The magnetic field inside a current carrying very long solenoid is

For a very long current-carrying solenoid, the magnetic field at any point well inside it is given by $B = \mu_0 n I$, where $\mu_0$ is the permeability of free space, $n$ is the number of turns per unit length, and $I$ is the steady current.

Since the parameters $n$ and $I$ are constant, the magnetic field has a constant magnitude. Furthermore, the direction of the magnetic field is parallel to the axis of the solenoid throughout its interior.

Therefore, the magnetic field inside a very long solenoid is uniform.