从 MySQL 获取元组的值到字符串数组中

Question

我想从数据库中选择所有行和列（没有条件）并将每个元组（例如：表中 index[0,0] 的值）存储在一个字符串数组中..

这是我正在使用的代码：

using (MySql.Data.MySqlClient.MySqlConnection connection = new MySql.Data.MySqlClient.MySqlConnection("S;Port=P;Database=DB;Uid=U;Pwd=P"))                      {
       connection.Open();
       MySql.Data.MySqlClient.MySqlCommand cmd=connection.CreateCommand();
       cmd.CommandText = "SELECT * FROM table_name";
       MySql.Data.MySqlClient.MySqlDataReader datr = cmd.ExecuteReader();            
       coun = Convert.ToInt32(cmd.ExecuteScalar());
       while (datr.Read()){
            for (int i = 0; i < coun; i++)  {                          
                 First_String[i] = datr[0].ToString();
                 Second_String[i] = datr[1].ToString();
                Third_String[i] = datr[2].ToString();
                        /* and so on...*/   
             }
       }
}

我收到一条错误消息：Object not "Object reference not set to an instance of an object."

做错了什么？

谢谢

Answer 1

试试这个代码

using (MySql.Data.MySqlClient.MySqlConnection connection = new MySql.Data.MySqlClient.MySqlConnection("S;Port=P;Database=DB;Uid=U;Pwd=P"))                      {
       connection.Open();
       MySql.Data.MySqlClient.MySqlCommand cmd=connection.CreateCommand();
       cmd.CommandText = "SELECT * FROM table_name";
       MySql.Data.MySqlClient.MySqlDataReader datr = cmd.ExecuteReader();            
       coun = Convert.ToInt32(cmd.ExecuteScalar());
       int counter = 0;
       string[] First_String = new string[datr.FieldCount];
       string[] Second_String = new string[datr.FieldCount];
       string[] Third_String = new string[datr.FieldCount];
       while (datr.Read()){                          
                 First_String[counter] = datr[0].ToString();
                 Second_String[counter] = datr[1].ToString();
                Third_String[counter] = datr[2].ToString();
                        /* and so on...*/   
                counter++;
       }
}

Answer 2

如果你经常使用 datareader，你可以使用这篇文章中的这个帮助类

SqlDataReader Best way to check for null values -sqlDataReader.IsDBNull vs DBNull.Value

/// <summary>
/// Helper class for SqlDataReader, which allows for the calling code to retrieve a value in a generic fashion.
/// </summary>
public static class SqlReaderHelper
{
    private static bool IsNullableType(Type theValueType)
    {
        return (theValueType.IsGenericType && theValueType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)));
    }

    /// <summary>
    /// Returns the value, of type T, from the SqlDataReader, accounting for both generic and non-generic types.
    /// </summary>
    /// <typeparam name="T">T, type applied</typeparam>
    /// <param name="theReader">The SqlDataReader object that queried the database</param>
    /// <param name="theColumnName">The column of data to retrieve a value from</param>
    /// <returns>T, type applied; default value of type if database value is null</returns>
    public static T GetValue<T>(this SqlDataReader theReader, string theColumnName)
    {
        // Read the value out of the reader by string (column name); returns object
        object theValue = theReader[theColumnName];

        // Cast to the generic type applied to this method (i.e. int?)
        Type theValueType = typeof(T);

        // Check for null value from the database
        if (DBNull.Value != theValue)
        {
            // We have a null, do we have a nullable type for T?
            if (!IsNullableType(theValueType))
            {
                // No, this is not a nullable type so just change the value's type from object to T
                return (T)Convert.ChangeType(theValue, theValueType);
            }
            else
            {
                // Yes, this is a nullable type so change the value's type from object to the underlying type of T
                NullableConverter theNullableConverter = new NullableConverter(theValueType);

                return (T)Convert.ChangeType(theValue, theNullableConverter.UnderlyingType);
            }
        }

        // The value was null in the database, so return the default value for T; this will vary based on what T is (i.e. int has a default of 0)
        return default(T);
    }
}

用法：

yourSqlReaderObject.GetValue<int?>("SOME_ID_COLUMN");
yourSqlReaderObject.GetValue<string>("SOME_VALUE_COLUMN");

Answer 3

添加这个方法

public static class DataReaderExtensions
{

    public static string SafeGetString(this SqlDataReader reader, int colIndex)
    {
       if(!reader.IsDBNull(colIndex))
           return reader.GetString(colIndex);
       else 
           return string.Empty;
    }
}

把你的代码改成这个

for (int i = 0; i < coun; i++)
{
 First_String[i] = datr.SafeGetString(0);
 Second_String[i] = datr.SafeGetString(1);
 Third_String[i] = datr.SafeGetString(2);
}