如何使表达式将值类型视为引用类型？ [英] How to make expression treat value type as a reference type?

问题描述

我想存储访问对象属性的表达式集合。例如：

I wanted to store a collection of expressions accessing object's properties. For example:

class Entity
{
    public int Id { get; set; }
    public Entity Parent { get; set; }
    public string Name { get; set; }
    public DateTime Date { get; set; }        
    public decimal Value { get; set; }
    public bool Active { get; set; }
}

static void Main(string[] args)
{
    var list = new List<Expression<Func<Entity, object>>>();
    list.Add(e => e.Id);
    list.Add(e => e.Name);
    list.Add(e => e.Parent);
    list.Add(e => e.Date);
    list.Add(e => e.Value);
    list.Add(e => e.Active);

    StringBuilder b = new StringBuilder();
    list.ForEach(f => b.AppendLine(f.ToString()));

    Console.WriteLine(b.ToString());
    Console.ReadLine();
}

此代码输出：

e => Convert(e.Id)
e => e.Name
e => e.Parent
e => Convert(e.Date)
e => Convert(e.Value)
e => Convert(e.Active)

它添加转换到值类型。

至于我想用LINQ to SQL使用这些表达式，我不需要这样的在表达式中转换，以便他们成功翻译成SQL。

As far as in the end I wanted to use those expressions with LINQ to SQL, I need not to have that Convert in expressions, for them to be successfully translated to SQL.

如何实现？

PS：此集合的表达式后来被用作 OrderBy 和 ThenBy 方法的参数。 / p>

P.S.: expressions from this collection are later used as arguments to OrderBy and ThenBy methods.

推荐答案

如果您在proeprty类型中创建一个泛型函数，可以避免转换：

If you create a function generic in the proeprty type you can avoid the Convert:

private static LambdaExpression GetExpression<TProp>
                                    (Expression<Func<Entity, TProp>> expr)
{
    return expr;
}

那么您可以更改列表的类型：

then you can change the type of list:

var list = new List<LambdaExpression>();
list.Add(GetExpression(e => e.Id));
list.Add(GetExpression(e => e.Name));

这将需要您创建您的 OrderBy 和 ThenBy 表达式使用反射例如

This will require you to create your OrderBy and ThenBy expressions using reflection e.g.

LambdaExpression idExpr = list[0];
Type keyType = idExpr.ReturnType;

var orderByMethod = typeof(Queryable).GetMethods()
    .Single(m => m.Name == "OrderBy" && m.GetParameters().Length == 2)
    .MakeGenericMethod(typeof(Entity), keyType);

var ordered = (IQueryable<Entity>)
                  orderByMethod.Invoke(null, new object[] { source, idExpr });

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