Start a Work Flow By Click on Button

 SPWorkflowManager objWorkflowManager = null; 

                                              SPWorkflowAssociationCollection objWorkflowAssociationCollection = null; 

                                              objWorkflowManager = listitem.Web.Site.WorkflowManager; 

                                              objWorkflowAssociationCollection = listitem.ParentList.WorkflowAssociations; 

  

                                              foreach (SPWorkflowAssociation objWorkflowAssociation in objWorkflowAssociationCollection) 

                                              { 

                                                  web.AllowUnsafeUpdates = true; 

                                                  objWorkflowManager.StartWorkflow(listitem, objWorkflowAssociation, objWorkflowAssociation.AssociationData, true); 

                                                  web.AllowUnsafeUpdates = false; 

                                                  message = "\\\"Mail has been sent to " + mailUsers.TrimEnd('.') + ", Workflow started.\\\"";