Why is the net force on a current carrying solenoid in uniform magnetic field equal to zero
You can consider a solenoid as a setup containing a few closely stacked circular loops.you can now broke the problem as to find the force on a circular loop due to a uniform field and integrate it furthet to find it for whole.
The force on a circular loop comes out to be zero since F=i.dl×B.
Therefore the force at any small element of the loop comes out to be radially outward and vectorially would add upto zero.
Similar will be the scenario for other loops and hence net force comes out to be zero.