从字符串列名称动态创建LINQ Select表达式 [英] Creating a LINQ Select expression dynamically from string column names

问题描述

我有一个看起来像这样的方法:

I have a method which looks something like this:

public string GetColVal(string aAcctField, bool callM1, bool callM2)
{
    if (callM1)
    {
        // create select clause here to select a string column with name
        // equal to the value of aAcctField from Table1
        // Expression<Func<Table1, string>> selector = ?
        return M1(selector);
    }

    if (callM2)
    {
        // create select clause here to select a string column with name
        // equal to the value of aAcctField from Table2
        // Expression<Func<Table2, string>> selector = ?
        return M2(selector);
    }
}

M1()是这样的:

public string M1(Expression<Func<Table1, string>> columnToSelect, string xType)
{
    // other logic 
    string acct = _cntx.Where(c => c.Type == xType)
                       .Select(columnToSelect)
                       .FirstOrDefault();
    return acct;
}

M2()也类似.请注意，方法过于简单.方法M1()和M2()可以完美地工作.我可以这样调用它们:

M2() is also something similar. Please note that the methods are over-simplified. And the methods M1() and M2() work perfectly. I can invoke them this way:

// MyColumn is a strongly typed column in Table1
string acct = M1(x => x.MyColumn, "some value");

---

但是，在方法GetColVal()中如何构造选择子句?关于selector的评论将帮助您了解我打算做什么.所以，请继续阅读评论.

---

But, inside the method GetColVal() how do I construct the select clauses? The comments regarding selector will help you understand what I intend to do. So, please go ahead and read the comment.

我已经尝试过了:

public string GetColVal(string aAcctField, bool callM1, bool callM2)
{
    if (callM1)
    {
        // create select clause here to select a string column with name
        // equal to the value of aAcctField from Table1
        Expression<Func<Table1, string>> selector = (w) => w.GetType().GetProperty(aAcctField).Name;
        return M1(selector);
    }
    ...
}

我得到一个例外:

LINQ to Entities无法识别该方法 'System.Reflection.PropertyInfo GetProperty(System.String)'方法， 而且该方法无法转换为商店表达式

LINQ to Entities does not recognize the method 'System.Reflection.PropertyInfo GetProperty(System.String)' method, and this method cannot be translated into a store expression

我看过这些:

• 为内部具有FirstOrDefault的Select创建动态LINQ表达式
• Linq访问属性(按变量)
• 使用C#中的反射从字符串获取属性值
• 和许多其他人.

• Create dynamic LINQ expression for Select with FirstOrDefault inside
• Linq access property by variable
• Get property value from string using reflection in C#
• and many others.

但是它们都不是我所需要的.

but none of them is quite something like what I need.

推荐答案

基本上，您需要使用 Expression.Lambda ，

Basically you need to use the Expression class methods like Expression.Lambda, Expression.PropertyOrField etc. to build the desired selector like:

static Expression<Func<T, TValue>> MemberSelector<T, TValue>(string name)
{
    var parameter = Expression.Parameter(typeof(T), "item");
    var body = Expression.PropertyOrField(parameter, name);
    return Expression.Lambda<Func<T, TValue>>(body, parameter);
}

要支持嵌套属性，请将var body = ...行更改为

To support nested properties, change var body = ... line to

var body = name.Split('.').Aggregate((Expression)parameter, Expression.PropertyOrField);

样品用量:

if (callM1)
{
    return M1(MemberSelector<Table1, string>(aAcctField));
}
...

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