Question

I have this code:

private void test(List<string> a)
{

}

I want to use for in for loop’s and to check that if there are duplicated items in the List a
Then mark it with null.

Answer 1

Not sure exactly what you want to do here but if you wanted to just get a collection of the distinct values in the list you could use LINQ to do this:

var distinctOnes = a.Distinct();