如果没有首先将 lambda 表达式转换为委托或表达式树类型，则不能将 lambda 表达式用作动态调度操作的参数 [英] Cannot use a lambda expression as an argument to a dynamically dispatched operation without first casting it to a delegate or expression tree type

问题描述

我正在使用 .NET4.5 和 VS2013，我有一个从数据库获取 dynamic 结果的查询.

I am working with .NET4.5 and VS2013, I have this query that gets dynamic result from db.

dynamic topAgents = this._dataContext.Sql(
    "select t.create_user_id as "User", sum(t.netamount) as "Amount" from transactiondetail t where t.update_date > sysdate -7 group by t.create_user_id")
    .QueryMany<dynamic>();

以下语句因编译错误而失败 不能使用 lambda 表达式作为动态调度操作的参数，而无需先将其转换为委托或表达式树类型甚至不允许我运行它

Following statement fails with compilation error Cannot use a lambda expression as an argument to a dynamically dispatched operation without first casting it to a delegate or expression tree type without even allowing me to run it

topAgents.ToList().Select(agent => new
{
    User = agent.User != null ? string.Format("{0}", agent.User).Replace("CORPNTGB\", "") : null,
    Amount = agent.Amount
});

虽然这个带有 foreach 的效果很好.

while this one with foreach works just fine.

var data = new List<List<object>>();
foreach (dynamic agent in topAgents)
{
    data.Add(new List<object>
    {
        agent.User != null ? string.Format("{0}", agent.User).Replace("CORPNTGB\", "") : null,
        agent.Amount
    });
}

在我 topAgents.ToList() 之后的我眼中，它们可以被解释为等效的，是不是因为我明确声明 var data = new List>(); 编译器允许第二条语句吗?

In my eyes after I topAgents.ToList() they could be interpreted as equivalent, is it because I explicitly state that var data = new List<List<object>>(); that second statement is allowed by compiler?

为什么编译器不允许 LINQ select，但允许每个`?

推荐答案

问题在于 topAgents 是 dynamic - 所以你的 ToList() 调用是动态的，Select 也是如此.这有以下问题:

The problem is that topAgents is dynamic - so your ToList() call is dynamic, and so is Select. That has issues that:

1. 不能将 lambda 表达式用于这样的动态调用；
2. 动态调用无论如何都找不到扩展方法.

幸运的是，操作不需要是动态的，只是因为 element 类型是动态的.你可以使用:

Fortunately, the operations don't need to be dynamic just because the element type is dynamic. You could use:

IEnumerable<dynamic> topAgents = ...;

... 或者只使用 var.这两个都应该没问题.

... or just use var. Both of those should be fine.

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