how to read edit control value continuously

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VS2008

Visual-Studio

MFC

, +

I have a edit control which has id is IDC_SearchTxtBox and corressponding string is m_strUN
When I click a button it reads edit control value sometime not read(why?)

if(m_strUN!="")
{
//todo
}
else
{
	MessageBox("Search text box is empty!", "SearchMessageBox",MB_ICONASTERISK | MB_OK);
}

and void abc::DoDataExchange(CDataExchange* pDX)
{
	CDialog::DoDataExchange(pDX);
	
	DDX_Text(pDX,IDC_SearchTxtBox,m_strUN);
}