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Berianidze Luka
Asked:2020-12-29 01:54:14 +0000 UTC2020-12-29 01:54:14 +0000 UTC

来自字符串的比较符号

  • 772

大家好，这样的东西很有意思，请问是否可以从string中获取比较符号(> < = >= <=)然后使用呢？

public class Main
{

   List<ListItem> ls = new List<ListItem>();
   private void FilterButtonClick(...)
   {
   if(Combobox1.SelectedText == "Id")
   {
       if(Combobox2.Selectedtext == ">=")
       {
       var its = ls.Where(z=>z.Id>=Convert.ToInt32(Textbox1.Text)).ToList();
       for(int i = 0;i<its.Count;i++)
       {
       datagridview.Rows.Add(Its[i].Name);
       }
       else if(Combobox2.Selectedtext == "<=")
       {
       var its = ls.Where(z=>z.Id<=Convert.ToInt32(Textbox1.Text)).ToList();
       for(int i = 0;i<its.Count;i++)
       {
       datagridview.Rows.Add(Its[i].Name);
       }
       }
   }
   else if(Combobox1.SelectedText == "Age")
   {
   if(Combobox2.Selectedtext == ">=")
       {
       var its = ls.Where(z=>z.Age>=Convert.ToInt32(Textbox1.Text)).ToList();
       for(int i = 0;i<its.Count;i++)
       {
       datagridview.Rows.Add(Its[i].Name);
       }
       else if(Combobox2.Selectedtext == "<=")
       {
       var its = ls.Where(z=>z.Age<=Convert.ToInt32(Textbox1.Text)).ToList();
       for(int i = 0;i<its.Count;i++)
       {
       datagridview.Rows.Add(Its[i].Name);
       }
       }
   }
   }

}
public class ListItem
{
string Name;
int Id;
int Age;
int birthday_date;
int Height;
int Weight;
}

我知道用 Switch 编写代码更好。但我在网站上写了，这样更方便。

c#

2 个回答

  1. Best Answer

    例如，对于int：

    Func<int, int, bool> ConstructRelation(string s)
{
    switch (s)
    {
        case ">":  return (Func<int, int, bool>)((a, b) => a > b);
        case "<":  return (Func<int, int, bool>)((a, b) => a < b);
        case "==": return (Func<int, int, bool>)((a, b) => a == b);
        case ">=": return (Func<int, int, bool>)((a, b) => a >= b);
        case "<=": return (Func<int, int, bool>)((a, b) => a <= b);
        default: throw new ArgumentException("Unknown relation");
    }
}

    像这样使用：

    Func<int, int, bool> rel = ConstructRelation(">");
Console.WriteLine(rel(5, 4)); // выводит True

    工作代码：http: //ideone.com/UWgKlU

    你可以使用这样的东西：

    Func<ListItem, int> firstGetter = null;
switch (Combobox1.SelectedText)
{
    case "Id": firstGetter = z => z.Id; break;
    case "Age": firstGetter = z => z.Age; break;
    // тут остальные значения
    default: throw new ArgumentException();
}

int secondValue = Convert.ToInt32(Textbox1.Text);

Func<int, int, bool> rel = ConstructRelation(Combobox2.Selectedtext);

var its = ls.Where(z => rel(firstGetter(z), secondValue));

foreach(var item in its)
{
    datagridview.Rows.Add(item.Name);
}
    • 3

  2. 通过脚本 API：

    > Install-Package Microsoft.CodeAnalysis.CSharp.Scripting

    [带有一堆依赖项的包，可能需要很长时间才能下载]

    之后，您可以即时构建代码：

    using System;
using Microsoft.CodeAnalysis.CSharp.Scripting;

namespace ConsoleApplication27
{
    public class Globals
    {
        public int X;
        public int Y;
    }

    class Program
    {

        static void Main(string[] args)
        {
            var globals = new Globals { X = 1, Y = 2 };
            string op = ">";
            Console.WriteLine(CSharpScript.EvaluateAsync<bool>($"X{op}Y", globals: globals).Result);
        }
    }
}

    在实际使用中，值得通过Compile()+RunAsync而不是. 来添加缓存和强制编译EvaluateAsync。

    对于三个比较标志，这就像对麻雀的大炮。如果对象很少，那么最简单的选择是通过比较符号+反射切换，以按名称提取属性。便宜，开朗和可读。

    如果有很多对象，那么您可以尝试组装一个 lambda 以进行动态比较：

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

namespace ConsoleApplication27
{
    class ListItem
    {
        public int Id { get; set; }
        public int Age { get; set; }
        public string Name { get; set; }
    }
    class Program
    {
        static readonly Dictionary<string, ExpressionType> opsMap =
            new Dictionary<string, ExpressionType>
            {
                {  ">=", ExpressionType.GreaterThanOrEqual },
                {  "==", ExpressionType.Equal },
                {  "<=", ExpressionType.LessThanOrEqual }
            };

        static void Main(string[] args)
        {
            string operation = ">=";
            string fieldToCompare = "Age";
            string textToCompare = "15";
            List<ListItem> ls = new List<ListItem>
            {
                new ListItem { Age = 10, Name = "a" },
                new ListItem { Age = 20, Name = "b" },
            };

            var listItemType = typeof(ListItem);
            var propToCompare = listItemType.GetProperty(fieldToCompare);

            var valueToCompare = Convert.ChangeType(textToCompare, propToCompare.PropertyType);

            // собираем лямбду x => x.Age >= 15
            var param = Expression.Parameter(listItemType);
            var predicate = Expression.Lambda<Func<ListItem, bool>>(
                Expression.MakeBinary(opsMap[operation],
                    Expression.Property(param, propToCompare),
                    Expression.Constant(valueToCompare)),
                param); // .Compile и кэш при необходимости

            // убрать AsQueryable если выше используется Compile
            var result = ls.AsQueryable().Where(predicate).ToList();
        }
    }
}
    • 3

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