读一个文档在加cstring写入另一个文档

void C加字符到txtDlg::OnBnClickedButton4()
{
CString str;
try
{
CFile file1(_T("D:\11\1.txt"), CFile::modeRead);
DWORD txtSize = (DWORD)file1.GetLength();
char* buf = new char[txtSize + 1];
memset(buf, 0, sizeof(char)* (txtSize + 1));
file1.Read(buf, txtSize);

           str = CString(buf);
           delete[] buf;
           buf = NULL;
           file1.Close();
        }
        catch (CException* e)
        {
           e->ReportError();
           e->Delete();
        }
        MessageBox(str); //能读取
        try
        {
           CFile   f(_T("D:\\11\\0.txt"), CFile::modeCreate | CFile::modeWrite | CFile::modeNoTruncate);
           f.SeekToEnd();
           CString a =_T("D:\\11\\0.txt ")+ str; //但无法写入
           f.Write(a, a.GetLength() * sizeof(TCHAR));
           f.Close();
        }
        catch (CException* e)
        {
           e->ReportError();
           e->Delete();
        }

}