How can I convert comma separated string into a List<int>

Here is one way of doing it:

List<int> TagIds = tags.Split(',').Select(int.Parse).ToList();

If you want to include some simple validation and skip over invalid values (instead of throwing an exception), here's something that uses TryParse:

string csv = "1,2,3,4,a,5";
int mos = 0;
var intList = csv.Split(',')
                    .Select(m => { int.TryParse(m, out mos); return mos; })
                    .Where(m => m != 0)
                    .ToList();

//returns a list with integers: 1, 2, 3, 4, 5

EDIT: Here is an updated query based on feedback by Antoine. It calls TryParse first to filter out any bad values, and then Parse to do the actual conversion.

string csv = "1,2,3,4,a,5,0,3,r,5";
int mos = 0;
var intList = csv.Split(',')
                    .Where(m => int.TryParse(m, out mos))
                    .Select(m => int.Parse(m))
                    .ToList();

//returns a list with integers: 1, 2, 3, 4, 5, 0, 3, 5

Edit 2: An updated query for C# 7.0, thanks to feedback from Charles Burns. Note that we get rid of the extra mos variable with this approach, so it's a bit cleaner.

string csv = "1,2,3,4,a,5,0,3,r,5";
var intList = csv.Split(',')
                 .Where(m => int.TryParse(m, out _))
                 .Select(m => int.Parse(m))
                 .ToList();

You can use LINQ w/ int.Parse() to convert the string[] to an IEnumerable<int> and then pass that result to the List<T> constructor:

var tagIds = new List<int>(tags.Split(',').Select(s => int.Parse(s)));