For one, it needs to be kept in mind that shirt dresses come in all shapes and styles. There is no limit as to what such a dress can look like. There can be variation in fabric, in how the dress is stitched and even in colors. This means that you could buy a light color and get it stitched in a very formal way. There is no way such a dress could look casual! Dresses that resemble dress shirts, for example, can be worn. These have collars like formal shirts and the only difference is that of the length. Such a dress can be made to fit your body comfortably and it would be perfect for wearing to an office with moderate heels.
Another style these dresses come in is the informal one. This is where the dress is fitted till the waist, after which it is rather loose like a frock. It gives the look of wearing a top with a separate mini skirt. This is the look that may be attributed to teenagers. Such a dress can be ideally worn with skinny jeans. Mostly, these dresses come with a belt in a matching color. Contrary to usual belts, the belts for shirt dresses are usually made out of cloth and are similar to a string that can be tied around the waist.
Another style is that which is similar to trench coats. Yes, these dresses are so versatile that they can even be worn in winters! In the colder season, they are available in materials like leather. A decently designed coat in the form of a shirt can be ideal to wear with jeans. The style can also be accentuated with the help of long boots and jeans. Depending on your preferences, you can also go for wearing leggings.
All in all, contrary to the popular belief, shirt dresses do not necessarily have to be informal. They can be designed in a number of ways and can be worn to various places, including formal offices!