<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_string(self):
 """ Gets a string representation of the object. The result is a comma-separated list of projection fields "field1,field2.field21,field2.field22.field221" :return: a string representation of the object. """

        builder = ""

        index = 0
        while index < self.__len__():
            if index > 0:
                builder = builder + ','
            builder = builder + super(ProjectionParams, self).__getitem__(index)
            index = index + 1

        return builder