C# 使用Visual Studio C从MS Access检索数据

好的，有点背景。我发现用C编写共享DLL是很困难的，而且这真的不值得费心，因为这只是一个学校的项目，基本上已经完成了，无论如何我还是愿意走这条路

所以我通过这个代码把数据放在MS Access中

     public void SetBal(double money)
    {
        bal = money; //balance equals whatever money that was sent to it
        string query = "Insert into Users" + "([Money])" + "Values (@Money)" + "where Users.UserID = 1";
        dbconn = new OleDbConnection(connection);
        OleDbCommand insert = new OleDbCommand(query, dbconn);
        insert.Parameters.Add("Money", OleDbType.Char).Value = bal;
        dbconn.Open();
        try
        {
            int count = insert.ExecuteNonQuery();
        }
        catch (OleDbException ex)
        {

        }
        finally
        {
            dbconn.Close();
        }

    }

好吧，那就行了。问题是当我试图从数据库检索数据时

    public double GetBal()
    {
        string query = "SELECT Users.Money FROM Users";
        bal = Convert.ToDouble(query);
        return bal;
    }

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我无法将查询结果转换为双精度。我不知道代码是否是错的，或者我只是走错了方向。提前感谢。

您需要创建/打开连接，然后执行查询。您是为insert做的，现在您需要使用select查询在get方法中创建相同的内容。您需要执行查询，然后将查询结果转换为双精度

您正在强制转换字符串SELECT Users。将用户的钱加倍，而不是查询结果。执行与更新查询相同的操作，但执行reader以获取所有值-。谢谢，我甚至不知道我怎么会没有意识到这一点。谢谢你的回复！嘿，谢谢你的回复，我非常感谢。我有个问题。在尝试之后和捕获之前，我了解一切。你能确切地解释一下发生了什么事吗。谢谢，我只是想更好地理解代码。我添加了一堆注释并简化了一点。希望这能帮助你理解它。

public double GetBal()
{
  // Make sure you change this to a real userID that you pass in.
  var query = "SELECT Users.Money FROM Users WHERE Users.UserID = 1";

  double balance = 0;

  using (var dbconn = new OleDbConnection(connectionString)) {
    var command = new OleDbCommand(query, dbconn);
    dbconn.Open();

    // Send the command (query) to the connection, creating an
    // OleDbReader in the process. We want it to close the database
    // connection in the process so we pass in that behavior as an
    // argument (CommandBehavior.CloseConnection)
    var myReader = command.ExecuteReader(CommandBehavior.CloseConnection);

    // this while loop will keep executing until there are no more rows
    // to read from the database. myReader.Read() moves to the next row
    // in the database too. The first read() puts you at the first row.
    while(myReader.Read()) 
    {
        // Use the reader's GetDouble() method to read the data and convert
        // it to a double. The 0 is there because it is the first column in
        // the results. for example to read the third column, it would be
        // myReader.GetDouble(2).
        balance = myReader.GetDouble(0));
    }
    // because there is only one row (query said where Users.UserID = 1) the
    // above loop will only execute once.

    // Close the reader so we can tell the command that the connection
    // can be closed...because CommandBehavior.CloseConnection was specified
    myReader.Close();
  }

  // return the value we got from the database
  return balance;
}