请问怎么通过扩展名判断文件是否被关联，即可被某个程序打开

请教如何通过扩展名判断文件是否被关联，即可被某个程序打开？
或者如何枚举系统中已经关联的所以扩展名？

------解决方案--------------------
参考:
BOOL CMainFrame::GetExeCommand(char *fullname,char* command)
{// ""%1" 1*" for .bat file
 // "xxx.exe %1"
 // ""\a b\c\yyy.exe" /p %1"
char ext[20];
char pathname[MAX_PATH];
char exename[MAX_PATH];
char *at=0;
strcpy(pathname,fullname);
// must ".xxx"
if((at=strrchr(fullname,'.'))==NULL) return FALSE;
strcpy(ext,at);

char open_key[MAX_PATH];
DWORD openkeylen=MAX_PATH;
char shellname[MAX_PATH]="shell";
// under shell may be different "open","cplopen","Open with"...
BYTE extfile[MAX_PATH];
DWORD cbextfile=MAX_PATH;
DWORD dwType=REG_SZ;
HKEY hkey;
HKEY hsubkey;
// from ".xxx" get "xxxfile"
DWORD dwErrorCode=RegOpenKeyEx(HKEY_CLASSES_ROOT,ext,0,KEY_READ,&hkey);
if (dwErrorCode != ERROR_SUCCESS)
{ // error not found
command[0]=0;
return FALSE;
}
else
{ // ".xxx" OK
// get "xxxfile"
dwErrorCode=RegQueryValueEx(hkey,NULL,0,&dwType,extfile,&cbextfile);
RegCloseKey(hkey);
char tmp[MAX_PATH];
sprintf(tmp,"%s\\%s",extfile,shellname);// "cppfile\\shell"
if (dwErrorCode == ERROR_SUCCESS) 
{// "cppfile\\shell"
dwErrorCode=RegOpenKeyEx(HKEY_CLASSES_ROOT,tmp,0,KEY_READ,&hsubkey);
if (dwErrorCode == ERROR_SUCCESS) 
{// "cppfile\\shell"
LONG enumErr=ERROR_SUCCESS;
DWORD j=0;
FILETIME ftLastWriteTime;
while (enumErr==ERROR_SUCCESS || enumErr==ERROR_MORE_DATA)
{
cbextfile=260;// re-init
openkeylen=MAX_PATH;// re-init
enumErr=RegEnumKeyEx(hsubkey,j,open_key,&openkeylen,
NULL,NULL,NULL,&ftLastWriteTime);
strcpy(tmp,open_key);
_strupr(tmp);
if(strstr(tmp,"OPEN")) // any "open"
{
sprintf(tmp,"%s\\%s\\%s\\command",extfile,shellname,open_key);// "cppfile\\shell"
break;
}
j++;
}
RegCloseKey(hsubkey);
if(enumErr!=ERROR_SUCCESS) return FALSE;
} // end with "shell" key
else
{ // no "shell" key
return FALSE;
}
// "shell\\...open...\\command"
dwErrorCode=RegOpenKeyEx(HKEY_CLASSES_ROOT,tmp,0,KEY_READ,&hsubkey);
cbextfile=260;// re-init
//extfile="%SystemRoot%\\system32\\NOTEPAD32.EXE %1"
dwErrorCode=RegQueryValueEx(hsubkey,NULL,0,&dwType,extfile,&cbextfile);
// if fullname contains space in its pathname
if(strchr(fullname,' ')) sprintf(exename,"\"%s\"",fullname);
else strcpy(exename,fullname);
// replace "%SystemRoot%" to "c:\\winnt.400" //NT
if(strstr((char *)extfile,"%SystemRoot%"))
{ 
char windir[MAX_PATH];
GetWindowsDirectory(windir,MAX_PATH);
at=strchr((char *)extfile,'\\');// after "%SystemRoot%"

strcat(windir,at);
strcpy((char *)extfile,windir);
}
// parameterse
strcpy(tmp,(char*)extfile);
if(at=strstr(tmp,"%1"))
{
*(at+1)='s';// "%s" 
sprintf(command,tmp,exename);
}
else
{
sprintf(command,"%s %s",tmp,exename);
}
RegCloseKey(hsubkey);
return TRUE;
}
else
{
command[0]=0;
return FALSE;
}
}
}

------解决方案--------------------