The Sonata that is considered the first major Sonata for the violin-piano repertoire is the Sonata in B-flat Kv.378. This piece was written most probably in early 1779 in Salzburg but certain sources say that this is not completely sure and suggest that it might as well have been written in 1780 or 1781 before he went to Vienna. Though not written with a dedication, Alfred Einstein suggests in his book "Mozart, his character, his work" that they were 'Doubtless for Marianne and his father' (P.255).
It was combined with the Kv.296 Sonata and four sonatas in his Op.II which were published in November 1781, dedicated to Josepha von Aurnhammer and were marked 'Six Sonatas pour le Clavecin ou Pianoforte , avec l'accompaniment d'un Violon'. With publishing this set, Mozart got great reviews, which was rather uncommon.
The first movement of this Sonata, like the Sonata Kv.306, opens with the same composition style in which the violin plays the same as the left hand of the piano and the right hand of the piano has the main line. This movement shows, definitely, many comparisons in composition style and treatment of the instruments from the Sonata Kv.306. Its character can be compared to the Kv.301 Sonata which is calm and storytelling. The development section is not as extended but starts with more counterpoint aspects than he wrote before and is more complex in that regard. The return of the A-section does not show the major changes like in for example the Sonata Kv.305.
The second movement is, again like the Sonata Kv.306 an Andante Cantabile and is linked by certain sources with the opera 'Die Entfuhrung aus dem Serail' that he wrote in the same period. Both instruments get their chance to show what they can do with the instrument melody-wise. A fermata which suggests an improvised cadensa is suitable to return back to the main theme, where the same Fermata is written not much further.
The third movement, in which the Rondo form returns, is a very virtuosic and uplifting movement. Both instruments are integrated and sometimes Mozart wrote in a Canonic style. The fast Allegro part is very virtuosic in the sense of chamber-music playing and for both instruments technically. Before the Rondo theme comes back to close the sonata Mozart wrote again in Canonic style and a bridge in which both the right hand and the violin play in a short written out cadence. The Sonata ends with the same virtuosity that is present from the beginning in this movement.