构建动态LINQ？

Question

我正在使用LINQ来查询数据.考虑一个用户只想报告3个字段中的1个字段的情况？(见下文)

谁能告诉我如何动态构建查询？

谢谢

DD

var query =
    from cl in db.tblClaims
    join cs in db.tblCases
        on cl.ref_no equals cs.ref_no
    where cl.claim_status == "Appeal"
        && cl.appeal_date >= Convert.ToDateTime(txtReferedFromDate.Text)
        && cl.appeal_date <= Convert.ToDateTime(txtReferedToDate.Text)
        && cs.referred_from_lho == dlLHO.Text
        && cs.adviser == dlAdviser.Text
    select new
    {
        Ref = cs.ref_no,
        ClaimType = cl.claim_type,
        ClaimStatus = cl.claim_status,
        AppealDate = cl.appeal_date
    };

gvReport.DataSource = query;

Answer 1

我们通过传入Criteria对象然后通过使用表达式树附加所需条件来构建查询.

IQueryable<Data.Story> query = ctx.DataContext.Stories;

if (criteria.StoryId != null) // StoryId
    query = query.Where(row => row.StoryId == criteria.StoryId);

if (criteria.CustomerId != null) // CustomerId
    query = query.Where(row => row.Project.CustomerId == criteria.CustomerId);

if (criteria.SortBy != null) // SortBy
    query = query.OrderBy(criteria.SortBy + " " + criteria.SortOrder.Value.ToStringForSql());

if (criteria.MaximumRecords != null) // MaximumRecords
    query = query.Take(criteria.MaximumRecords.Value);

var data = query.Select(row => StoryInfo.FetchStoryInfo(row));

您还可以访问http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-查看Dynamic Linq库. library.aspx

Answer 2

你可以这样做:

var query = from cl in db.tblClaims
                        join cs in db.tblCases
                            on cl.ref_no equals cs.ref_no
                        where cl.claim_status == "Appeal"
                        select new
                        {
                            Ref = cs.ref_no,
                            ClaimType = cl.claim_type,
                            ClaimStatus = cl.claim_status,
                            AppealDate = cl.appeal_date,
                            cs.referred_from_lho,
                            cs.adviser
                        };

if(!string.IsNullOrEmpty(txtReferedFromDate.Text) 
   && !string.IsNullOrEmpty(txtReferedToDate.Text))
  query = query.Where(cl => 
                   cl.appeal_date >= Convert.ToDateTime(txtReferedFromDate.Text) 
                && cl.appeal_date <= Convert.ToDateTime(txtReferedToDate.Text));

if(!string.IsNullOrEmpty(dlLHO.Text))
  query = query.Where(cl => cl.referred_from_lho == dlLHO.Text);

if(!string.IsNullOrEmpty(dlAdviser.Text))
  query = query.Where(cl => cl.adviser == dlAdviser.Text);

gvReport.DataSource = 
      query.Select(o => new { o.Ref, o.ClaimType, o.ClaimStatus, o.AppealDate });

如果它们具有要过滤的值,则仅将这些字段用作过滤器,否则它们将不会在查询中使用.