Count(), Any()

Count(), Any().cs

void M<T>(ICollection<T> source) {
  var a = source.Any(); // OK, statically known, that source is ICollection<T> and call is optimized
  var b = source.Count(); // Warning, source.Count is better
}

bool M1<T>(IEnumerable<T> source) {
  return source.Any(); // OK - one iteration in a worst case
}

int M1<T>(IEnumerable<T> source) {
  return source.Count(); // OK - see above
}

void M2<T>(IEnumerable<T> source) {
  if(source.Any()) { // // Warning - two iterations in a worst case
    foreach(var item in source) {…
  }//if
}