Check IAoInitialize.Shutdown Method:
The ArcObject environment should be initialized on startup and then shutdown on closing.
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using ESRI.ArcGIS;
using ESRI.ArcGIS.esriSystem;
// Initialize the ArcObject components runtime environment
// This class must be the first ArcObject created
private IAoInitialize m_AoInitialize = new AoInitializeClass();
////////////////////////////////////////////////////
// Call this from the closing event
public void FreeMapResources()
{
//Shut down the initialize
Utility.COMSupport.AOUninitialize.Shutdown();
//Check for the object
if(m_AoInitialize != null)
{
//Shut it down
m_AoInitialize.Shutdown();
}
}