在C#中使用SendMessage

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();
}