetType().GetProperty(propertyName);
    return (T)property .GetCustomAttributes(attrType, false).First();
}

码:

var name = player.GetAttributeFrom<DisplayAttribute>("PlayerDescription").Name;
var maxLength = player.GetAttributeFrom<MaxLengthAttribute>("PlayerName").Length;

Answer 2

试试这个:

((DisplayAttribute)
  (myPlayer
    .GetType()
    .GetProperty("PlayerName")
    .GetCustomAttributes(typeof(DisplayAttribute),true)[0])).Name;

Answer 3

以下是一些可用于获取 MaxLength 或任何其他属性的静态方法。

using System;
using System.Linq;
using System.Reflection;
using System.ComponentModel.DataAnnotations;
using System.Linq.Expressions;

public static class AttributeHelpers {

public static Int32 GetMaxLength<T>(Expression<Func<T,string>> propertyExpression) {
    return GetPropertyAttributeValue<T,string,MaxLengthAttribute,Int32>(propertyExpression,attr => attr.Length);
}

//Optional Extension method
public static Int32 GetMaxLength<T>(this T instance,Expression<Func<T,string>> propertyExpression) {
    return GetMaxLength<T>(propertyExpression);
}


//Required generic method to get any property attribute from any class
public static TValue GetPropertyAttributeValue<T, TOut, TAttribute, TValue>(Expression<Func<T,TOut>> propertyExpression,Func<TAttribute,TValue> valueSelector) where TAttribute : Attribute {
    var expression = (MemberExpression)propertyExpression.Body;
    var propertyInfo = (PropertyInfo)expression.Member;
    var attr = propertyInfo.GetCustomAttributes(typeof(TAttribute),true).FirstOrDefault() as TAttribute;

    if (attr==null) {
        throw new MissingMemberException(typeof(T).Name+"."+propertyInfo.Name,typeof(TAttribute).Name);
    }

    return valueSelector(attr);
}

}

使用静态方法...

var length = AttributeHelpers.GetMaxLength<Player>(x => x.PlayerName);

或者在实例上使用可选的扩展方法......

var player = new Player();
var length = player.GetMaxLength(x => x.PlayerName);

或者对任何其他属性使用完整的静态方法（例如 StringLength）...

var length = AttributeHelpers.GetPropertyAttributeValue<Player,string,StringLengthAttribute,Int32>(prop => prop.PlayerName,attr => attr.MaximumLength);

受到这里答案的启发... /sf/answers/2275094951/