Question

Here is the string I am looking to modify:
170-0175-00B-BEARING PLATE MACHINING.asm:2

I want to keep “170-0175-00B“. So I need to remove the third hyphen and whatever is after it.

Answer 1

A rapid solution

string test = "170-0175-00B-BEARING PLATE MACHINING.asm:2";
int num = 2;
int index = test.IndexOf('-');
while(index > 0 && num > 0)
{
    index = test.IndexOf('-', index+1);
    num--;
}
if(index > 0)
    test = test.Substring(0, index);

of course, if you are searching for the last hyphen then is more simple to do

int index = test.LastIndexOf('-');
if(index > 0)
    test = test.Substring(0, index);