C# traverse dictionary to delete elements

//从后往前删
for (int i = dict.Count - 1; i >= 0; i--)
			{
				var item = dict.ElementAt(i);
				dict.Remove(item.Key);
			}
从前往后删
for (int i = 0; i < dict.Count )
			{
				var item = dict.ElementAt(i);
                if(true)
				    dict.Remove(item.Key);
                else
                    i++
			}