In a small bowl, whisk together salt, vinegar, garlic, and mustard. Whisking constantly, slowly whisk in oil until incorporated. Whisk in chives and pepper.
Bring a pot of salted water to a boil. Drop in haricots verts and cook until just tender, 2 to 3 minutes. Drain, cool and chop into bite-size pieces.
In a large bowl, toss together haricots verts, corn and carrot. Toss in dressing and season with salt and pepper.