Question

我有一个3层的项目，所以所有的Linq-Querys都在DAL中。现在我在Desig-Layer中实现了一个过滤功能，并希望在那里轻松过滤，但是如何？

            Business.UserHandling uh = new Business.UserHandling();
            List<DAL.Benutzer> users = uh.GetUserOverview();
            gridUserOverview.AutoGenerateColumns = false;
            gridUserOverview.DataSource = users;
            gridUserOverview.DataBind();

我如何使用“users = users.Where（”bla = 1“）”？

之类的内容

我试过了：

                Business.UserHandling uh = new Business.UserHandling();
            List<DAL.Benutzer> users = uh.GetUserOverview();

            var filters = new Dictionary<string, object>();
            filters.Add(Request.QueryString["value"], Request.QueryString["text"]);
            users = users.Where(user => filters.All(filter => user.GetType().GetProperty(filter.Key).GetValue(user, null) == filter.Value)).ToList();

            gridUserOverview.AutoGenerateColumns = false;
            gridUserOverview.DataSource = users;
            gridUserOverview.DataBind();

但它失败了...我过滤了“UserID = 1”，我有2个用户，过滤器应该是这些过滤器中的1个。但是返回的是0个用户。

Answer 1

为了使用动态过滤器，您可以构建字典并通过反射验证过滤器：

var filters = new Dictionary<string, object>();
filters.Add("bla", 1);
users.Where(user => filters.All(filter => user.GetType().GetProperty(filter.Key).GetValue(user, null) == filter.Value));

修改支持所有可转换为字符串的类型：

var users = new List<User>();
users.Add(new User {Age = 1 });
users.Add(new User { Age = 2 });
var filters = new Dictionary<string, string>();
filters.Add("Age", "1");
var filtered = users.Where(user => filters.All(filter =>
       {
           var propertyInfo = user.GetType().GetProperty(filter.Key);
           return Equals(propertyInfo.GetValue(user, null) , Convert.ChangeType(filter.Value, propertyInfo.PropertyType));
       }));

Assert.AreEqual(1, filtered.Count());

Answer 2

您可以更改GetUserOverview以接受表达式，这样您就可以进行自定义过滤。

实施例

public List<Benutzer> GetUserOverview(Func<Benutzer, bool> filter)
{
    //get your users however you were getting them
    var _result = new List<Benutzer>();

    _result = _result.Where(filter);
    return _result;
}

那会让你这样做：

List<DAL.Benutzer> users = uh.GetUserOverview(user => user.bla == 1);

<强>更新

如果您需要动态构建过滤器，可以这样做：

Expression<Func<Benutzer, bool> finalFilter;

if (doFirstFilter)
{
    Expression<Func<Benutzer, bool> firstFilter = (user) => user.blah == 1;
}

if (doSecondFilter)
{
    Expression<Func<Benutzer, bool> secondFitler = 
        (user) => user.other == "whatever";
    finalFilter = Expression.Lambda<Func<Benutzer, bool>>(
        Expression.And(firstFilter, secondFilter));
}

var users = uh.GetUserOverview(finalFilter.Compile());

你可以使用表达式做各种疯狂的事情。

Answer 3

您是否尝试过从here

获取的有用代码

public static IQueryable<T> AddEqualityCondition<T, V>(this IQueryable<T> queryable,string propertyName, V propertyValue)
        {
            ParameterExpression pe = Expression.Parameter(typeof(T), "p");
            IQueryable<T> x = queryable.Where<T>(
              Expression.Lambda<Func<T, bool>>(
                Expression.Equal(Expression.Property(
                  pe,
                  typeof(T).GetProperty(propertyName)),
                  Expression.Constant(propertyValue, typeof(V)),
                  false,
                  typeof(T).GetMethod("op_Equality")),
              new ParameterExpression[] { pe }));

            return (x);
        }

简单的linq问题：如何过滤源？

3 个答案: