在 C# 中从 SQL 数据库中读取值

Question

i have just started learning C# and i can write data to the database without a problem. But i'm having problems with reading, the SQL executes fine but i'm having issues with storing it. How would i store the four columns that should be returned and then show them as a message box? Thanks.

SqlCommand myCommand = new SqlCommand("select * from Requests where Complete = 0", myConnection);
SqlDataReader myReader = myCommand.ExecuteReader();
while (myReader.Read())

Console.WriteLine(myReader["Username"].ToString());
Console.WriteLine(myReader["Item"].ToString());
Console.WriteLine(myReader["Amount"].ToString());
Console.WriteLine(myReader["Complete"].ToString());

Answer 1

One problem is missing braces after the while

while (myReader.Read())
{  // <<- here
    Console.WriteLine(myReader["Username"].ToString());
    Console.WriteLine(myReader["Item"].ToString());
    Console.WriteLine(myReader["Amount"].ToString());
    Console.WriteLine(myReader["Complete"].ToString());
}  // <<- here

if you skip the braces only the first line will be processed in each loop, the rest will be processed after the loop, then myReader is past the last row.

Answer 2

Don't forget to use the using(){} block :

using (SqlConnection connection = new SqlConnection(connectionString))
using (SqlCommand command = new SqlCommand("select * from Requests where Complete = 0", connection))
{
    connection.Open();  
    using (SqlDataReader reader = command.ExecuteReader())
    {
        while (reader.Read())
        {
            Console.WriteLine(reader["Username"].ToString());
            Console.WriteLine(reader["Item"].ToString());
            Console.WriteLine(reader["Amount"].ToString());
            Console.WriteLine(reader["Complete"].ToString());
        }
    }
}

Answer 3

Personally I'd write a class with 4 properties (with matching names and types), then use "dapper" (http://code.google.com/p/dapper-dot-net/):

var data = connection.Query<Request>(
    "select * from Requests where Complete = 0").ToList();

With something like:

public class Request {
    public string Username{get;set;}
    ...
    public bool Complete {get;set;}
}

Dapper is free, simple, has parameterisation to avoid SQL-injection, and is very very fast.

Answer 4

我知道它有点晚了，但你可以使用本地字符串变量，或字符串数​​组或列表将数据插入数据库中，然后在你的控制台写入行中调用它

Answer 5

I would create an object with properties that holds those values and then pass that object around as needed.

public class YourObjectName
{
   public string Username { get; set; }
   public string Item { get; set; }
   public string Amount { get; set; }
   public string Complete { get; set; }
}

YourObjectName a = new YourObjectName();
a.Username = Reader['Username'].ToString();