c# lambda 中的动态 Where 子句

Question

我有一个页面允许用户通过多个字段进行搜索。可以一个或全部使用。用户可以将每个字段的运算符设置为等于、包含、开始于等...看起来像这样

我正在使用 EntityFrame Work 并使用像这样的 lamba 检索数据:

listOfPeople = adDB.People.Where(x => x.LastName.StartsWith(lastName) && x.FirstName.StartsWith(firstName)).OrderBy(x => x.LastName)

问题是，如何根据用户提供的数据动态创建 where 子句？

Answer 1

您可以使用类似于 Func 工厂的方法来执行此操作，因为 where 子句接受 Func。

例子:

public class Program
{
    public static void Main(string[] args)
    {
        var people = new[]
        {   
            new Person {FirstName = "Hello", LastName = "World"}, 
            new Person {FirstName = "Foo", LastName = "Bar"},
        };

        Console.WriteLine(people.Where(FuncFactory.GetFilterFunc<Person>(FilterType.Contains, x => x.FirstName, "ello")).Any());
        Console.WriteLine(people.Where(FuncFactory.GetFilterFunc<Person>(FilterType.Equals, x => x.FirstName, "ello")).Any());
        Console.WriteLine(people.Where(FuncFactory.GetFilterFunc<Person>(FilterType.Contains, x => x.LastName, "ar")).Any());
        Console.WriteLine(people.Where(FuncFactory.GetFilterFunc<Person>(FilterType.Equals, x => x.LastName, "ar")).Any());

        Console.ReadKey();
    }
}

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public enum FilterType
{
    Contains,
    Equals
}

public static class FuncFactory
{
    public static Func<T, bool> GetFilterFunc<T>(FilterType filterType, Func<T, IComparable> propFunc, string filter)
    {
        switch (filterType)
        {
            case FilterType.Contains:
                return x => (propFunc(x) as string).Contains(filter);
            case FilterType.Equals:
                return x => (propFunc(x) as string).Equals(filter);
            default:
                throw new ArgumentException("Invalid FilterType");
        }
    }
}