定石3:メモリ消費の実測比較— Stopwatch + GC.GetTotalMemory

// ✅定石3:メモリ・実行時間の実測比較
const string sql = "SELECT id, name, memo, amount, created_at FROM big_log";
 
// --- DataAdapter版---
GC.Collect(); GC.WaitForPendingFinalizers();
long memBefore = GC.GetTotalMemory(true);
var sw1 = Stopwatch.StartNew();
 
DataTable dt = new DataTable();
using (var conn = new SqlConnection(connStr))
using (var adapter = new SqlDataAdapter(sql, conn))
{
    adapter.Fill(dt);   // 10万件をメモリに全部ロード
}
sw1.Stop();
long memAfter = GC.GetTotalMemory(false);
Console.WriteLine($"DataAdapter: {sw1.ElapsedMilliseconds}ms / {(memAfter - memBefore)/ 1024 / 1024}MB");
 
// --- DataReader版---
GC.Collect(); GC.WaitForPendingFinalizers();
memBefore = GC.GetTotalMemory(true);
var sw2 = Stopwatch.StartNew();
 
int rowCount = 0;
using (var conn = new SqlConnection(connStr))
using (var cmd = new SqlCommand(sql, conn))
{
    conn.Open();
    using (var reader = cmd.ExecuteReader())
    {
        while (reader.Read())
        {
            rowCount++;
            // CSVに1行ずつ書き出すイメージ（メモリには溜めない）
        }
    }
}
sw2.Stop();
memAfter = GC.GetTotalMemory(false);
Console.WriteLine($"DataReader : {sw2.ElapsedMilliseconds}ms / {(memAfter - memBefore)/ 1024 / 1024}MB");