如何投放前pression＆LT;＆Func键LT; T，日期时间＆GT;＆GT;到防爆pression＆LT;＆Func键LT; T，对象＆gt;＆GT; [英] How to cast Expression<Func<T, DateTime>> to Expression<Func<T, object>>

问题描述

我一直在寻找，但我无法找到如何从类型转换

I've been searching but I can't find how to cast from the type

Expression<Func<T, DateTime>>

要的类型：

Expression<Func<T, object>>

所以，我必须再次到如此巨大的知识转动;）

So I must turn again to the SO vast knowledge ;)

推荐答案

您不能只是投他们之间，因为他们是不是同一种东西。但是，可以有效地恩pression树中添加一个转换：

You can't just cast between them, as they're not the same kind of thing. However, you can effectively add a conversion within the expression tree:

using System;
using System.Linq.Expressions;

class Test
{
    // This is the method you want, I think
    static Expression<Func<TInput,object>> AddBox<TInput, TOutput>
        (Expression<Func<TInput, TOutput>> expression)
    {
        // Add the boxing operation, but get a weakly typed expression
        Expression converted = Expression.Convert
             (expression.Body, typeof(object));
        // Use Expression.Lambda to get back to strong typing
        return Expression.Lambda<Func<TInput,object>>
             (converted, expression.Parameters);
    }

    // Just a simple demo
    static void Main()
    {
        Expression<Func<string, DateTime>> x = text => DateTime.Now;
        var y = AddBox(x);        
        object dt = y.Compile()("hi");
        Console.WriteLine(dt);
    }        
}

这篇关于如何投放前pression＆LT;＆Func键LT; T，日期时间＆GT;＆GT;到防爆pression＆LT;＆Func键LT; T，对象＆gt;＆GT;的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！