如何从一个列表中的数据表中返回数据类型？

Question

我有一个数据表，它给了我的列名，列的数据类型和最大字符长度。 returntype应该是一个列表，我怎样才能在一个列表中链接列名和数据类型。请帮忙。

try 
     { 
      List<string> tablelist = new List<string>(); 
      DataTable dt; 
      conn.Open(); 
      SqlCommand cmd = new SqlCommand("SELECT column_name,data_type,character_maximum_length FROM "+ dbPropertiesobj.DBName +".information_schema.columns WHERE table_name = '"+ TableName+"' ", conn); 
      SqlDataAdapter sqlda = new SqlDataAdapter(cmd); 
      dt = new DataTable(); 
      sqlda.Fill(dt); 
      for (int i = 0; i < dt.Rows.Count; i++) 
      { 
       string dtrow = dt.Rows[i].ItemArray[0].ToString(); //gets only the columnname 
       tablelist.Add(dtrow); 
      } 
      return tablelist; // need the datatype and maximum character length along with the name. 
     } 
     catch (Exception ex) 
     { 
      return null; 
     } 

Answer 1

你有很多东西与此代码回事。我完全重写了它，这不是最简单的代码，甚至也不是最好的代码，但它是为了说明需要了解更多或正在处理的领域。

研究异常处理，尤其是何时使用它以及如何使用它。切勿写入吞噬错误的尝试块。如果此方法返回null，你打算做什么？表名是不好的？连接失败了吗？用户可能会也可能不会修复这个错误，但是您会吞噬它。现在没有人知道它是什么。

更糟糕的是，甚至可能没有人知道在这里发生的错误取决于您对null做了什么。永远不要吞下这样的错误。应用程序应该在发生错误的地方失败。还要小心返回这样的空值。当您返回空值，然后执行其他操作时，应用程序可能会在某个地方出现故障，而原始错误现在很难找到并修复。编写框架时，您可能偶尔会以这种风格返回空值。对于普通的应用程序，通常不需要它，几乎也不是一个好主意。

良好质量的生产代码通常只包含很少的异常处理，因为您应该使用条件来处理任何您可以预见的事情。任何你无法预见的东西通常也无法处理。你可能有很多尝试...... finally块存在清理资源，但一个应用程序应该包含很少的实际try..catch黑色。通常，您会让错误传播回调用堆栈，最终处理程序在应用程序关闭之前通知用户。

以下仍然不是您可以编写的最佳代码。我保持原来的合理性，并删除了一些快捷方式，使其更清晰。研究差异，并从那里去

public class SomeClass 
{ 

    //Use parameters rather than accessing module level properties 
    private IList<ColumnInformation> GetColumnInformationForTable(string dbName, string tableName) 
    { 
     // Favor object oriented styles and meaningful names. Your method does not return a list of tables 
     // it returns a list of column meta data 
     List<ColumnInformation> columnInformations = new List<ColumnInformation>(); 

     // Avoid SQL conncatenation if at all possible. NEVER concatenate where parameters into SQL commands and NEVER EVER with single quotes. 
     // Here table name requires concatenation but the select parameter TableName does not. 
     string selectCmdString = "SELECT column_name,data_type,character_maximum_length FROM " + dbName + ".information_schema.columns WHERE table_name = @TableName"; 

     // Use parameters. Get everything ready first, don't open connections prematurely and only wrap error prone code in try blocks. 
     SqlCommand cmd = new SqlCommand(selectCmdString, conn); 
     SqlParameter tableNameParameter = new SqlParameter("@TableName", tableName); 
     cmd.Parameters.Add(tableNameParameter); 

     // Use a DataReader since you cannot modify this data anyway. 
     // This also shows an appropriate use of a try block to ensure a connection gets closed, 
     // but better yet, open your reader with the CommandBehavior set to close 
     // and get rid of this try block altogether 
     try 
     { 
      //Reconsider use of a module or global level connection. May be better to create a new here. 
      conn.Open(); 
      SqlDataReader reader = cmd.ExecuteReader(); 
      //Favor OOP styles rather than indexes and arrays and repeated calls to determine things like Rows.Count in a loop 
      while(reader.Read()) 
      { 
       // Favor explicit member access rather than index acess. 
       //YOUR HOMEWORK! Study DataReader access and rewrite the code below to handle possible nulls in length field. Use a method based on evaluating conditionals, DO NOT use a method based on a try block. 
       ColumnInformation columnInformation = new ColumnInformation(reader["column_name"].ToString(), reader["data_type"].ToString(), (int)reader["character_maximum_length"].ToString()); 
       columnInformations.Add(columnInformation); 
      } 
      reader.Close(); 
     } 
     finally 
     { 
      // The only reason to use the try is to make sure the connection gets closed here. A better approach 
      // is to use the CommandBehavior.CloseConnection option and get rid of the try finally block completely. 
      // But NEVER just wrap a bunch of code in try blocks arbitrarily, swallow any errors and return a null. 
      conn.Close(); 
     } 


     return columnInformations; 
    } 


} 

public class ColumnInformation 
{ 
    private string _columnName; 
    private string _dataType; 
    private int _columnLength; 

    public string ColumnName 
    { 
     get { return _columnName; } 
    } 

    public string DataType 
    { 
     get { return _dataType; } 
    } 

    public int ColumnLength 
    { 
     get { return _columnLength; } 
    } 

    public ColumnInformation(string columnName, string dataType, int columnLength) 
    { 
     _columnName = columnName; 
     _dataType = dataType; 
     _columnLength = columnLength; 
    } 
} 

Answer 2

为什么不是3元组对象列表（即带3个值的元组）？不知道申请宣告该名单的确切语法，但每个条目会是这样的：

Tuple.Create(column_name, datatype, max_length)