使用lambda防爆pression来选择字段名不同领域 [英] Using Lambda Expression to Select different fields from field names

问题描述

我需要从数据库表中的两个字段（被检索使用LINQ到SQL），一个场是一个日期时间（并且是一个固定的场），另一种是始终小数，但该字段可以不同。

I need to get two fields from a database table (retrieved using linq-to-sql), one field is a datetime (and is a fixed field) and the other is always a decimal, but the field can be different.

该表保存被处理，每天两次，并在不同的货币，以便能有字段，如AM_USD，PM_USD，AM_EUR等货币数据，我需要获取数据，如对PM_USD日期或对日期的列表AM_EUR。

The table holds currency data which is processed twice a day and in different currencies so could have fields such as AM_USD, PM_USD, AM_EUR etc. And I need to get data such as a list of the date against PM_USD or the date against AM_EUR.

我希望能够调用例如使用一个lambda前pression（这是一个剥离出来的例子）中的数据：

I would like to be able to call the data using a lambda expression for example (this is a stripped out example):

data = TableData.Select(x=>new {x.DateTimeAdded, x.[**field name as string**]});

我一直在试图写一个函数来做到这一点，和我惨淡经营失败。

I have been trying to write a function to do this, and am failing dismally.

我设法最接近的是：

private Func<TableData, KeyValuePair<DateTime, decimal>> CreateSelect(string FieldName)
{
    var parameterExp = Expression.Parameter(typeof(TableData), "sel");
    var dateParameter = Expression.Parameter(typeof(DateTime), "DateTimeAdded");
    var fieldParameter = Expression.Parameter(typeof(decimal), FieldName);
    ConstructorInfo constructorInfo = typeof(KeyValuePair<DateTime, decimal>).GetConstructor(new[] { typeof(DateTime), typeof(decimal) });
    NewExpression constructExpression = Expression.New(constructorInfo, new ParameterExpression[] { dateParameter, fieldParameter});

    var lambda = Expression.Lambda<Func<TableData, KeyValuePair<DateTime, decimal>>>( constructExpression, parameterExp);

    return lambda.Compile();
}

这失败System.InvalidOperationException：LAMBDA参数不在范围内。

Which fails with "System.InvalidOperationException: Lambda Parameter not in scope".

我敢肯定，我缺少明显的东西，或绕了错误的方式。

I'm sure I missing something obvious, or going about it wrong way.

任何想法？

谢谢
ŧ

推荐答案

x.Foo 是成员的的X （属性或字段），而不是一个参数：

x.Foo is a member of x (property or field), not a parameter:

private Func<TableData, KeyValuePair<DateTime, decimal>> CreateSelect(string FieldName)
{
    var parameterExp = Expression.Parameter(typeof(TableData), "sel");
    var dateProp = Expression.PropertyOrField(parameterExp, "DateTimeAdded"); 
    var fieldProp = Expression.PropertyOrField(parameterExp, FieldName);
    ConstructorInfo constructorInfo = typeof(KeyValuePair<DateTime, decimal>).GetConstructor(new[] { typeof(DateTime), typeof(decimal) });
    NewExpression constructExpression = Expression.New(constructorInfo, new [] { dateProp, fieldProp});

    var lambda = Expression.Lambda<Func<TableData, KeyValuePair<DateTime, decimal>>>( constructExpression, parameterExp);

    return lambda.Compile();
}

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