C# と VB.NET の質問掲示板

■No102457 (KOZ) に返信
> セルの幅やフォントを設定していろいろ試してみるしかないかと。
> TextRenderer.MeasureText メソッドの値が目安になるかもしれないですが、やってみないとわかりません。

イケたかも。

using ClosedXML.Excel;
using System;
using System.Drawing;
using System.Windows.Forms;

internal class Program
{
    static void Main(string[] args) {

        const string filename = @"Z:\MergedRowHeight.xlsx";
        const string longText = "This is a long text that will be merged across multiple cells.";

        using (var workbook = new XLWorkbook()) {
            var worksheet = workbook.Worksheets.Add("Sheet1");
            var A1 = worksheet.Cell("A1");
            A1.Value = longText;
            A1.Style.Alignment.SetWrapText(true);

            var A2_A3 = worksheet.Range("A2:A3");
            A2_A3.Merge();
            A2_A3.Value = longText;
            A2_A3.Style.Alignment.Vertical = XLAlignmentVerticalValues.Top;
            A2_A3.Style.Alignment.SetWrapText(true);
            worksheet.Row(2).Height = A2_A3.GetHeight(longText)
                                    - worksheet.Row(3).Height;

            workbook.SaveAs(filename);
        }
    }
}

internal static class ExcelUtl
{
    const int DPI = 96;
    const double PointsPerInch = 72.0;
    const double PixelsPerChar = 7;

    public static double GetHeight(this IXLRange range, string text) {
        return range.Cell(1, 1).GetHeight(text);
    }

    public static double GetHeight(this IXLCell cell, string text) {
        var font = CreateFont(cell.Style.Font);
        var width = (int)(PixelsPerChar * cell.WorksheetColumn().Width);
        using (var g = Graphics.FromHwnd(IntPtr.Zero)) {
            var size = new Size(width, short.MaxValue);
            TextFormatFlags flags =
                TextFormatFlags.Left | TextFormatFlags.Top | TextFormatFlags.WordBreak;
            size = TextRenderer.MeasureText(g, text, font, size, flags);
            return PixelsToPoints(size.Height);
        }
    }

    public static int PointsToPixels(double points) {
        return (int)(points * DPI / PointsPerInch);
    }

    public static double PixelsToPoints(int pixels) {
        return (int)(pixels * PointsPerInch / DPI);
    }

    public static Font CreateFont(IXLFont font) {
        FontStyle style = FontStyle.Regular;
        if (font.Bold) { style |= FontStyle.Bold; }
        if (font.Italic) { style |= FontStyle.Italic; }
        if (font.Underline != XLFontUnderlineValues.None) { style |= FontStyle.Underline; }
        if (font.Strikethrough) { style |= FontStyle.Strikeout; }
        return new Font(font.FontName,
                            (float)font.FontSize, style, GraphicsUnit.Point);
    }
}

まぁ試してみてダメだったら調整してください。