custom validation on work item in team foundation server-2010

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C#

C#4.0

TFS

hi friends,
i am working on Team Foundation Server-2010, so i want to perform the custom validations on desired work item. how can we deploy that using C#
(note:not using power tools) please help to me.

1)suppose if previous sprint end date has not closed.then not possible to create new sprint.

any body help to me how can we perform particular validation on work item.

thanks in advance

thanks,
nareshrajuu