System.LINQ.Dynamic:选择("new(...)")到List< T>中. (或< T>的任何其他可枚举的集合) [英] System.LINQ.Dynamic: Select(" new (...)") into a List<T> (or any other enumerable collection of <T>)

问题描述

说我有一个包含四个列的数据表，公司(字符串)，基金(字符串)，州(字符串)，值(双精度):

Say I have a DataTable with four columns, Company (string), Fund (string), State (string), Value(double):

    table1.Rows.Add("Company 1","Fund 1","NY",100));
    table1.Rows.Add("Company 2","Fund 1","CA",200));
    table1.Rows.Add("Company 3","Fund 1","FL",300));
    table1.Rows.Add("Company 4","Fund 2","CA",400));
    table1.Rows.Add("Company 5","Fund 1","NY",500));
    table1.Rows.Add("Company 6","Fund 2","CA",600));
    table1.Rows.Add("Company 7","Fund 3","FL",700));

我想使用System.LINQ.Dynamic构建一个动态查询，该查询按公司，基金或州进行分组，然后按条件选择我的组作为第一列，并求和(值):

I want to use System.LINQ.Dynamic to build a dynamic query which groups on either Company, Fund, or State, and then selects my group by criteria as the first column, and sum(value):

string groupbyvalue="Fund";
var q1= table1.AsEnumerable().AsQueryable()
              .GroupBy(groupbyvalue,"it")
              .Select("new ("+groupbyvalue+" as Group, Sum(Value) as TotalValue)");

在上面的查询中，所选的groupbyvalue(组)将始终为字符串，并且总和始终为双精度，因此我希望能够转换为类似List的内容，其中Result是具有属性的对象组(字符串)和TotalValue(双精度).

In the above query, the selected groupbyvalue (Group) will always be a string, and the sum will always be a double, so I want to be able to cast into something like a List, where Result is an object with properties Group (string) and TotalValue (double).

对此我感到很麻烦，有人可以阐明吗?

I'm having a lot of trouble with this, can anyone shed some light?

推荐答案

首先，您将在Select子句中以Key的形式访问当前分组的值:

First, you'll access the current grouped value as Key in your Select clause:

.Select("new (Key as Group, Sum(Value) as TotalValue)");

这应该可以使您的查询正常工作.更加棘手的问题是如何将返回的对象(具有从DynamicClass继承的动态生成的类型)转换为静态类型.

That should make your query work. The harder question is how to turn the returned objects, which will have a dynamically generated type that inherits from DynamicClass, into a static type.

选项1:使用反射访问动态对象的Group和TotalValue属性.

Option 1: Use reflection to access the dynamic object's Group and TotalValue properties.

选项2:使用编译的表达式树进行轻量级代码生成以访问Group和TotalValue属性.

Option 2: Use compiled expression trees for lightweight code generation to access the Group and TotalValue properties.

选项3:修改动态库以支持强类型的结果.事实证明这很简单:

Option 3: Modify the Dynamic library to support a strongly-typed result. This turns out to be rather simple:

1. 在ExpressionParser.Parse()中，在专用字段中捕获type参数:

1. In ExpressionParser.Parse(), capture the type argument in a private field:

private Type newResultType;
public Expression Parse(Type resultType)
{
    newResultType = resultType;
    int exprPos = token.pos;
    // ...

在ExpressionParser.ParseNew()末尾附近，我们将尝试使用newResultType，然后将其默认为动态类型:

Near the end of ExpressionParser.ParseNew(), we'll try to use newResultType before defaulting to a dynamic type:

Expression ParseNew()
{
    // ...
    NextToken();
    Type type = newResultType ?? DynamicExpression.CreateClass(properties);
    MemberBinding[] bindings = new MemberBinding[properties.Count];
    for (int i = 0; i < bindings.Length; i++)
        bindings[i] = Expression.Bind(type.GetProperty(properties[i].Name), expressions[i]);
    return Expression.MemberInit(Expression.New(type), bindings);
}

最后，我们需要一个Select()的强类型版本:

Finally, we need a strongly typed version of Select():

public static IQueryable<TResult> Select<TResult>(this IQueryable source, string selector, params object[] values)
{
    if (source == null) throw new ArgumentNullException("source");
    if (selector == null) throw new ArgumentNullException("selector");
    LambdaExpression lambda = DynamicExpression.ParseLambda(source.ElementType, typeof(TResult), selector, values);
    return source.Provider.CreateQuery<TResult>(
        Expression.Call(
            typeof(Queryable), "Select",
            new Type[] { source.ElementType, typeof(TResult) },
            source.Expression, Expression.Quote(lambda)));
}

与原始Select()相比，唯一的变化是我们引用的TResult.

The only changes from the original Select() are places we reference TResult.

现在我们只需要一个命名类型即可返回:

Now we just need a named type to return:

    public class Result
    {
        public string Group { get; set; }
        public double TotalValue { get; set; }
    }

更新后的查询将如下所示:

And your updated query will look like this:

    IQueryable<Result> res = table1.AsQueryable()
        .GroupBy(groupbyvalue, "it")
        .Select<Result>("new (Key as Group, Sum(Value) as TotalValue)");

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