何时必须显式打开SqlConnection

我写了以下代码（为了简洁起见，进行了修剪）：

using (SqlConnection cn = new SqlConnection("Server=test;Database=test;User=test;Password=test"))
using (SqlDataAdapter da = new SqlDataAdapter())
using (DataSet ds = new DataSet())
{
    string groupsQuery = @"SELECT GroupName FROM tblGroups ORDER BY GroupName";
    da.SelectCommand = new SqlCommand(groupsQuery, cn);
    da.Fill(ds);
    foreach (System.Data.DataRow row in ds.Tables[0].Rows)
    {
        string group = row["GroupName"].ToString();
        this.GroupList.Add(group);
    }
}

我忘了调用cn.Open()，但令我惊讶的是，代码运行得很好。我怀疑SqlDataAdapter在变魔术，所以我去找到了源代码。

SqlDataAdapter从DbDataAdapter继承了Fill方法。

Fill调用FillInternal，它将其逻辑封装在一个块中，如下所示：

try {
    QuietOpen(activeConnection, out originalState);
    //... do the fill ...
}
finally {
    QuietClose(activeConnection, originalState);
}

QuietOpen和QuietClose非常简单：

static private void QuietClose(IDbConnection connection, ConnectionState originalState) {
    // close the connection if:
    // * it was closed on first use and adapter has opened it, AND
    // * provider's implementation did not ask to keep this connection open
    if ((null != connection) && (ConnectionState.Closed == originalState)) {
        // we don't have to check the current connection state because
        // it is supposed to be safe to call Close multiple times
        connection.Close();
    }
}
// QuietOpen needs to appear in the try {} finally { QuietClose } block
// otherwise a possibility exists that an exception may be thrown, i.e. ThreadAbortException
// where we would Open the connection and not close it
static private void QuietOpen(IDbConnection connection, out ConnectionState originalState) {
    Debug.Assert(null != connection, "QuietOpen: null connection");
    originalState = connection.State;
    if (ConnectionState.Closed == originalState) {
        connection.Open();
    }
}

我很好奇，我什么时候在不需要的SqlConnection上调用Open？我应该总是明确地做，还是让.NET"安静地"做它的事情？

此外，任何关于SqlDataAdapter为什么这样做的详细信息的参考都将非常好。