Write a C# method that validates a start/end date range for an API request.

Implement ValidateDateRange(DateOnly? start, DateOnly? end, int maxDays) returning a list of error messages. Both dates are required. Start must be on or before end. The inclusive range cannot exceed maxDays.

Input: start 2026-01-01, end 2026-01-31, maxDays 31 Output: no errors

• Collect all validation errors instead of returning after the first one.
• Both dates are required.
• The date range is inclusive.
• maxDays must be positive.

I would collect errors in a list so the API can return all field issues at once. First validate required values and maxDays. Only if both dates exist do I compare ordering and length. The inclusive day count is end.DayNumber - start.DayNumber + 1, which avoids time-of-day problems because DateOnly is already date-based.

public static IReadOnlyList<string> ValidateDateRange(
    DateOnly? start,
    DateOnly? end,
    int maxDays)
{
    var errors = new List<string>();

    if (start is null)
    {
        errors.Add("Start date is required.");
    }

    if (end is null)
    {
        errors.Add("End date is required.");
    }

    if (maxDays <= 0)
    {
        errors.Add("Maximum range must be positive.");
    }

    if (start is not null && end is not null)
    {
        if (start > end)
        {
            errors.Add("Start date must be on or before end date.");
        }
        else if (maxDays > 0)
        {
            var days = end.Value.DayNumber - start.Value.DayNumber + 1;
            if (days > maxDays)
            {
                errors.Add($"Date range cannot exceed {maxDays} days.");
            }
        }
    }

    return errors;
}

The method is O(1) time and O(1) space, excluding the small error list.

• Missing start and end.
• Start after end.
• Same-day range.
• Range exactly equal to maxDays.
• Invalid maxDays value.

• Use maxDays 1 with same start and end to catch inclusive math.
• Use a range one day over the max.
• Use missing dates to verify all required errors are returned.

• Calculating exclusive day count accidentally.
• Returning only one error when the API expects all field errors.
• Using DateTime and creating timezone noise for date-only input.
• Checking range length before handling missing dates.