public void SyncWithoutRelinquishing(Document doc)
{
    // Set options for accessing central model
    TransactWithCentralOptions transOpts = new TransactWithCentralOptions();
    SynchLockCallback transCallBack = new SynchLockCallback();
    // Override default behavior of waiting to try again if the central model is locked
    transOpts.SetLockCallback(transCallBack);

    // Set options for synchronizing with central
    SynchronizeWithCentralOptions syncOpts = new SynchronizeWithCentralOptions();
    // Sync without relinquishing any checked out elements or worksets
    RelinquishOptions relinquishOpts = new RelinquishOptions(false);
    syncOpts.SetRelinquishOptions(relinquishOpts);
    // Do not automatically save local model after sync
    syncOpts.SaveLocalAfter = false;
    syncOpts.Comment = "Changes to Workset1";

    try
    {
        doc.SynchronizeWithCentral(transOpts, syncOpts);
    }
    catch (Exception e)
    {
        TaskDialog.Show("Synchronize Failed", e.Message);
    }
}

// This class implements the ICentralLockedCallback to override Revit's default
// behavior for how to handle when the central model is locked by another user
class SynchLockCallback : ICentralLockedCallback
{
    // If unable to lock central, give up rather than waiting
    public bool ShouldWaitForLockAvailability()
    {
        return false;
    }

}

Public Sub SyncWithoutRelinquishing(doc As Document)
    ' Set options for accessing central model
    Dim transOpts As New TransactWithCentralOptions()
    Dim transCallBack As New SynchLockCallback()
    ' Override default behavior of waiting to try again if the central model is locked
    transOpts.SetLockCallback(transCallBack)

    ' Set options for synchronizing with central
    Dim syncOpts As New SynchronizeWithCentralOptions()
    ' Sync without relinquishing any checked out elements or worksets
    Dim relinquishOpts As New RelinquishOptions(False)
    syncOpts.SetRelinquishOptions(relinquishOpts)
    ' Do not automatically save local model after sync
    syncOpts.SaveLocalAfter = False
    syncOpts.Comment = "Changes to Workset1"

    Try
        doc.SynchronizeWithCentral(transOpts, syncOpts)
    Catch e As Exception
        TaskDialog.Show("Synchronize Failed", e.Message)
    End Try
End Sub

' This class implements the ICentralLockedCallback to override Revit's default
' behavior for how to handle when the central model is locked by another user
Private Class SynchLockCallback
    Implements ICentralLockedCallback
    ' If unable to lock central, give up rather than waiting
    Public Function ShouldWaitForLockAvailability() As Boolean Implements ICentralLockedCallback.ShouldWaitForLockAvailability
        Return False
    End Function

End Class