用RTF格式解决DrawText函数或Graphics.DrawString方法不能设置行距的问题

绘制多行文字时会用到DrawText函数或Graphics.DrawString方法，但它们都无法设置行距，也无法设置段落间距。下图是用DrawString方法绘制的两段文字，可以看到中文测试段落的行距非常小。

我们可以利用RTF格式来设置行距和段落间距，下图是测试结果：

C#源代码：

using System;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;namespace WindowsFormsApplication1
{public partial class Form1 : Form{private struct RECT{public int Left;public int Top;public int Right;public int Bottom;}private struct CHARRANGE{public int cpMin;public int cpMax;}private struct FORMATRANGE{public IntPtr hDC;public IntPtr hDCTarget;public RECT rc;public RECT rcPage;public CHARRANGE chrg;}private struct PARAFORMAT2{public int cbSize;public uint dwMask;public short wNumbering;public short wReserved;public int dxStartIndent;public int dxRightIndent;public int dxOffset;public short wAlignment;public short cTabCount;[MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]public int[] rgxTabs;public int dySpaceBefore;public int dySpaceAfter;public int dyLineSpacing;public short sStyle;public byte bLineSpacingRule;public byte bOutlineLevel;public short wShadingWeight;public short wShadingStyle;public short wNumberingStart;public short wNumberingStyle;public short wNumberingTab;public short wBorderSpace;public short wBorderWidth;public short wBorders;}[DllImport("user32", CharSet = CharSet.Auto)]private static extern IntPtr SendMessage(IntPtr hWnd, int msg, int wParam, ref PARAFORMAT2 lParam);[DllImport("user32", CharSet = CharSet.Auto)]private static extern IntPtr SendMessage(IntPtr hWnd, int msg, int wParam, ref FORMATRANGE lParam);public const int WM_USER = 0x0400;public const int EM_FORMATRANGE = WM_USER + 57;public const int EM_SETPARAFORMAT = WM_USER + 71;public const int PFM_LINESPACING = 0x00000100;public const int PFM_SPACEBEFORE = 0x00000040;public const int PFM_SPACEAFTER = 0x00000080;public Form1(){InitializeComponent();this.Paint += new System.Windows.Forms.PaintEventHandler(this.Form1_Paint);}private void Form1_Load(object sender, EventArgs e){this.Size = new Size(400, 200);}private void Form1_Paint(object sender, PaintEventArgs e){RichTextBox richTextBox = new RichTextBox();richTextBox.Text = "第一段是中文字符测试内容，第二段是英文字符测试内容，中文的行距明显更小。\r\nYou have to believe in yourself. That's the secret of success. - Charles Chaplin";richTextBox.SelectAll();richTextBox.Font = this.Font;richTextBox.BackColor = this.BackColor;int intTwipsPerPixel = (int)(1440 / e.Graphics.DpiX);PARAFORMAT2 paraFormat = new PARAFORMAT2();paraFormat.cbSize = Marshal.SizeOf(paraFormat);paraFormat.bLineSpacingRule = 3;paraFormat.dyLineSpacing = 20 * intTwipsPerPixel;  //行距paraFormat.dySpaceAfter = 10 * intTwipsPerPixel; //段落后的间距paraFormat.dwMask = PFM_LINESPACING | PFM_SPACEAFTER;richTextBox.SelectAll();SendMessage(richTextBox.Handle, EM_SETPARAFORMAT, 0, ref paraFormat);IntPtr hDC = e.Graphics.GetHdc();FORMATRANGE formatRange = new FORMATRANGE();formatRange.chrg.cpMin = 0;formatRange.chrg.cpMax = richTextBox.Text.Length;formatRange.rc.Left = 20 * intTwipsPerPixel;formatRange.rc.Right = (this.ClientSize.Width-20) * intTwipsPerPixel;formatRange.rc.Top = 20 * intTwipsPerPixel;formatRange.rc.Bottom = (this.ClientSize.Height-20) * intTwipsPerPixel;formatRange.rcPage = formatRange.rc;formatRange.hDC = hDC;formatRange.hDCTarget = hDC;SendMessage(richTextBox.Handle, EM_FORMATRANGE, 1, ref formatRange);e.Graphics.ReleaseHdc(hDC);richTextBox.Dispose();}}
}

VB.Net源代码：

Imports System.Runtime.InteropServicesPublic Class Form1Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As IntPtr, ByVal wMsg As Integer, ByVal wParam As Integer, ByRef lParam As FORMATRANGE) As IntegerPrivate Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As IntPtr, ByVal wMsg As Integer, ByVal wParam As Integer, ByRef lParam As PARAFORMAT2) As IntegerPrivate Structure RECTDim Left As IntegerDim Top As IntegerDim Right As IntegerDim Bottom As IntegerEnd StructurePrivate Structure CHARRANGEDim cpMin As IntegerDim cpMax As IntegerEnd StructurePrivate Structure FORMATRANGEDim hDC As IntPtrDim hdcTarget As IntPtrDim rc As RECTDim rcPage As RECTDim chrg As CHARRANGEEnd StructurePrivate Structure PARAFORMAT2Dim cbSize As IntegerDim dwMask As UInt32Dim wNumbering As ShortDim wReserved As ShortDim dxStartIndent As IntegerDim dxRightIndent As IntegerDim dxOffset As IntegerDim wAlignment As ShortDim cTabCount As Short<MarshalAs(UnmanagedType.ByValArray, SizeConst:=32)> _Dim rgxTabs As Integer()Dim dySpaceBefore As IntegerDim dySpaceAfter As IntegerDim dyLineSpacing As IntegerDim sStyle As ShortDim bLineSpacingRule As ByteDim bOutlineLevel As ByteDim wShadingWeight As ShortDim wShadingStyle As ShortDim wNumberingStart As ShortDim wNumberingStyle As ShortDim wNumberingTab As ShortDim wBorderSpace As ShortDim wBorderWidth As ShortDim wBorders As ShortEnd StructurePrivate Const WM_USER As Integer = &H400Private Const EM_FORMATRANGE As Integer = WM_USER + 57Private Const EM_SETPARAFORMAT As Integer = WM_USER + 71Private Const PFM_LINESPACING As Integer = &H100Private Const PFM_SPACEBEFORE As Integer = &H40Private Const PFM_SPACEAFTER As Integer = &H80Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.LoadMe.Size = New Size(400, 200)End SubPrivate Sub Form1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles Me.PaintDim richTextBox As New Windows.Forms.RichTextBoxrichTextBox.Text = "第一段是中文字符测试内容，第二段是英文字符测试内容，中文的行距明显更小。" & vbCrLf & "You have to believe in yourself. That's the secret of success. - Charles Chaplin"richTextBox.SelectAll()richTextBox.Font = Me.FontrichTextBox.BackColor = Me.BackColorDim intTwipsPerPixel = 1440 / e.Graphics.DpiXDim paraFormat As New PARAFORMAT2paraFormat.cbSize = Marshal.SizeOf(paraFormat)paraFormat.bLineSpacingRule = 3paraFormat.dyLineSpacing = 20 * intTwipsPerPixel '行距paraFormat.dySpaceAfter = 10 * intTwipsPerPixel '段落后的间距paraFormat.dwMask = PFM_LINESPACING Or PFM_SPACEAFTERrichTextBox.SelectAll()SendMessage(richTextBox.Handle, EM_SETPARAFORMAT, 0, paraFormat)Dim hDC As IntPtr = e.Graphics.GetHdc()Dim formatRange As FORMATRANGEformatRange.chrg.cpMin = 0formatRange.chrg.cpMax = Len(richTextBox.Text)formatRange.rc.Left = 20 * intTwipsPerPixelformatRange.rc.Right = (Me.ClientSize.Width - 20) * intTwipsPerPixelformatRange.rc.Top = 20 * intTwipsPerPixelformatRange.rc.Bottom = (Me.ClientSize.Height - 20) * intTwipsPerPixelformatRange.rcPage = formatRange.rcformatRange.hDC = hDCformatRange.hdcTarget = hDCSendMessage(richTextBox.Handle, EM_FORMATRANGE, 1, formatRange)e.Graphics.ReleaseHdc(hDC)richTextBox.Dispose()End SubEnd Class

程序中用到的bLineSpacingRule，其取值可以参考：
https://learn.microsoft.com/en-us/windows/win32/api/richedit/ns-richedit-paraformat2

bLineSpacingRule = 0：单倍行距
bLineSpacingRule = 1：1.5倍行距
bLineSpacingRule = 2：2倍行距
bLineSpacingRule = 3：固定行距，用dyLineSpacing指定行距

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