代码之家 › 专栏 › 技术社区 › David Robbins

如何生成一个表达式树来查询具有List<T>属性的对象?

expression-trees linq-to-objects c#

1

David Robbins · 技术社区 · 16 年前

请原谅我笨拙的解释,但我有一个类包含一个列表:

public class Document
    {
        public int OwnerId { get; set; }
        public List<User> Users { get; set; }

        public Document() { }
    }

    public class User
    {
        public string UserName { get; set; }
        public string Department { get; set; }
    }

目前我使用PredicateBuilder对我的对象执行动态查询。如何将以下LINQ语句转换为表达式树:

var predicate= PredicateBuilder.True<User>();
predicate= predicate.And<User>(user => user.Deparment == "HR"); 

var deptDocs = documents.AsQueryable()
          .Where(doc => doc.Users
          .AsQueryable().Count(predicate) > 0)
          .ToList();

var deptDocs = documents.HasUserAttributes("Department", "HR").ToList(); . 据我所知,我的问题是无法评估 user => user.Deparment == "HR" 如果我要发出一个Expression.Invoke。

我想我在这方面取得了一些进展:

var predicate = PredicateBuilder.True<User>();
predicate = predicate.And<User>(user => user.Department == "FIN");

Expression<Func<Document, bool>> userSelector =
            doc => doc.Users
            .AsQueryable()
            .Any(predicate);

var docParm = Expression.Parameter(typeof(Document), "appDoc");
var body = Expression.Invoke(userSelector, docParm);
var lambda = Expression.Lambda<Func<Document, bool>>(body, docParm);

var allPredicate = PredicateBuilder.True<Document>();            
allPredicate = allPredicate.And<Document>(doc => doc.OwnerId == 1);
allPredicate = allPredicate.And<Document>(lambda);

var hrDocs = this.docs.AsQueryable().Where(allPredicate).ToList();

有没有更有效的方法?如何生成创建谓词的表达式-user=>用户。部门-有泛型?

2 回复 | 直到 16 年前

1

0

Craig Stuntz 16 年前

听起来你想 this toolkit ?

2

0

David Robbins 16 年前

var predicate = PredicateBuilder.True<Document>();
predicate = predicate.And<Document>(User.SubQuery("UserName", "DAVER"));
predicate = predicate.And<Document>(AdHoc<Document>("OwnerId", 1));

var finDocs = docs.AsQueryable().Where(predicate).ToList();

我有一个使用此方法的扩展类:

public static Expression<Func<T, bool>> AdHoc<T>
            (string columnName, object compValue)
{
    //  Determine type of parameter
    ParameterExpression parameter = Expression.Parameter(typeof(T), "x");
    //  Target to compare to
    Expression property = Expression.Property(parameter, columnName);
    //  The value to match
    Expression constant = Expression.Constant(compValue, compValue.GetType());

    Expression equality = Expression.Equal(property, constant);
    Expression<Func<T, bool>> predicate =
    Expression.Lambda<Func<T, bool>>(equality, parameter);

    return predicate;
}

在我的用户类中,我有一个静态方法:

public static Expression<Func<Document, bool>> SubQuery(string property, 
                                                         string targetValue)
        {
            var predicate = PredicateBuilder.True<User>();
            predicate = predicate.And<User>(Extensions.AdHoc<User>(property, targetValue));

            Expression<Func<Document, bool>> userSelector =
                                    doc => doc.Users
                                        .AsQueryable()
                                        .Any(predicate);

            var docParm = Expression.Parameter(typeof(Document), "appDoc");
            var body = Expression.Invoke(userSelector, docParm);

            var docPredicate = PredicateBuilder.True<Document>();
            docPredicate = docPredicate.And<Document>(Expression.Lambda<Func<Document, bool>>(body, docParm));

            return docPredicate;
        }

缺点是我在用户类本身中包含了子查询功能。它完成了任务,但如果有人有任何建议或更好的方法来使用泛型,这样我就不必在我的用户类中包含这个静态方法,我很想听听你的意见。