SendMessage是一个在user32.dll中声明的API函数，在C#中导入如下：
using System.Runtime.InteropServices;
[DllImport("user32.dll", EntryPoint="SendMessageA")]
public static extern int SendMessage (IntPtr hwnd, int wMsg, IntPtr wParam, IntPtr lParam);
本文描述其参数 lParam 的用法，主要是数据类型之间的转化。
● 一种最简单的处理方式是声明多个SendMessage函数(overload)，用所需的数据类型直接替换IntPtr。例如：
//声明:
[DllImport("user32.dll", EntryPoint="SendMessageA")]
private static extern int SendMessage (IntPtr hwnd, int wMsg, IntPtr wParam, string lParam);
[DllImport("user32.dll", EntryPoint="SendMessageA")]
private static extern int SendMessage (IntPtr hwnd, int wMsg, IntPtr wParam, ref Rectangle lParam);
//调用:
string s = "hello, floodzhu";
SendMessage(this.textBox1.Handle, WM_SETTEXT, IntPtr.Zero, s);
Rectangle rect = new Rectangle();
SendMessage(this.richTextBox1.Handle, EM_GETRECT, (IntPtr)0, ref rect);
● 对要求返回字符串的类型（out string）可以用 StringBuilder 代替，此时不需要 out/ref。例如：
[DllImport("user32.dll", EntryPoint="SendMessageA")]
private static extern int SendMessage (IntPtr hwnd, int wMsg, int wParam, StringBuilder lParam);
private void button1_Click(object sender, System.EventArgs e)
{
const int buffer_size = 1024;
StringBuilder buffer = new StringBuilder(buffer_size);
SendMessage(this.textBox1.Handle, WM_GETTEXT, buffer_size, buffer);
//MessageBox.Show(buffer.ToString());
}
● 如果想用 InPtr 类型统一处理的话，可以借助于 Marshal 或者 GCHandle 的相关方法。例如：
[DllImport("user32.dll", EntryPoint="SendMessageA")]
private static extern int SendMessage (IntPtr hwnd, int wMsg, IntPtr wParam, IntPtr lParam);
private void button2_Click(object sender, System.EventArgs e)
{
Rectangle rect = new Rectangle();
IntPtr buffer = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(Rectangle)));
Marshal.StructureToPtr(rect, buffer ,true);
SendMessage(this.richTextBox1.Handle, EM_GETRECT, (IntPtr)0, buffer);
rect = (Rectangle)Marshal.PtrToStructure(buffer, typeof(Rectangle));
Marshal.FreeHGlobal(buffer);
}
或者
private void button2_Click(object sender, System.EventArgs e)
{
Rectangle rect = new Rectangle();
GCHandle gch = GCHandle.Alloc(rect);
SendMessage(this.richTextBox1.Handle, EM_GETRECT, (IntPtr)0, (IntPtr)gch);
rect = (Rectangle)Marshal.PtrToStructure((IntPtr)gch, typeof(Rectangle));
gch.Free();
}