Concept:

A solenoid is an instrument that consists of copper coiling over a cylinder designed to create a strong magnetic field inside the coil because of the flow of current through the coil.
By wrapping the same wire many times around a cylinder, the magnetic field due to the flow of current can become quite strong.
Hence we can say that the strength of the magnetic field will change as a current through coil or number of turns changes.
Magnetic field strength is independent of the diameter of the cylinder of a solenoid.
The strength of the magnetic field in a solenoid will be directly proportional to the number of turns and amount of current flowing through a wire and will be inversely proportional to its length and it is given as

Where N = number of turns and I = current, l = length of the solenoid

Explanation:

Inside a long current carrying a straight solenoid, magnetic field lines are straight and parallel.
The strength of parallel magnetic field lines indicates that the magnetic field is uniform everywhere inside a long solenoid.​
i.e. The strength of the magnetic field inside a long current-carrying solenoid is uniform everywhere.