返回选定的指定列

Question

我想从某个(Blobs)表中只选择几列.我有以下字段:Id,RowVersion,Size,Signature,Blob,我想只选择前四个.我这样做:( --->是一个错误的地方)

public List<BlobDetails> GetAllBlobsNames()  
{  
    RichTekstModelDataContext dc = new RichTekstModelDataContext();

    var allBlobs = from b in dc.Blobs
               orderby b.RowVersion descending
               select new {b.Id, b.Size, b.Signature, b.RowVersion};

---> allBlobs.ToList<BlobDetails>();
}

public class BlobDetails   
{  
    public int Id { get; set; }  
    public string Signature { get; set; }  
    public int Size { get; set; }  
    public System.Data.Linq.Binary RowVersion { get; set; }     
}

当我尝试返回BlobDetails时发生错误 - 因为VS.08不知道如何从匿名类型(allBlobs)转换为List.

我不想选择所有值,因为Blob字段可能很重,我不想一直发送它.

你知道如何正确地做到这一点吗？

Answer 1

如果BlobDetails 不是 LINQ实体,那么您可以直接执行:

var qry = from b in dc.Blobs
          orderby b.RowVersion descending
          select new BlobDetails {
              Id = b.Id, Size = b.Size,
              Signature = b.Signature, RowVersion = b.RowVersion};

return qry.ToList();

然而; 如果BlobDetails 是 LINQ实体,则需要使用subrefuge:

var qry = from b in dc.Blobs
          orderby b.RowVersion descending
          select new {b.Id, b.Size, b.Signature, b.RowVersion};

var typedQry = from b in qry.AsEnumerable()
               select new BlobDetails {
                  Id = b.Id, Size = b.Size,
                  Signature = b.Signature, RowVersion = b.RowVersion};
return typedQry.ToList();

该AsEnumerable断LINQ组成,使其工作.

Answer 2

使用"select new {",您将创建一个无法隐式转换为BlobDetails的匿名类型.相反,在select中明确声明您正在创建的类型:

var allBlobs = from b in dc.Blobs
           orderby b.RowVersion descending
           select new BlobDetails {Id = b.Id, .... };
allBlobs.ToList();