c# - 将 PropertyInfo 转换为 Func<> 委托(delegate)

Question

我有许多数据模型对象，它们的主键字段用 DataAnnotations.KeyAttributes 装饰:

public class Example
{
  [Key]
  [Column("FirstKey", Order = 1)]
  public Guid FirstKey { get; set; }

  [Key]
  [Column("SecondKey", Order = 2)]
  public Guid SecondKey { get; set; }

  [Column("Unimportant", Order = 3)]
  public int UnimportantField {get; set; }
}

我可以使用 PropertInfo 选择数据模型对象的关键字段:

public static IEnumerable<PropertyInfo> GetKeyProperties<T>(this T source)
{
  PropertyInfo[] properties = source.GetType().GetProperties();
  return properties.Where(p => Attribute.IsDefined(p, typeof(KeyAttribute)));
}

我还可以手动构建一个包含给定类的键字段的匿名对象:

public static object GetKeyObjectForExample(Example example)
{
  return new { example.FirstKey, example.SecondKey };
}

我想创建一个通用方法来构造一个 Func 委托(delegate)，该委托(delegate)将返回一个包含键字段的匿名对象。

public static Func<T, object> GetKeyFunction<T>(this T source)
{
  //construct Func<T, object> using IEnumerable<PropertyInfo>?
}

因此，为给定的 Example 实例调用 GetKeyFunction 方法将返回一个 Func 委托(delegate)。此委托(delegate)应生成与 GetKeyObjectForExample 方法等效的输出。

如何实现 GetKeyFunction 方法？

Answer 1

不确定您要实现的确切目标，因为即使使用动态或匿名对象，您仍然需要知道关键字段的名称，但像这样的方法可以解决您声明的方法(错误处理等省略):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Dynamic;

namespace ConsoleApplication11
{
    class KeyAttribute : Attribute
    {
    }
    public class Example
    {
        [Key]
        public Guid FirstKey { get; set; }

        [Key]
        public Guid SecondKey { get; set; }

        public int UnimportantField { get; set; }
    }

    static class Program
    {
        static void Main(string[] args)
        {
            Example e = new Example()
            {
                FirstKey = Guid.NewGuid(),
                SecondKey = Guid.NewGuid()
            };

            var func = GetKeyFunction<Example>();

            var key = func(e);
            Console.WriteLine(key.FirstKey);
            Console.WriteLine(key.SecondKey);
        }

        public static Func<T, dynamic> GetKeyFunction<T>()
        {
            return (t) =>
                {
                    var dyn = (IDictionary<string, object>)new ExpandoObject();
                    foreach (var p in GetKeyProperties(typeof(T)))
                    {
                        dyn.Add(p.Name, p.GetValue(t));
                    }
                    return dyn;
                };
        }

        public static IEnumerable<PropertyInfo> GetKeyProperties(Type t)
        {
            PropertyInfo[] properties = t.GetProperties();
            return properties.Where(p => p.GetCustomAttributes<KeyAttribute>().Any());
        }
    }
}

编辑

阅读一些评论后，您可能会喜欢上这种方法，并向您的动态对象添加一个 Equals 和 GetHashCode 实现(此实现假定关键字段的名称在相等性方面很重要)。感觉像确定对象相等性的代码和动态太多了，但是......

与上面相同的代码，但有一个辅助类来确定字典相等性和哈希码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Dynamic;

namespace ConsoleApplication11
{
    class KeyAttribute : Attribute
    {
    }
    public class Example
    {
        [Key]
        public Guid FirstKey { get; set; }

        [Key]
        public Guid SecondKey { get; set; }

        public int UnimportantField { get; set; }
    }

    static class Program
    {
        static void Main(string[] args)
        {
            Example e = new Example()
            {
                FirstKey = Guid.NewGuid(),
                SecondKey = Guid.NewGuid(),
                UnimportantField = 0
            };

            var func = GetKeyFunction<Example>();

            var key = func(e);
            Console.WriteLine(key.FirstKey);
            Console.WriteLine(key.SecondKey);

            Console.WriteLine(key.Equals(key));
            Console.WriteLine(key.GetHashCode());

            Example e2 = new Example()
            {
                FirstKey = Guid.NewGuid(),
                SecondKey = Guid.NewGuid(),
                UnimportantField = 1
            };

            var key2 = func(e2);
            Console.WriteLine(key2.FirstKey);
            Console.WriteLine(key2.SecondKey);
            Console.WriteLine(key.Equals(key2));
            Console.WriteLine(key2.GetHashCode());
        }

        public static Func<T, dynamic> GetKeyFunction<T>()
        {
            return (t) =>
                {
                    var dyn = (IDictionary<string, object>)new ExpandoObject();
                    foreach (var p in GetKeyProperties(typeof(T)))
                    {
                        dyn.Add(p.Name, p.GetValue(t));
                    }
                    dyn.Add("Equals", (Func<object, bool>)((o) => 
                        {
                            var comparer = new DictionaryComparer<string,object>();
                            return comparer.Equals(dyn as IDictionary<string,object>, o as IDictionary<string,object>);
                        }
                        ));

                    dyn.Add("GetHashCode", (Func<int>)(() =>
                        {
                            var comparer = new DictionaryComparer<string, object>();
                            return comparer.GetHashCode(dyn as IDictionary<string, object>);
                        }
                        ));
                    return dyn;
                };
        }

        public static IEnumerable<PropertyInfo> GetKeyProperties(Type t)
        {
            PropertyInfo[] properties = t.GetProperties();
            return properties.Where(p => p.GetCustomAttributes<KeyAttribute>().Any());
        }
    }

    public class DictionaryComparer<TKey, TValue> : EqualityComparer<IDictionary<TKey, TValue>>
    {
        public DictionaryComparer()
        {
        }
        public override bool Equals(IDictionary<TKey, TValue> x, IDictionary<TKey, TValue> y)
        {
            // early-exit checks
            if (object.ReferenceEquals(x, y))
                return true;

            if (null == x || y == null)
                return false;

            if (x.Count != y.Count)
                return false;

            // check keys are the same
            foreach (TKey k in x.Keys)
                if (!y.ContainsKey(k))
                    return false;

            // check values are the same
            foreach (TKey k in x.Keys)
            {
                TValue v = x[k];
                if (object.ReferenceEquals(v, null))
                    return object.ReferenceEquals(y[k], null);


                if (!v.Equals(y[k]))
                    return false;
            }
            return true;
        }    

        public override int GetHashCode(IDictionary<TKey, TValue> obj)
        {
            if (obj == null)
                return 0;

            int hash = 0;

            foreach (KeyValuePair<TKey, TValue> pair in obj)
            {
                int key = pair.Key.GetHashCode(); // key cannot be null
                int value = pair.Value != null ? pair.Value.GetHashCode() : 0;
                hash ^= ShiftAndWrap(key, 2) ^ value;
            }

            return hash;
        }

        private static int ShiftAndWrap(int value, int positions)
        {
            positions = positions & 0x1F;


            // Save the existing bit pattern, but interpret it as an unsigned integer. 
            uint number = BitConverter.ToUInt32(BitConverter.GetBytes(value), 0);
            // Preserve the bits to be discarded. 
            uint wrapped = number >> (32 - positions);
            // Shift and wrap the discarded bits. 
            return BitConverter.ToInt32(BitConverter.GetBytes((number << positions) | wrapped), 0);
        }
    }
}