All the time keep forgetting this, while is pretty simple to create a list of string values separated by any character

string.Join(", ", myCollection.Select(p => p.myProperty));

the full LINQ version with aggregate, despite being less performing for that simple case, can provide a flexible way to handle more complex scenario

myCollection
     .Select(p => p.myProperty)
     .Aggregate((current, next) => current + ", " + next);