Algorithms

private void ShowQuestionsDialog()
{
    using (QuestionsForm form = new QuestionsForm())
    {
        if (form.ShowDialog() == DialogResult.OK)
        {
            // user answered all questions and clicked OK
        }
        else
        {
            // user canceled
        }
    }
}

private void OkButton_Click(object sender, EventArgs e)
{
    if (/* check all questions have been answered */)
    {
        DialogResult = DialogResult.OK;
        Close();
    }
    else
    {
        MessageBox.Show("Please answer all questions.");
    }
}

struct Record
{
    public int T { get; set; }
    public int H { get; set; }
    public int L { get; set; }
}

Record[] UnsortedRecords =
{
    new Record { T = 0, H = 0, L = 2 },
    new Record { T = 1, H = 2, L = 1 },
    new Record { T = 0, H = 1, L = 4 },
    new Record { T = 0, H = 0, L = 0 },
    new Record { T = 2, H = 4, L = 2 },
    new Record { T = 2, H = 3, L = 6 },
};

public static IEnumerable<Record> SortRecords(IEnumerable<Record> source)
{
    return source
        .GroupBy(r => r.T, (key, items) => items
            .GroupBy(r => r.H)
            .SelectMany(g => g)
        )
        .SelectMany(g => g);
}

IEnumerable<Record> SortedRecords = SortRecords(UnsortedRecords);

/*
Sorted records:

T0 H0 L2 
T0 H0 L0 
T0 H1 L4 
T1 H2 L1 
T2 H4 L2 
T2 H3 L6 
*/

Enumerable.GroupBy[^]:

The IGrouping<TKey,TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey,TElement>. Elements in a grouping are yielded in the order they appear in source.

source.GroupBy(r => new { r.T, r.H }).SelectMany(g => g);

/*
Result:

T0 H0 L2 
T0 H0 L0 
T1 H2 L1 
T0 H1 L4 
T2 H4 L2 
T2 H3 L6 
*/