Hello The problem is not for selecting a object but a group of parts of this object .For example,i have an object with 3 parts .I make a group,i select the group in proprieties window or in the sequencer or other way But when i try to move the object with mouse i have diificulty with moving all and the group is deselecting and one of the 3 part always becoming selected in place of the group .Which the best solution? I need this because several times i import file the object is very often out of the window box or on the border of this perspective box ,so when i try to move ,it is difficult(excuse my poor english)

Make sure that you don't double-click on the grouped objects. If you do, you will "jump-in" on the group and they will become un-grouped (you are jumping into the group, not another modeler). Just click on one of the objects within the group, hold and drag and the whole group should move.

Also look in the sequencer, make sure the group isn't expanded.

Thanks I just try some files with check in the sequencer if the group is expanded.I think it was my problem with lot of my selecting-deselecting problems.Thanks again