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A Single-Instance Application Class
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This article was contributed by <a href="mailto:giancarlo@saria.com">Giancarlo Iovino</a>.
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<p><strong>Introduction</strong></p>
<p>Making <em>single-instance </em>an application means enabling
the application to recognize, at startup, if another running
instance of itself exists so that, in this case, the application
stops its execution. Generally, before quitting, the new instance
retrieves the main window handle of the existing instance via the
FindWindow() or FindWindowEx() function and uses this handle to
bring the application window to foreground.<br>
The problem can be solved in several ways. The most known
solutions are:</p>
<ul>
<li>Using a CMutex (a synchronization object global to the
system).</li>
<li>Using shared data sections.</li>
</ul>
<p>The small class described in this article, CSingleInstanceApp,
uses the first method to implement the single-instance behaviour
in a MFC application. It makes very simple to add the feature to
a new project, eliminating the necessity of rewriting a
consistent amount of code.</p>
<p><strong>Comparison with other similar classes</strong></p>
<p>In this section, I summarize the main features that
distinguish my class from other similar classes you can find in
this site. I hope this will help the reader to decide which code
suits his needs better.</p>
<ul>
<li>CSingleInstanceApp is an extension of CWinApp. This
reduces the work needed to add the single-instance
behaviour.</li>
<li>It uses a safe and stable method to build the class name,
avoiding conflicts with other running applications.</li>
</ul>
<p><strong>Implementation details</strong></p>
<p>For details about how to use a CMutex to implement the
single-instance you can read the article "<a
href="../../../../misc/single_instance.shtml">Single Instance of
an application Class</a>" by Kevin Lussier. In this article,
I will discuss only the method I have used to build the window
class name string. I assume that you know what a window class is
and the reason why Windows requires the class registration before
creating a window.<br>
The key point to implement single-instance is that we need the
registered class name to be the same in every instance of the
application. Moreover, it is very important that two different
applications don't use the same class name. The framework uses
AfxRegisterWndClass() to register the window class but,
unfortunately, this function registers a class name that is
different in different instances of the same application (you can
see it using the GetClassName() function). Thus, we cannot pass
this string to FindWindow(). But <em>what is the class name
registered by AfxRegisterWndClass()</em>?<br>
To answer this question, once again, we have to browse in the MFC
source code. We discover an interesting thing:
AfxRegisterWndClass() generates a name for the class with the
following code:</p>
<pre><font color="#800000">// generate a synthetic name for this class
HINSTANCE hInst = AfxGetInstanceHandle();
if (hCursor == NULL && hbrBackground == NULL && hIcon == NULL)
wsprintf(lpszName, _T("Afx:%x:%x"), (UINT)hInst, nClassStyle);
else
wsprintf(lpszName, _T("Afx:%x:%x:%x:%x:%x"), (UINT)hInst, nClassStyle,
(UINT)hCursor, (UINT)hbrBackground, (UINT)hIcon);</font></pre>
<p><font size="3">Well, the idea behind the CSingleInstanceApp
class is to mimic this way to generate the name string. <em>hInst</em>
(the application instance handle), <em>nClassStyle</em> (the
window class style), <em>hCursor</em> (the window cursor handle)
and <em>hbrBackground</em> (the window background brush handle)
are the same in every instance of the application. Only <em>hIcon</em>
changes, so simply we cut it off from the class name string.
Moreover, we have to make the class name application-dependent by
adding to it the application name returned by the AfxGetAppName()
function. This is exactly what the CheckSingleInstance() member
function does. This is an excerpt from CheckSingleInstance():</font></p>
<pre><font color="#800000" size="3">if (hCursor == NULL && hbrBackground == NULL && hIcon == NULL)
m_strClassName.Format(_T("%s:%x:%x"), lpstrAppName,
(UINT)hInst, nClassStyle);
else
m_strClassName.Format(_T("%s:%x:%x:%x:%x"), lpstrAppName,
(UINT)hInst, nClassStyle, (UINT)hCursor, (UINT)hbrBackground);</font></pre>
<p><strong>How to use CSingleInstanceApp</strong></p>
<p>Using this class requires the following steps:</p>
<ul>
<li>Include the "CSingleInstanceApp.c" and
".h" in your project. </li>
<li>At the beginning of the application class header file
#include "SingleInstanceApp.h". In the class
declaration replace CWinApp with CSingleInstanceApp.</li>
<li>Add the following as the first line of the InitInstance()
body of your application class:</li>
</ul>
<pre><font color="#800000">if (!CheckSingleInstance(IDR_MAINFRAME))
return FALSE;</font></pre>
<ul>
<li>Add the following as the first line of the
PreCreateWindow() body of your frame window class:</li>
</ul>
<pre><font color="#800000">cs.lpszClass = ((CMyApp*)AfxGetApp())->GetClassName();</font></pre>
<p><strong>Operations for CSingleInstanceApp</strong></p>
<pre><font color="#800000">BOOL CheckSingleInstance(UINT nID);
CString GetClassName() const;</font></pre>
<h4>Author's note</h4>
<p>I'm continuously working to improve this class. I'll be
grateful to you if you mail me your comments, advice, or bug
apparition reports!</p>
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<p><!-- first the filename (zip files) --><A HREF="single_instance_app.zip">Download demo project and source - 36KB</A>
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<p>Date Posted: 10 June 1998
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