C# LINQ - 根据运行时定义的属性选择动态对象

Question

我一直在尝试创建一个表达式，它可以将强类型的 EF Core 实体投影到一个动态对象中，该对象包含一个在运行时使用 REST API 调用定义的列表。

这是我到目前为止所拥有的：

Expression<Func<Message, dynamic>> DynamicFields(IEnumerable<string> fields)

{

    var xParameter = Expression.Parameter(typeof(Message), "o");

    var xNew = Expression.New(typeof(ExpandoObject));

    var bindings = fields.Select(o => {

        var mi = typeof(Message).GetProperty(o);

        var xOriginal = Expression.Property(xParameter, mi);

        return Expression.Bind(mi, xOriginal);

    });

    var xInit = Expression.MemberInit((dynamic)xNew, bindings);

    return Expression.Lambda<Func<Message, dynamic>>(xInit, xParameter);

}

感觉就像我非常接近，但这在运行时会爆炸，说明 X 属性不是 ExpandoObject 的成员。我尝试改变动态和 ExpandoObject 的使用，但似乎没有任何效果 - 这甚至可能吗？

如果我为 Message 切换出 dyanmic / ExpandoObject，它工作得很好，但会返回 Message 类的一个实例，其所有属性都为默认值。

以前有人做过吗？

Answer 1

不能直接投影到ExpandoObject/ dynamic。但是您可以在运行时投影到动态创建的类型。参见例如Microsoft.EntityFrameworkCore.DynamicLinq包 - 动态数据类。如果您以 nuget 的形式安装该软件包（您可以自己开发类似的功能，但您应该基本上实现该软件包的所有功能），则可以按如下方式实现该方法：using System.Linq.Dynamic.Core;static Expression<Func<TSource, dynamic>> DynamicFields<TSource>(IEnumerable<string> fields){&nbsp; &nbsp; var source = Expression.Parameter(typeof(TSource), "o");&nbsp; &nbsp; var properties = fields&nbsp; &nbsp; &nbsp; &nbsp; .Select(f => typeof(TSource).GetProperty(f))&nbsp; &nbsp; &nbsp; &nbsp; .Select(p => new DynamicProperty(p.Name, p.PropertyType))&nbsp; &nbsp; &nbsp; &nbsp; .ToList();&nbsp; &nbsp; var resultType = DynamicClassFactory.CreateType(properties, false);&nbsp; &nbsp; var bindings = properties.Select(p => Expression.Bind(resultType.GetProperty(p.Name), Expression.Property(source, p.Name)));&nbsp; &nbsp; var result = Expression.MemberInit(Expression.New(resultType), bindings);&nbsp; &nbsp; return Expression.Lambda<Func<TSource, dynamic>>(result, source);}

Answer 2

我对如何在不使用Expressions. 很显然，因为Reflection在C＃中是缓慢，因为在此进一步解释第二条，我决定用一个叫库FastMember去获得对象的属性。==============安装 FastMember在 PackageManagerConsole 中运行以下命令来安装库。Install-Package FastMember -Version 1.4.1写函数我们的函数/方法将成为任何泛型类型的扩展方法，例如TEntity，它必须是 aclass并且将字段数组作为参数告诉函数TEntity要返回哪些属性。using FastMember;using Dynamic;using System;using System.Linq;using System.Collections.Genericpublic static class Utilities{&nbsp; &nbsp; public static dynamic GetDynamicFields<TEntity>(this TEntity entity, params string[] fields)&nbsp; &nbsp; &nbsp; &nbsp; where TEntity : class&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; dynamic dynamic = new ExpandoObject();&nbsp; &nbsp; &nbsp; &nbsp; // ExpandoObject supports IDictionary so we can extend it like this&nbsp; &nbsp; &nbsp; &nbsp; IDictionary<string,object> dictionary = dynamic as IDictionary<string,object>;&nbsp; &nbsp; &nbsp; &nbsp; TypeAccessor typeAccessor = TypeAccessor.Create(typeof(TEntity));&nbsp; &nbsp; &nbsp; &nbsp; ObjectAccessor accessor = ObjectAccessor.Create(entity);&nbsp; &nbsp; &nbsp; &nbsp; IDictionary<string,Member> members = typeAccessor.GetMembers().ToDictionary(x => x.Name);&nbsp; &nbsp; &nbsp; &nbsp; for (int i = 0; i < fields.Length; i++)&nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (members.ContainsKey(fields[i]))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; var prop = members[fields[i]];&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (dictionary.ContainsKey(prop.Name))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; dictionary[prop.Name] = accessor[prop.Name];&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; else&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; dictionary.Add(prop.Name, accessor[prop.Name]);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; return dynamic;&nbsp; &nbsp; }}如何使用使用该方法很简单，可以这样做：Message msg = new Message();dynamic result = msg.GetDynamicFields("Id","Text");在 c# 中，dynamic是ExpandoObject. 在查看代码后，ExpandoObject我意识到它实现了IDictionary<string,object>. 因此，这意味着我们可以扩展它并在运行时向它添加动态属性，从而使我们更容易。测试用例时间使用NUnit在上述方法上运行测试用例大约需要 40毫秒，这听起来足够好。希望此解决方案对您有用，或者可能为您提供有关如何解决问题的想法。