Question

我有List并且希望能够定义Expression的列表，该列表选择在此Expression列表中定义的新对象。

但我不知道，如何将表达式列表传递给data.Select（）。像这样：data.Select（SelectExpressionList）

public class Test
{
    public void Process()
    {
        var foo = new Foo<User>();

        foo.data.Add(new User() { id = 1, LastName = "User1", Other = "A" });
        foo.data.Add(new User() { id = 2, LastName = "User2", Other = "B" });
        foo.data.Add(new User() { id = 3, LastName = "User3", Other = "C" });

        foo.AddExpression(u => u.id);
        foo.AddExpression(u => u.LastName);

        var result = foo.CreateNewByExpression(); // I want List<> object {id, LastName}

    }

}

public class User
{
    public int id { get; set; }
    public string LastName { get; set; }

    public string Other { get; set; }


}
public class Foo<T>
{

    public List<T> data = new List<T>();

    public List<Expression> SelectExpressionsList = new List<Expression>();
    public List<object> CreateNewByExpression()
    {
        return data.Select(....); // I want List<> object defined by SelectExpressions
    }

    public void AddExpression<TKey>(Expression<Func<T, TKey>> exp)
    {
        SelectExpressionsList.Add(exp);
    }


}

希望，它清楚，我想要实现的目标。感谢。

修改它不必是匿名类型。我想要只有“选定”属性的列表...

Answer 1

您可以使用ExpandoObject。查看我的working example。

using System;
using System.Linq;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Dynamic;

public class Program
{
    public static void Main()
    {       
        var foo = new Foo<User>();

        foo.data.Add(new User() { id = 1, LastName = "User1", Other = "A" });
        foo.data.Add(new User() { id = 2, LastName = "User2", Other = "B" });
        foo.data.Add(new User() { id = 3, LastName = "User3", Other = "C" });

        foo.AddExpression(u => u.id);
        foo.AddExpression(u => u.LastName);

        var result = foo.CreateNewByExpression(); 
        Console.WriteLine(result.First().LastName);
    }
}

public class User
{
    public int id { get; set; }

    public string LastName { get; set; }

    public string Other { get; set; }

}

public class Foo<T>
{
    public List<T> data = new List<T>();

    public List<Expression<Func<T, object>>> SelectExpressionsList = new List<Expression<Func<T, object>>>();

    public IEnumerable<dynamic> CreateNewByExpression()
    {
        foreach (var item in data)
        {
            IDictionary<string, object> result = new ExpandoObject();
            foreach (var exp in SelectExpressionsList)
            {
                string name = GetMember(exp.Body).Member.Name;
                result[name] = exp.Compile()(item);
            }

            yield return result as ExpandoObject;
        }
    }

    public void AddExpression(Expression<Func<T, object>> exp)
    {
        SelectExpressionsList.Add(exp);
    }

    public static MemberExpression GetMember(Expression expression)
    {
        MemberExpression exp = null;
        switch (expression.NodeType)
        {
            case ExpressionType.MemberAccess:
                exp = expression as MemberExpression;
                break;
            case ExpressionType.Convert:
                exp = ((UnaryExpression)expression).Operand as MemberExpression;
                break;
            default:
                break;
        }

        return exp;
    }
}

如何从List＆lt;＆gt;创建新对象通过Expression array

1 个答案: