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why we need lock to read or write variables, if we don't use lock, sometimes a deadlock will occur.

writing variable:

private void WriteValue(string name,object value)
        {
            Variables variables = Dts.Variables;

            try
            {
                Dts.VariableDispenser.LockOneForWrite(name, ref variables);
                variables[name].Value = value;
            }
            finally
            {
                if (variables.Locked)
                {
                    variables.Unlock();
                }
            }

        }

reading variable:

private object ReadValue(string name)
        {
            Variables variables = Dts.Variables;

            try
            {
                Dts.VariableDispenser.LockOneForRead(name, ref variables);
                return variables[name].Value;
            }
            finally
            {
                if (variables.Locked)
                {
                    variables.Unlock();
                }
            }
        }

 more information:

http://msdn.microsoft.com/en-us/library/microsoft.sqlserver.dts.runtime.variabledispenser.lockforwrite.aspx