Wie rufe ich Datenanmerkungen aus Code ab? (programmatisch)

Erweiterungsmethode:

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();
}

Code:

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

versuchen Sie Folgendes:

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

Hier sind einige statische Methoden, die Sie verwenden können, um MaxLength oder ein anderes Attribut abzurufen.

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);
}

}

Mit der statischen Methode...

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

Oder die optionale Erweiterungsmethode für eine Instanz verwenden...

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

Oder verwenden Sie die vollständige statische Methode für ein beliebiges anderes Attribut (z. B. StringLength) ...

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

Inspiriert von der Antwort hier ... https://stackoverflow.com/a/32501356/324479