C# .NET - How to close opened QBW file in Quickbooks from integrated C# desktop application

I had developed Quickbooks integration desktop application in c#. It is working fine with one QBW file which is opened in Quickbooks. But What I want is - Connecting one QBW file and then adding data to it and then close this QBW file and then connecting another QBW file and sending data to that.
I am getting, error when connecting to another QBW file - A QuickBooks company data file is already open and it is different from the one requested or there are multiple company files open.

You'd have to post some of your code.  Something is not permanently closing the connection to the file.

strCompanyFile = "D;/Test..QBW";
Item objAcc = new Item("US", 6, 0, strCompanyfile, "Test QB");                        

Item.sessionManager.OpenConnection(strCompanyfile, "Test QB");
Item.sessionManager.BeginSession(strCompanyfile, ENOpenMode.omDontCare);

then added Item to this company file and then

if (dsItem.Tables[0].Rows[i]["QBCompanyId"] != dsItem.Tables[0].Rows[i + 1]["QBCompanyId"])
 {
   Item.sessionManager.EndSession();
   Item.sessionManager.CloseConnection();
  }

This complete code is in loop and should work for next company as from database the next company is D:/ABC.QBW so the strCompanyFile will be "D:/ABC.QBW" and the same code will execute then.

But when I am runnign this code it is showing error that - A QuickBooks company data file is already open and it is different from the one requested or there are multiple company files open.

This line never is never used?

Item objAcc = new Item("US", 6, 0, strCompanyfile, "Test QB");  

Item.sessionManager looks like it is static because I don't see where you create an instance of "Item".  Are you sure it should be like this?  I would expect database oriented classes to implement IDisposable so everything can be destroyed when the object is done with the connection.

Put a breakpoint in here:  I don't think it is "always" closing your session. 

if (dsItem.Tables[0].Rows[i]["QBCompanyId"] != dsItem.Tables[0].Rows[i + 1]["QBCompanyId"])
 {
   Item.sessionManager.EndSession();
   Item.sessionManager.CloseConnection();
  }           

This code is in my Item class -

public Item(string country, short majorVersion, short minorVersion, string filePath, string applicationName)
      {
        if (country.Length > 0) Item.QBVersionn.country = country;
        if (majorVersion > 0) Item.QBVersionn.majorVersion = majorVersion;
        if (minorVersion > 0) Item.QBVersionn.minorVersion = minorVersion;
        sessionManager = null;
        sessionManager = new QBSessionManager();

        if (filePath.Length > 0 && File.Exists(filePath)) CompanyFilePath = filePath;
        if (filePath.Length > 0 && File.Exists(filePath)) this.logFilePath = filePath.Substring(0, filePath.LastIndexOf("\\") + 1) + "QBInterface.log";
        if (applicationName.Length > 0) ApplicationName = applicationName;
      }

Can anybody help me to resolve this issue