Get eeror cannot build Custom SSIS Task in 3.5 .NET and 4.0 for the same code run in 2.0 perfectly.

I want to create a same custom DLL (SSIS Task ) in VS 4.0 becouase i have one helper class which has already functions in 4.0 implementation and uses MSCRM 2011 DLL - Microsoft.Xrm.Sdk.DLL which used 4.0 .NET.

I have to use that existing custom DLL in SSIS Task.

1. When i am bulding  my SSIS Task in 4.0 .NET framwork with my existing custom DLL (which is in.NET 4.0 .NET framework) but could not able to add this SSIS Task DLL in GAC and if i put it manually in GAC and C:\Program Files\Microsoft SQL Server\100\DTS\Tasks or BINN then could not find these SSIS Custom DLL in SSIS Tool box "Choose Items" hence could not use in package canvas.Could you suggest me if any option use custom SSIS Task in Package ?


2. Also below line shows error :

read the existing variable in package  but works perfectly in .NET 2.O with Script Task in c#.NET :

 Dts.VariableDispenser.LockForRead("User::CustomerCount");
 Dts.VariableDispenser.LockForRead("User::MaxRecordCount");
 Dts.VariableDispenser.GetVariables(ref vars);

Any option?

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Sanjay