protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.InsertRows(
        table: "Countries",
        rows: new [] { // Note: we don't get compiler suggestions here
            new { Name = "Netherlands", Population = 16680000 },
        });
}

protected override void Down(MigrationBuilder migrationBuilder)
{
    migrationBuilder.DeleteRows(
        table: "Countries",
        rows: new [] { "Netherlands" }); // Note: this is an array of the primary keys
}