But I'm not not searching. I want to perform some operation on the subsets.(which is not sum) So how will string search algorithms help me?
I looked up one algortihm Boyer-Moore algorithm but it's not of any help.

That is really dependent of the nature of function applied and if there is overlap with previous answers and recursive function, you can sometimes reverse it to build dynamic programming solution or if it is difficult use memoized recursive function. I have recently done some subrange sums, but not in C.

for eg. if we need to find a sub array such that the (sum of the sub array) mod b is atleast equal to k, then what are the sort of functions that we can use?

Are all values positive, if not bow you define modulo for negative values?
How many values there are in array?
How big is b, how is range of values, are they random or for example typically small?

I'm trying to write a program in C# that downloads a file from my DropBox account but I keep getting an error with var task = Task.Run((Func<Task>)Download("", "largetest.mpk", folderName)); The ...