返回选择指定的列 [英] Return selected specified columns

问题描述

我想从某个（BLOB）的表只选择几个列。我有这样的字段：ID，RowVersion，大小，签名，斑点，我想只选择第4位。我这样做，是这样的：（--->是一个错误的地方）

I would like to select only few columns from a certain (Blobs) table. I have fields like: Id, RowVersion, Size, Signature, Blob, and I want to select only first four. I do it like this: (---> is an error place)

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; }     
}

当我试图返回BlobDetails错误occures - 作为VS.08没有按知道如何从匿名类型（allBlobs）转换到列表。

Error occures when I am trying to return BlobDetails - as VS.08 doesn't know how to convert from Anonymous Type (allBlobs) to List.

我不想选择所有值，因为Blob字段可以是相当沉重的，我不希望所有的时间发送。

I do not want to select all values, because Blob field can be quite heavy and I don't want to send it all the time.

你有什么想法如何正确地做呢？

Do you have any idea how to do it properly?

推荐答案

如果 BlobDetails 不是的LINQ实体，那么你可以直接做到这一点：

If BlobDetails isn't the LINQ entity, then you can do it directly:

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();

不过，如果 BlobDetails 是 LINQ的实体，你需要使用subtrefuge：

However; if BlobDetails is a LINQ entity, you need to use subtrefuge:

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();

的

AsEnumerable 打破了LINQ构图，使得它的工作。

The AsEnumerable breaks the LINQ composition, making it work.

这篇关于返回选择指定的列的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！