entity-framework - 实体框架：条件过滤器

Question

假设我有客户表，我想通过以下方式对其进行过滤：

• 国家: 所有, 美国, 英国, 加拿大
• 收入：全部、低、高、中
• 年龄：所有，青少年，成人，高级

如果我必须为这个过滤器构建一个 SQL 字符串，它会是这样的：

if (Country != "All") sql += "country = " + Country
if (Income != "All") sql += "and income = " + Income
if (Age != "All") sql += "and age = " + Age;

因此，基本上，用户可以按一些但不是必须的所有字段进行过滤。

你如何使用实体框架来做到这一点？

谢谢 ！

Answer 1

LINQ to Entity queries return IQueryable's, so you can build your query this way:

IQueryable<Person> query = context.People;

if (Country != "All")
{
    query = query.Where(p => p.Country == Country);
}

if (Income != "All")
{
    query = query.Where(p => p.Income == Income);
}

if (Age != "All")
{
    query = query.Where(p => p.Age == Age);
}

List<Person> fetchedPeople = query.ToList();

This case is almost too simple, but this is very helpful in more complex situations when you need to add filtering dynamically.

Answer 2

您可以通过这种方式包含条件参数：

return Customers.Where(
                customer =>
                customer.Name == Name &&
                (Age == "All" || customer.Age == Age) &&
                (Income == "All" || customer.Income == Income) &&
                (Country == "All" || customer.Country == Country)
                ).ToList();

如果某个条件为真（例如 country 等于All），则所有参数条件都为真，并且该参数不过滤结果。

Answer 3

您可以使用扩展方法来帮助代码的可读性和可维护性。

• LinqExtensions.cs

public static class LinqExtensions
{
    public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool condition, Expression<Func<TSource, bool>> predicate)
    {
        return condition ? source.Where(predicate) : source;
    }
}

• 重构代码

List<Person> peoples = context.People
    .WhereIf(Country != "All", p => p.Country == Country)
    .WhereIf(Income != "All", p => p.Income == Income)
    .WhereIf(Age != "All", p => p.Age == Age)
    .ToList();