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我如何连接到数据库和C #循环记录集?

What's the simplest way to connect and query a database for a set of records in C#?

2016年12月09日32分46秒

@Goyuix -- that's excellent for something written from memory. tested it here -- found the connection wasn't opened. Otherwise very nice.

using System.Data.OleDb;
...

using (OleDbConnection conn = new OleDbConnection())
{
    conn.ConnectionString = "Provider=sqloledb;Data Source=yourServername\\yourInstance;Initial Catalog=databaseName;Integrated Security=SSPI;";

    using (OleDbCommand cmd = new OleDbCommand())
    {
        conn.Open();
        cmd.Connection = conn;
        cmd.CommandText = "Select * from yourTable";

        using (OleDbDataReader dr = cmd.ExecuteReader())
        {
            while (dr.Read())
            {
                Console.WriteLine(dr["columnName"]);
            }
        }
    }
}

2016年12月08日32分46秒

Very roughly and from memory since I don't have code on this laptop:

using (OleDBConnection conn = new OleDbConnection())
{
  conn.ConnectionString = "Whatever connection string";

  using (OleDbCommand cmd = new OleDbCommand())
  {
    cmd.Connection = conn;
    cmd.CommandText = "Select * from CoolTable";

    using (OleDbDataReader dr = cmd.ExecuteReader())
    {
      while (dr.Read())
      {
        // do something like Console.WriteLine(dr["column name"] as String);
      }
    }
  }
}

2016年12月08日32分46秒

That's definitely a good way to do it. But you if you happen to be using a database that supports LINQ to SQL, it can be a lot more fun. It can look something like this:

MyDB db = new MyDB("Data Source=...");
var q = from db.MyTable
select c;
foreach (var c in q)
Console.WriteLine(c.MyField.ToString());

2016年12月08日32分46秒

This is an alternative way (DataReader is faster than this one):

string s = "";
SqlConnection conn = new SqlConnection("Server=192.168.1.1;Database=master;Connect Timeout=30;User ID=foobar;Password=raboof;");
SqlDataAdapter da = new SqlDataAdapter("SELECT TOP 5 name, dbid FROM sysdatabases", conn);
DataTable dt = new DataTable();

da.Fill(dt);

for (int i = 0; i < dt.Rows.Count; i++)
{
s += dt.Rows[i]["name"].ToString() + " -- " + dt.Rows[i]["dbid"].ToString() + "\n";
}

MessageBox.Show(s);

2016年12月08日32分46秒

If you are intending on reading a large number of columns or records it's also worth caching the ordinals and accessing the strongly-typed methods, e.g.

using (DbDataReader dr = cmd.ExecuteReader()) {
  if (dr.Read()) {
    int idxColumnName = dr.GetOrdinal("columnName");
    int idxSomethingElse = dr.GetOrdinal("somethingElse");

    do {
      Console.WriteLine(dr.GetString(idxColumnName));
      Console.WriteLine(dr.GetInt32(idxSomethingElse));
    } while (dr.Read());
  }
}

2016年12月08日32分46秒

If you are querying a SQL Server database (Version 7 and up) you should replace the OleDb classes with corresponding classes in the System.Data.SqlClient namespace (SqlConnection, SqlCommand and SqlDataReader) as those classes have been optimized to work with SQL Server.

Another thing to note is that you should 'never' select all as this might lead to unexpected results later on if you add or remove columns to this table.

2016年12月08日32分46秒

I guess, you can try entity framework.

using (SchoolDBEntities ctx = new SchoolDBEntities())
{
     IList<Course> courseList = ctx.GetCoursesByStudentId(1).ToList<Course>();
     //do something with courselist here
}

2016年12月08日32分46秒