c＃lambda表达式如果条件在where里面

Question

我有一个场景，我使用lambda表达式按名称，地址和电话号码过滤数据。 但是当一个属性没有值时，我的过滤器会考虑属性为null的位置，并给出错误的结果。 如果属性为null，是否有任何选项可以使用if条件从三种情况中删除where条件。 我的意思是有任何情况下添加where / check仅用于具有值的属性的对象吗？

var filterCriteria ={
name: "abc",
address:"",
phone:""
}

var Details = _context.LogEntities
                    .Where(p => p.name == (filterCriteria.name))
                    .Where(p => p.address== (filterCriteria.address))
                    .Where(p => p.phone== (filterCriteria.phone))

                return Json(Details);

Answer 1

如果您仍希望在名称，地址或电话为空时返回记录，则只需在条件中检查是否为空。

var Details = _context.LogEntities
                    .Where(p => p.name == null || p.name == (filterCriteria.name))
                    .Where(p => p.address == null || p.address == (filterCriteria.address))
                    .Where(p => p.phone == null || p.phone == (filterCriteria.phone))

                return Json(Details);

Answer 2

您可以编写一个扩展方法来有条件地应用过滤器：

public static IQueryable<T> WhereIf<T>(
   this IQueryable<T> source, bool condition, 
   Expression<Func<T, bool>> predicate)
{
    if (condition)
        return source.Where(predicate);
    else
        return source;
}

您的查询变为：

using static System.String;

...

var employees = _context
   .LogEntities
   .WhereIf(!IsNullOrEmpty(filterCriteria.name), e => e.name == filterCriteria.name)
   .WhereIf(!IsNullOrEmpty(filterCriteria.address), e => e.address == filterCriteria.address)
   .WhereIf(!IsNullOrEmpty(filterCriteria.phone), e => e.phone == filterCriteria.phone)
   .ToList();

Answer 3

在进行过滤时，如果您想要的是在过滤器中考虑null属性以匹配集合中相应属性中的any值，则应该针对过滤器属性检查null，而不是对集合项的属性进行检查。 所以对于像这样的集合

var myCollection = [
{name: "abc",address:"qwerty",phone:"123"},
{name: "abc",address:"xyz",phone:"456"},
{name: "efg",address:"cde",phone:"654"}]

和过滤器一样

var filterCriteria ={name: "abc",address:"",phone:""}

期望的结果如：

[{name: "abc",address:"qwerty",phone:"123"},
{name: "abc",address:"xyz",phone:"456"}]

你的代码应该是

var Details = _context.LogEntities
                .Where(p => (filterCriteria.name) == null || p.name == (filterCriteria.name))
                .Where(p => (filterCriteria.address) == null || p.address== (filterCriteria.address))
                .Where(p => (filterCriteria.phone) == null || p.phone== (filterCriteria.phone))