如何从代码中检索数据注释？ （以编程方式）

Question

我正在使用 System.ComponentModel.DataAnnotations 为我的 Entity Framework 4.1 项目提供验证。

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 The selected "Player Name" is Frank.

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

为什么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();
}

代码：

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

受到此处答案的启发...
https://stackoverflow.com/a/32501356/324479

Answer 4

这就是我做类似事情的方式

/// <summary>
/// Returns the DisplayAttribute of a PropertyInfo (field), if it fails returns null
/// </summary>
/// <param name="propertyInfo"></param>
/// <returns></returns>
private static string TryGetDisplayName(PropertyInfo propertyInfo)
{
    string result = null;
    try
    {
        var attrs = propertyInfo.GetCustomAttributes(typeof(DisplayAttribute), true);
        if (attrs.Any())
            result = ((DisplayAttribute)attrs[0]).Name;
    }
    catch (Exception)
    {
        //eat the exception
    }
    return result;
}

Answer 5

因为 https://stackoverflow.com/a/7027791/7173655 上的接受答案仍然使用魔术常量，所以我分享我的基于链接答案的代码：

扩展方法：

public static TA GetAttributeFrom<TC,TA>(string propertyName) where TA : Attribute {
    return (TA)typeof(TC).GetProperty(propertyName)
        .GetCustomAttributes(typeof(TA), false).SingleOrDefault();
}

没有魔术常量的使用（确保重构确实伤害较小）：

var nameMaxLength = device.GetAttributeFrom<StringLengthAttribute>(nameof(device.name)).MaximumLength;

Answer 6

我认为这个例子 https://github.com/TeteStorm/DataAnnotationScan 非常有用。

我只是为了让 EF 在我的模型装配中使用数据注释，但请随意 fork 并根据需要进行更改。

更改以下方法 HasEFDataAnnotation 并享受乐趣！

https://github.com/TeteStorm/DataAnnotationScan

        private static bool HasEFDataAnnotaion(PropertyInfo[] properties)
        {
            return properties.ToList().Any((property) =>
            {
                var attributes = property.GetCustomAttributes(false);
                Attribute[] attrs = System.Attribute.GetCustomAttributes(property);
                return attrs.Any((attr) =>
                {
                    return attr is KeyAttribute || attr is ForeignKeyAttribute || attr is IndexAttribute || attr is RequiredAttribute || attr is TimestampAttribute
                    || attr is ConcurrencyCheckAttribute || attr is MinLengthAttribute || attr is MinLengthAttribute
                    || attr is MaxLengthAttribute || attr is StringLengthAttribute || attr is TableAttribute || attr is ColumnAttribute
                    || attr is DatabaseGeneratedAttribute || attr is ComplexTypeAttribute;
                });
            });
        }

Answer 7

修复了使用元数据类与 MetadataTypeAttribute 的问题，来自此处

     public  T GetAttributeFrom<T>( object instance, string propertyName) where T : Attribute
    {
        var attrType = typeof(T);
        var property = instance.GetType().GetProperty(propertyName);
        T t = (T)property.GetCustomAttributes(attrType, false).FirstOrDefault();
        if (t == null)
        {
            MetadataTypeAttribute[] metaAttr = (MetadataTypeAttribute[])instance.GetType().GetCustomAttributes(typeof(MetadataTypeAttribute), true);
            if (metaAttr.Length > 0)
            {
                foreach (MetadataTypeAttribute attr in metaAttr)
                {
                    var subType = attr.MetadataClassType;
                    var pi = subType.GetField(propertyName);
                    if (pi != null)
                    {
                        t = (T)pi.GetCustomAttributes(attrType, false).FirstOrDefault();
                        return t;
                    }


                }
            }

        }
        else
        {
            return t;
        }
        return null; 
    }