Conditional where with or criteria linqtosql

Question

我已经了解了如何使用linq to sql进行条件查询，还了解了如何使用OR where子句。不幸的是，我想不出如何同时做这两件事。我可以做一个条件where子句，如下所示：

var ResultsFromProfiles = from AllPeeps in SearchDC.aspnet_Users
                          select AllPeeps;


if (SearchFirstNameBox.Checked)
{
    ResultsFromProfiles = ResultsFromProfiles.Where(p => p.tblUserProfile.FirstName.Contains(SearchTerm));
}

if (SearchLastNameBox.Checked)
{
    ResultsFromProfiles = ResultsFromProfiles.Where(p => p.tblUserProfile.LastName.Contains(SearchTerm));
}

这将使我得到名字和姓氏包含搜索词的所有配置文件。

或者我可以这样做：

var ResultsFromProfiles = from p in SearchDC.aspnet_Users
                          where p.tblUserProfile.LastName.Contains(SearchTerm) ||
                                p.tblUserProfile.FirstName.Contains(SearchTerm)
                          select p;

这将使我获得名字或姓氏包含搜索词的任何配置文件。

我有一堆复选框，用户可以在其中指定他们想要搜索搜索词的字段，所以我希望能够构建一个查询，该查询将像上面的第一个代码片段中那样有条件地添加它们，但将它们作为OR添加，以便它们像第二个代码片段一样工作。这样，它将在指定字段中的任何位置搜索任何匹配项。

有什么建议吗？

Answer 1

是的，使用PredicateBuilder。它允许您使用And或or语义构建动态查询。它是免费和稳定的--我到处都在用它。

Answer 2

为此，一种方法是操作查询的LINQ表达式树。在本例中，您需要构建一个lambda表达式，并将其替换为对Where的调用。下面是一个基于列表的工作示例，但是操作表达式树的代码是相同的，而与查询提供程序无关。

List<User> Users = new List<User>();
Users.Add(new User() { FirstName = "John", LastName = "Smith" });
Users.Add(new User() { FirstName = "Jane", LastName = "Smith" });


string Query = "John";

var Queryable = Users.AsQueryable();

var Results = (from u in Queryable
               select u);

//initial method call... the lambda u => false is a place-holder that is about to be replaced
MethodCallExpression WhereExpression = (MethodCallExpression)Results.Where(u => false).Expression;

//define search options
Expression<Func<User, string, bool>> FilterLastName = (u, query) => u.LastName.Contains(query);
Expression<Func<User, string, bool>> FilterFirstName = (u, query) => u.FirstName.Contains(query);

//build a lambda based on selected search options... tie the u parameter to UserParameter and the query parameter to our Query constant
ParameterExpression UserParameter = Expression.Parameter(typeof(User), "u");
Expression Predicate = Expression.Constant(false);  //for simplicity, since we're or-ing, we'll start off with false || ...

//if (condition for filtering by last name)
{
    Predicate = Expression.Or(Predicate, Expression.Invoke(FilterLastName, UserParameter, Expression.Constant(Query)));
}

//if (condition for filtering by first name)
{
    Predicate = Expression.Or(Predicate, Expression.Invoke(FilterFirstName, UserParameter, Expression.Constant(Query)));
}

//final method call... lambda u => false is the second parameter, and is replaced with a new lambda based on the predicate we just constructed
WhereExpression = Expression.Call(WhereExpression.Object, WhereExpression.Method, WhereExpression.Arguments[0], Expression.Lambda(Predicate, UserParameter));

//get a new IQueryable for our new expression
Results = Results.Provider.CreateQuery<User>(WhereExpression);

//enumerate results as usual
foreach (User u in Results)
{
    Console.WriteLine("{0} {1}", u.FirstName, u.LastName);
}

Working with expression trees通常可以通过使用访问者模式来简化，但我省略了这一点，以便您可以更清楚地看到必须完成的工作。

Answer 3

这里有一个建议。

我还没有尝试过编译和运行它，而且LinqToSQL充满了惊喜，所以不能保证:-)

var ResultsFromProfiles = from AllPeeps in SearchDC.aspnet_Users select AllPeeps;

IEnumerable<AspNet_User> total = new AspNew_User[0];

if (SearchFirstNameBox.Checked)
{    
    total = total.Concat(ResultsFromProfiles.Where(p => p.tblUserProfile.FirstName.Contains(SearchTerm));}
}

if (SearchLastNameBox.Checked)
{
   total = total.Concat(ResultsFromProfiles.Where(p => p.tblUserProfile.LastName.Contains(SearchTerm));
}

total = total.Distinct();