Your daughter's desire to choose from more options for how she dresses is normal and healthy. If you feel like you are in a battle over her desire to have more choice and more freedom about her dress, you have to check your sense of struggle against your ultimate goal. So, ask yourself, "is this something worth battling over?" We lived in Montana, where the wind chill factor can get as low as 20 degrees below zero, and my daughter wore dresses every day. This lasted until she started first grade. Then she never wanted to wear dresses because no one else wore dresses to school. So this struggle is temporary. I noticed other ladies mentioned the tights designed with snaps at the crotch to accommodate potty training. These work great. What I wanted to respond to was the matter of "battles". The best advice anyone ever gave me was "pick your battles" and "if you have a battle, make sure that you always win the battle." At the age of 2, your daughter is asserting her desire to pick clothes for herself, to refuse the things you've chosen as her options, because she wants real control over her choices. If her choice in this matter does not pose a risk to her health or safety, it is an ideal situation to begin the process of informing her about things she should consider as she makes choices for herself. It's also an ideal time to talk with her about what she likes and doesn't like, and to share with her what you like and dislike (and why).
In my experience, my difficulty was always my reaction to my daughter's choices and behavior, not the actual outcome of her choice or behavior. Her independence often seemed to resist my authority and disrespect me. But it was focused on her own needs. When I consider how she needs to be self-sufficient, confident, happy, and stable, I realize that I should stop trying to be my daughter's boss. She just needs guidance to make the right choices. This principle has worked for me for the last 12 years. This kind of struggle doesn't actually go away. Remember: you are the safest avenue for your children to try on assertiveness and as long as they feel safe with you, they will always work out new behaviors with you first. Let children assert themselves and they will become both confident and independent. And when you find yourself reacting, remember, it's not about you. :-) Good luck.