How to find exact matching number from comma separated string in LINQ

I have a column in database, it contains error codes like "2,3,7,5,6,17" and I have taken it as a string and I want to exact match digit using linq.

For example I want to find match 7 (This number is taking dynamically) inside this string then it will return true. But the problem is, string also contain 17 so its fetching row which contains 17 also even if 7 is not present. I have used Contains() for that. Kindly give me the exact solution of it so I can return rows where we got exact match inside this string.

Note: This string I am fetching from the Database using LINQ

Image to show Rows fetched from db

In this image you can see the number 17 row also fetched but it doesn't contain 7. I want only for the 7 (exact match only)

Code that I have used

List<TableModel> result = from DN in dbContext.Example where DN.ErrorType.Contains(ErrorTypetxt).Select({})

DN.ErrorType is a complete string fetched from the db i.e 2,3,7,5,6,17

ErrorTypetxt is a 7 that I want to exact match..

How to resolve this?

var num = 7;
var start = $"{num},";
var end = $",{num}";
var mid = $",{num},";
var same = $"{num}";

var query =
  from dn in source
  where
    dn.ErrorType.StartsWith(start) ||
    dn.ErrorType.Contains(mid) ||
    dn.ErrorType.EndsWith(end) ||
    dn.ErrorType == same
  select dn;

Or

var num = 7;
var mid = $",{num},";

var query =
  from dn in source
  let modified = "," + dn.ErrorType + ","
  where modified.Contains(mid)
  select dn;

var expected = "7";
var queryResult = "1,2,17,7";
var splitResult = queryResult.Split(',');
bool result = !string.IsNullOrEmpty(splitResult.FirstOrDefault(s => string.Equals(s, expected)));
//returns true

You can solve this using split first and then use the contain.
Please check this
https://dotnetfiddle.net/80k13h

Update after the first comment:
use this structure

TestTables
.Where(x => 
    ("," + x.Subjects + ",").Contains(",7,") ||
    (x.Subjects.EndsWith("7") && x.Subjects.Length == 1) ||
    (x.Subjects.StartsWith("7,") && x.Subjects.Length == 2) ||
    (x.Subjects.Contains(",7,") && x.Subjects.Length > 2 && 
        x.Subjects[0] != '7' && x.Subjects[x.Subjects.Length - 1] != '7')
)
.ToList()

This generates the following SQL in linqpad as give the expected result.