Moving file from folder to folder

:20:S10734539/940

am reading swift messages from a folder, and at line :20: I perform some sort of validation to know if the file and reading is valid. In this case if line :20: has 940 in it, the windows service reads and moves it to a success folder when done. But an invalid file will not have 940 on line :20: , the windows service is meant to move the invalid file to an invalid file location. I have written a code to do that, but it not able to move the file. I get an error message 'file in use' find below snippets of my code.

    if (Directory.Exists(@CBN_INFLOW940))
DirectoryInfo dr = new DirectoryInfo(CBN_INFLOW940);
                        FileInfo[] fi = dr.GetFiles();
                        string[] files = Directory.GetFiles(CBN_INFLOW940);

int lengthchk = 0;

if (files.Length < 1)
    check = false;

while (files.Length > lengthchk)
{
    StringBuilder sb = new StringBuilder();
    logger.Info(fi[lengthchk].Name + ": read from folder");
    string narrationgen = "";
    bool isvalidrtgs = false;
    DateTime createddate = fi[lengthchk].CreationTime.Date;
    FileStream stream = null;
    try
    {
    stream = File.Open(files[lengthchk], FileMode.Open, FileAccess.ReadWrite, FileShare.None);
    StreamReader sf = new StreamReader(stream);

     }

and

if (line.Contains(":20:"))
{
        firstchk = 1;
        if (!line.Contains('/'))
        {
            string[] fnamesplit = fi[lengthchk].Name.Split('.');
            string newfname = fnamesplit[0] + "_bk" + ".txt";
            string destlocation = Invalidfilelocation940 + newfname;
            string sourcelocation = CBN_INFLOW940 + fi[lengthchk]; //  + "\\"
            File.Move(sourcelocation, destlocation);
            return;
        }
        string[] narr = line.Split('/');
        string filecode = narr[1];

        if (filecode.Trim() != "940")
        {
            string[] fnamesplit = fi[lengthchk].Name.Split('.');
            string newfname = fnamesplit[0] + "_bk" + ".txt";
            string destlocation = Invalidfilelocation940 + newfname;
            string sourcelocation = CBN_INFLOW940 + "\\" + fi[lengthchk];
            File.Move(sourcelocation, destlocation);

            return;
        }
}