DataReader vs DataAdapter

SqlDataReader — forward-only streaming

Fast and lightweight. Reads one row at a time and doesn't hold the whole result set in memory. Best for simply looping through and printing/using data once.

using var reader = command.ExecuteReader();
while (reader.Read())
{
    Console.WriteLine(reader["FirstName"]);
}

SqlDataAdapter + DataTable — an in-memory snapshot

Loads the entire result into a DataTable you can pass around, bind to a grid (very common in Windows Forms), filter, or sort without touching the database again.

using var connection = new SqlConnection(connectionString);
var adapter = new SqlDataAdapter("SELECT * FROM Students", connection);

var table = new DataTable();
adapter.Fill(table);

foreach (DataRow row in table.Rows)
{
    Console.WriteLine(row["FirstName"]);
}

Which one should you use?