It's not safe to call ToList on IsReadOnly collection

gistfile1.cs

var cdict = new ConcurrentDictionary<int, int>();

var t = Task.Run(() => {
    for (int i = 0; i < 100000; i++)
        cdict[i] = i;
});

IEnumerable<KeyValuePair<int, int>> kvps = new ReadOnlyDictionary<int, int>(cdict);

((ICollection<KeyValuePair<int, int>>)kvps).IsReadOnly.Dump(); // true

while (!t.IsCompleted)
    kvps.ToList(); // almost always throws ArgumentException in CopyTo