Error in post_executation in SSIS script task.

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SSIS

I have a SSIS package with script task element. Input columns are directed from excel source through data conversion. 2 variables are taken as Readonly Variables in script task.

All the rows processed well through my code but after processing while executing Post_Execute methode. There is no code in post_ececute.But the error is occuring. Error showing

Unable to cast COM object of type 'System.__ComObject' to interface type 'Microsoft.SqlServer.Dts.Runtime.Wrapper.IDTSVariables100'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{D1A469BC-F371-4D15-832F-BACB50C162E9}' failed due to the following error: The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

I am using VS2015 and SQL Server 2017.

What I have tried:

Tried changing variables from readonly to readwrite variables but doesn't work.