Get and Set Dynamics CRM Attribute Values in c#

Set OptionsetValue:

entity.Attributes.Add("attributename", new OptionSetValue(1));

Set Money Attribute:

Decimal  creditlimit=0;

entity.Attributes.Add("attributename", new Money(creditlimit));

Set Lookup:

EntityReference _event = new EntityReference();
                        _event.Id = guid;
                        _event.LogicalName = "entityname";
                        _event.Name = name;
                        entity.Attributes["attributename"] = _event;

DateTime:

entity.Attributes.Add("attributename", Start_date);