Question

When I connect to a SQL datasource in my C# application, I can fill a dataset using the following code. Note that I am not explictly opening a connection to the datasource.

SqlConnection cw_con = new SqlConnection("Server=Server;Database=Database;User=User;password=password");
SqlCommand cmd = new SqlCommand("SELECT * FROM Example WHERE value = value");
cmd.Connection = cw_con;

//Create DataSet
DataSet cw_ds = new DataSet("cw_ds");
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = cmd;
//Execute Command and Fill DataSet
da.Fill(cw_ds);
cw_ds.Clear();

However, If I want to execute a nonquery such as a INSERT INTO or UPDATE why do I have to explicitly open the connection by using connection.Open();?

SqlConnection cw_con = new SqlConnection("Server=Server;Database=Database;User=User;password=password");
cw_con.Open();
SqlCommand cmd = new SqlCommand("UPDATE example SET value = value WHERE value = value");
cmd.Connection = cw_con;
cmd.ExecuteNonQuery();
cw_con.Close();

Answer 1

That is because a DataAdapter opens the connection implicitely when it’s closed initially. It’ll close it afterwards if it was closed before, otherwise the connection stays open.

From MSDN:

The Fill method retrieves the data from the data source using a SELECT
statement. The IDbConnection object associated with the select command
must be valid, but it does not need to be open. If the IDbConnection
is closed before Fill is called, it is opened to retrieve data and
then closed. If the connection is open before Fill is called, it
remains open.

When using a SqlCommand you must explicitely open and close connections yourself.

As a side note:

1. Use parameters to avoid SQL-Injection
2. Use using-statement for anything implementing IDisposable, especially Connections since it ensures that it gets disposed/closed as soon as possible, even in the case of an error