如何将数据阅读器转换为动态查询结果

Question

我有一个视图，它通常从 WebMatrix 查询（IEnumerable<dynamic> 数据类型）中获取查询结果，并在表格中显示结果：

@model MySite.Models.Entity
@foreach(var row in Model.Data)
{
    <tr>
        @foreach (var column in row.Columns)
        {
            <td>@column<span>:</span> @row[column]</td>
        }
    </tr>
}

这是我查询数据库的模型：

public class Entity
{
    public dynamic Data {get; set; }
    public Entity(String table)
    {
        if (table == "User" || table == "Group)
        {
            WebMatrix.Data.Database db = new WebMatrix.Data.Database();
            db.Open(ConString);
            Data = db.Query("SELECT * FROM " + table);
        }
        else
        {
            using (OdbcConnection con = ne4w OdbcConnection(ConString))
            {
                OdbcCommand com = new OdbcCommand("Select * From " + table);
                command.CommandType = System.Data.CommandType.Text;
                connection.Open();
                OdbcDataReader reader = command.ExecuteReader();

以下是我通过阅读其他各种帖子尝试过的所有不同的事情：

                // Atempt 1
                Data = reader;
                // Error in view, 'Invalid attempt to call FieldCount when reader is closed' (on 'var row `in` Model.Data')

                // Atempt 2
                Data = reader.Cast<dynamic>;
                // Error: 'Cannot convert method group "Cast" to non-delegate type "dynamic". Did you intend to invoke the method?

                // Atempt 3
                Data = reader.Cast<IEnumerable<dynamic>>;
                // Error same as Atempt 2

                // Atempt 4
                Data = reader.Cast<IEnumerable<string>>;
                // Error same as Atempt 2
            }
        }
    }
}

我正在寻找让读者对象成为IEnumerable<dynamic> 对象的最佳方法。

请注意，这是一个简化的示例，虽然这两种查询类型的原因并不明显，但它们在我的代码中是必需的。

Answer 1

您缺少基本的 C# 语法。

Data = reader;
// You cant do this. You have to loop the reader to get the values from it.
// If you simply assign reader object itself as the data you wont be 
// able to get data once the reader or connection is closed. 
// The reader is typically closed in the method.

Data = reader.Cast<dynamic>;
// You should call the Cast method. And preferably execute the resulting query. 
// As of now you're merely assigning method reference to a variable
// which is not what you want. 
// Also bear in mind that, as I said before there's no real benefit in casting to dynamic

Data = reader.Cast<IEnumerable<dynamic>>;
// Cast method itself returns an IEnumerable. 
// You dont have to cast individual rows to IEnumerable

Data = reader.Cast<IEnumerable<string>>;
// Meaningless I believe. 
// The data you get from database is not always strings

你犯的主要错误是没有调用方法。这就是你想要的：

Data = reader.Cast<IDataRecord>().ToList();
                               ^^ // notice the opening and closing parentheses

---

根据更容易处理的内容（例如，在前端显示），您可以采取多种方式。

1. 返回数据记录。

   public IEnumerable<IDataRecord> SelectDataRecord()
   {
       ....
   
       using (var reader = cmd.ExecuteReader())
           foreach (IDataRecord record in reader as IEnumerable)
               yield return record; //yield return to keep the reader open
   }
   

2. 返回 ExpandoObjects。 也许这就是你想要的？

   public IEnumerable<dynamic> SelectDynamic()
   {
       ....
   
       using (var reader = cmd.ExecuteReader())
       {
           var names = Enumerable.Range(0, reader.FieldCount).Select(reader.GetName).ToList();
           foreach (IDataRecord record in reader as IEnumerable)
           {
               var expando = new ExpandoObject() as IDictionary<string, object>;
               foreach (var name in names)
                   expando[name] = record[name];
   
               yield return expando;
           }
       }
   }
   

3. 属性包的返回顺序

   public IEnumerable<Dictionary<string, object>> SelectDictionary()
   {
       ....
   
       using (var reader = cmd.ExecuteReader())
       {
           var names = Enumerable.Range(0, reader.FieldCount).Select(reader.GetName).ToList();
           foreach (IDataRecord record in reader as IEnumerable)
               yield return names.ToDictionary(n => n, n => record[n]);
       }
   }
   

4. 普通对象数组的返回序列

   public IEnumerable<List<object>> SelectObjectArray()
   {
       ....
   
       using (var reader = cmd.ExecuteReader())
       {
           var indices = Enumerable.Range(0, reader.FieldCount).ToList();
           foreach (IDataRecord record in reader as IEnumerable)
               yield return indices.Select(i => record[i]).ToList();
       }
   }
   

5. 返回数据行

   public IEnumerable<DataRow> SelectDataRow()
   {
       ....
   
       using (var reader = cmd.ExecuteReader())
       {
           var table = new DataTable();
           table.BeginLoadData();
           table.Load(reader);
           table.EndLoadData();
           return table.AsEnumerable(); // in assembly: System.Data.DataSetExtensions
       }
   }
   

6. 最后但同样重要的是，如果有帮助，您可以返回强类型序列，而无需任何手动管道。您可以使用表达式树在运行时编译代码。参见this，例如

Answer 2

尝试循环阅读器结果：

OdbcDataReader reader = command.ExecuteReader();

while(reader.Read())
{
    var item = reader["yourField"].ToString();
}

看起来您只是想将一个对象设置为 command.ExecuteReader(); 的未执行 IQueriable 结果。