动态lambda表达式（OrderBy）和可空属性类型(Dynamic lambda expression (OrderBy) and nullable property type)

 我正在尝试动态创建表达式，通过Entity Framework对数据库中的数据进行排序。 但我遇到了一个问题，无法克服它。 也许让我解释一下我想做什么。 我的目标是创建这样的表达式： 
 
x => x.Property
 
 其中“Property”是我想动态指定的属性名称。 
 
 现在，让我们转到类，它代表数据库中的表（我简化它以使事情更清晰）： 
 
public class MyModelClass
{
    public long MyLongProperty { get; set; }
    public decimal? MyNullableDecimalProperty { get; set; } // IMPORTANT: it's nullable
}
 
 这是我的代码，我正在尝试创建前面描述的表达式： 
 
// Db is EntityFramework context
IQueryable<MyModelClass> list = Db.MyModels.Select(x => x);

// x =>
var argument = Expression.Parameter(list.ElementType, "x");

// x.MyNullableDecimalProperty
var propertyToOrder = Expression.Property(argument, "MyNullableDecimalProperty");

// x => x.MyNullableDecimalProperty
var finalExpression = Expression.Call(
    typeof (Queryable),
    "OrderBy",
    new[] { list.ElementType, typeof(IComparable) },
    list.Expression,
    Expression.Lambda<Func<MyModelClass, IComparable>>(propertyToOrder, argument));

list = list.Provider.CreateQuery<MyModelClass>(finalExpression);
 
 第4个语句出现问题（var finalExpression = Expression.Call（...））。 我得到一个例外： 
 
 
 
 “System.Nullable`1 [System.Decimal]”类型的表达式不能用于返回类型“System.IComparable”。 
 
 
 据我所知，问题在于我使用“IComparable”类型，其中“MyNullableDecimalProperty”是Nullable，而Nullable不是用户IComparable接口。 当我通过“MyLongProperty”订购或更换“IComparable”时，不会抛出异常。 
 
 所以我的问题： 
 
 
 
 我应该使用什么类型来使它适用于任何可以为空的属性？ 
 
 
 是否可以使用一种类型，它将适用于所有属性，无论它们是可空的还是不可空的。 
 
 
 注意：我知道我可以使用ex。 动态Linq库，但我对这个解决方案不感兴趣 - 我想学习如何在不使用第三方库的情况下克服它。 

I'm trying to dynamically create expression that will sort data from database through Entity Framework. But I encountered one problem and cannot overcome it. Maybe let me explain what I'm trying to do. My goal is to create expression like this:
 
x => x.Property
 
Where "Property" is the name of property which I'd like to specify dynamically.
 
Now, let's go to the class, which represents the table in database (I simplified it to make things more clear):
 
public class MyModelClass
{
    public long MyLongProperty { get; set; }
    public decimal? MyNullableDecimalProperty { get; set; } // IMPORTANT: it's nullable
}
 
It is my code, where I'm trying to create expression described earlier:
 
// Db is EntityFramework context
IQueryable<MyModelClass> list = Db.MyModels.Select(x => x);

// x =>
var argument = Expression.Parameter(list.ElementType, "x");

// x.MyNullableDecimalProperty
var propertyToOrder = Expression.Property(argument, "MyNullableDecimalProperty");

// x => x.MyNullableDecimalProperty
var finalExpression = Expression.Call(
    typeof (Queryable),
    "OrderBy",
    new[] { list.ElementType, typeof(IComparable) },
    list.Expression,
    Expression.Lambda<Func<MyModelClass, IComparable>>(propertyToOrder, argument));

list = list.Provider.CreateQuery<MyModelClass>(finalExpression);
 
Problem occurs at 4th statement (var finalExpression = Expression.Call(...)). I get an exception:
 
 
 
Expression of type „System.Nullable`1[System.Decimal]” cannot be used for return type „System.IComparable”.
 
 
As far I understand the problem is with me using "IComparable" type where "MyNullableDecimalProperty" is Nullable and Nullable doesn't user IComparable interface. The exception isn't thrown when I'm ordering by "MyLongProperty" or when I replace "IComparable".
 
So my questions:
 
 
 
What type should I use to make it work with any nullable properties?
 
 
Is it possible to use one type and it will work with all properties whether they are nullable or non-nullable.
 
 
Notice: I know I can use for ex. Dynamic Linq library, but I'm not interested in this solution - I'd like to learn how to overcome it without using 3rd party libraries.

原文：https://stackoverflow.com/questions/32098485