如何使用SqlDataReader获取浮点值？ [英] How to get float value with SqlDataReader?

问题描述

在我的数据库中，我具有NextStatDistanceTime值作为浮点数。当执行 浮动时间= reader.GetFloat（0）; 行时，给出的错误为

In my database, I have NextStatDistanceTime value as a float. When "float time = reader.GetFloat(0);" line excecuted, it gives an error of

系统无效的强制转换异常

system invalid cast exception

如何在此代码中从sql命令获取浮点值？

How can I get float value from sql command in this code?

这是我的代码：

using (SqlConnection conn = new SqlConnection(@"<myconnectionstring>"))
{
    float totaltime = 0;
    for (int i = startStationIndex; i < endStationIndex; i++)
    {
        SqlCommand command = new SqlCommand("SELECT NextStatDistanceTime FROM [MetroDatabase].[dbo].[MetroStation] WHERE StationIndex = " + i + "", conn);
        try
        {
            conn.Open();
            command.ExecuteNonQuery();
            using (SqlDataReader reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    float time = reader.GetFloat(0);
                    totaltime = totaltime + time;
                    conn.Close();
                }
            }                        
        }
        catch (Exception ex)
        {
            result = ex.Message;
            Console.WriteLine(ex.Message);
        }
    }

}

推荐答案

我想是时候放一张桌子了。

It's time for a little table, I think.

T-SQL type name | .NET equivalent | C# type name | DataReader method
----------------+-----------------+--------------+------------------------
FLOAT           | System.Double   | double       | IDataReader.GetDouble()
REAL            | System.Single   | float        | IDataReader.GetFloat()

请注意， GetFloat 名称错误-应该是 GetSingle ，因为 float 是C＃特定的名称。例如，在VB.NET中这没有任何意义。

Note that GetFloat has the wrong name -- it should be GetSingle, because float is a C#-specific name. It makes no sense in VB.NET, for example.

因此，如果您的数据库列的类型为 FLOAT ，请使用 GetDouble 而不是 GetFloat 进行读取。数据读取器方法不执行转换；有一个通用的 GetValue 方法可将值作为对象获取，然后可以进一步转换。

So, if your database column is of type FLOAT, read it using GetDouble, not GetFloat. The data reader methods do not perform conversions; there is a generic GetValue method to get the value as an object that you can then convert further.

顺便说一句，这并不是唯一的妙处-.NET浮点类型支持非规范化的值，而T-SQL类型则没有，因此，.NET代码中可能会有无法成功存储在数据库中的浮点数，即使这些类型匹配。

Incidentally, this is not the only subtlety -- the .NET floating-point types support denormalized values, whereas the T-SQL types do not, so it is possible to have floating-point numbers in your .NET code that can't be successfully stored in the database, even if the types match.

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