lParam 和 wParam 是宏定義，一般在消息函數中帶這兩個類型的參數，通常用來存儲窗口消息的參數。
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
wParam 通常用來存儲小段信息，如，標志
lParam 通常用於存儲消息所需的對象
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
typedef unsigned int UINT;
typedef long LONG;
typedef UINT WPARAM;
typedef LONG LPARAM;
lParam 和 wParam 是 Win16 系統遺留下來的產物，在 Win16 API 中 WndProc 有兩個參數，一個 WORD 類型的 16 位整型變量，另一個是 LONG 類型的 32 位整型變量。根據匈牙利命名法(Hungarian notation)，16 位的變量就被命名為 wParam，而 32 位的變量就被命名為 lParam。
到了 Win32 API 中，原來的 16 位變量也被擴展為 32 位，因此 lParam 和 wParam 的大小完全相同。在 Win32 API 的早期，為了保證和 Win16 API 的代碼兼容，MS 定義了 MPARAM 和 LPARAM 兩個宏。保留 w 前綴的原因一方面是由於 WPARAM 宏也以 W 開頭，另一方面是希望提醒程序員注意到可移植性。到了現在，Win16 早已退出歷史舞台，但是這個前綴仍然約定俗成的沿用了下來。
Once upon a time, Windows was 16-bit. Each message could carry with it two pieces of data, called WPARAM and LPARAM. The first one was a 16-bit value ("word"), so it was called W. The second one was a 32-bit value ("long"), so it was called L.
You used the W parameter to pass things like handles and integers. You used the L parameter to pass pointers.
When Windows was converted to 32-bit, the WPARAM parameter grew to a 32-bit value as well. So even though the "W" stands for "word", it isn't a word any more. (And in 64-bit Windows, both parameters are 64-bit values!)
It is helpful to understand the origin of the terms. If you look at the design of window messages, you will see that if the message takes a pointer, the pointer is usually passed in the LPARAM, whereas if the message takes a handle or an integer, then it is passed in the WPARAM. (And if a message takes both, the integer goes in the WPARAM and the pointer goes in the LPARAM.)
“在Win 3.x中，WPARAM是16位的，而LPARAM是32位的，兩者有明顯的區別。因為地址通常是32位的，所以LPARAM 被用來傳遞地址，這個習慣在Win32 API中仍然能夠看到。在Win32 API中，WPARAM和LPARAM都是32位，所以沒有什么本質的區 別。Windows的消息必須參考幫助文件才能知道具體的含義。如果是你定義的消息，願意怎么使這兩個參數都行。但是習慣上，我們願意使用LPARAM傳 遞地址，而WPARAM傳遞其他參數。”
在 MSDN 網站中關於 Windows Data Types 中有如下定義：
LPARAM: A message parameter. This type is declared in WinDef.h as follows: typedef LONG_PTR LPARAM;
WPARAM: A message parameter. This type is declared in WinDef.h as follows: typedef UINT_PTR WPARAM;
LPARAM is a typedef for LONG_PTR which is a long (signed 32-bit) on win32 and __int64 (signed 64-bit) on x86_64.
WPARAM is a typedef for UINT_PTR which is an unsigned int (unsigned 32-bit) on win32 and unsigned __int64 (unsigned 64-bit) on x86_64
(x86_64 is how Microsoft now refer to amd64)
In c#, you should use IntPtr for LPARAM and UIntPtr for WPARAM.
在 C# 與 C++ 的互操作中，可以使用 IntPtr 來聲明 LPARAM 類型變量，使用 UIntPtr 來聲明 WPARAM 類型的變量。
當 WPARAM, LPARAM 和 LRESULT 在 32 位和 64 位 Windows 系統中傳遞的時候會發生什么？
如果是從 64 位 Windows 系統到 32 位系統，那么只有一個選擇：截斷 truncation。
如果是從 32 位到 64 位，那么對 WPARAM 采用補零擴展（zero-extended），對 LPARAM 和 LRESULT 采用符號擴展 (sign-extended)。
擴展方式不同的原因主要是因為 WPARAM 被定義為 “字 (WORD)” 也就是 “UINT_PTR”，而 LPARAM 和 LRESULT 被定義為 “LONG”，也就是 "LONG_PTR"。
What happens to WPARAM, LPARAM, and LRESULT when the travel between 32-bit and 64-bit windows?
The integral types WPARAM, LPARAM, and LRESULT are 32 bits wide on 32-bit systems and 64 bits on 64-bit systems. What happens when a 32-bit process sends a message to a 64-bit window or vice versa ?
There's really only one choice when converting a 64-bit value to a 32-bit value: Truncation. When a 64-bit process sends a message to a 32-bit window, the 64-bit WPARAM and LPARAM values are truncated to 32 bits. Similarly, when a 64-bit window returns an LRESULT back to a 32-bit sender, the value is truncate.
But converting a 32-bit value to a 64-bit value includes a choice: Do you zero-extend or sign-extend?
The answer is obvious if you remember the history of WPARAM, LPARAM, and LRESULT or if you just look at the header file.
The WPARAM is zero-extend, while LPARAM and LRESULT are sign-extended.
If you remember that WPARAM used to be a WORD and LPARAM and LRESULT used to be LONG, then this follows from the fact that WORD is an unsigned type (therefore zero-extended) and LONG is a signed type (therefore sign-extend).
Even if you didn't know that, you could look it up in the header file.
typedef UINT_PTR WPARAM;
typedef LONG_PTR LPARAM;
typedef LONG_PTR LRESULT;
UINT_PTR is an unsigned type (therefore zero-extended) and LONG_PTR is a signed type (therefore sign-extended).
參考資料
- 弱智問題：請問什么是 lParam 和 wParam ?
- Windows Data Types
- what are the definitions for LPARAM and WPARAM?
- Message.LParam Property (System.Windows.Forms)
- WPARAM 與 LPARAM 的區別
- What do the letters W and L stand for in WPARAM and LPARAM ?
- What happens to WPARAM, LPARAM, and LRESULT when they travel between 32-bit and 64-bit windows ?