c# - 从 SqlDataReader 填充通用列表

Question

如何将 SqlDataReader 返回的值添加到通用列表中？我有一种方法，使用 SqlDataReader 从 Category 表中获取 CategoryID 。我想将所有 CategoryID 添加到通用列表中。

这不起作用，因为它只返回一个 categoryID 并且这是最后一个。我想将所有 categoryID 添加到列表中，然后返回它们。

我该怎么做？

SqlConnection connection = null;
SqlDataReader reader = null;
SqlCommand cmd = null;

try
{
    connection = new SqlConnection(connectionString);
    cmd = new SqlCommand("select CategoryID from Categories", connection );

    connection.Open();

    List<int> catID = new List<int>();
    dr = cmd.ExecuteReader();
    while (dr.Read())
    {
        catID.Add(Convert.ToInt32(dr["CategoryID"].ToString()));
    }
}
finally
{
    if (connection  != null)
        connection.Close();
}
return catID;

Answer 1

尝试这样，它更好，更安全，使用延迟加载，更少的代码，工作，...：

public IEnumerable<int> GetIds()
{
    using (var connection = new SqlConnection(connectionString))
    using (var cmd = connection.CreateCommand())
    {
        connection.Open();
        cmd.CommandText = "select CategoryID from Categories";
        using (var reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                yield return reader.GetInt32(reader.GetOrdinal("CategoryID"));
            }
        }
    }
}

然后：

List<int> catIds = GetIds().ToList();

Answer 2

您当前的代码应该可以工作，假设 catID 确实在 try 块之前声明，否则将无法编译。

Answer 3

AS BrokenGlass 解释说这是演示

SqlConnection connection = null;
        SqlDataReader dr= null;
        SqlCommand cmd = null;
List<int> catID = new List<int>();
        try
        {
            connection = new SqlConnection(connectionString);
            cmd = new SqlCommand("select CategoryID from Categories", connection );

            connection.Open();



            dr = cmd.ExecuteReader();
            while (dr.Read())
            {
                catID.Add(Convert.ToInt32(dr["CategoryID"].ToString()));
            }


        }
        finally
        {
            if (connection  != null)
                connection.Close();
        }
        return catID;

，您将声明更改

SqlDataReader reader = null;

为

SqlDataReader dr= null; // Because you are using dr in the code not reader

Answer 4

这应该可行，但我建议您将 using 与您的 connections 一起使用

    SqlConnection connection = null;
    SqlDataReader reader = null;
    SqlCommand cmd = null;
    List<int> catID = new List<int>();
    try
    {
        connection = new SqlConnection(connectionString);
        cmd = new SqlCommand("select CategoryID from Categories", connection );

        connection.Open();



        dr = cmd.ExecuteReader();
        while (dr.Read())
        {   
            catID.Add(Convert.ToInt32(dr["CategoryID"].ToString()));
        }


    }
    finally
    {
        if (connection  != null)
            connection.Close();
    }
    return catID;

Answer 5

        List<int> s = new List<int>();
        conn.Open();
        SqlCommand command2 = conn.CreateCommand();
        command2.CommandText = ("select turn from Vehicle where Pagged='YES'");
        command2.CommandType = CommandType.Text;
        SqlDataReader reader4 = command2.ExecuteReader();
        while (reader4.Read())
        {
            s.Add(Convert.ToInt32((reader4["turn"]).ToString()));
        }
        conn.Close();