Generate random string

Current code is generate string in defined length and include chars, numbers and symbols.

public static string GetRandomString(int size)
{
    StringBuilder builder = new StringBuilder();
    Random random = new Random();
    int[] symbol = new[] { 33, 36, 38, 64, 94 };
    int[] digits = new[] { 48, 49, 50, 51, 52, 53, 54, 55, 56, 57 };
    int symbolsAmount = size / 3;
    int digitAmount = size / 3;
    for(int i = 0; i < size; i++)
    {

        int switcher = random.Next(3);
        char ch;
        if(switcher == 0 && symbolsAmount > 0)
        {
            int id = random.Next(symbol.Length);
            ch = Convert.ToChar(symbol[id]);
            symbolsAmount--;
        }
        else if(switcher == 1 && digitAmount > 0)
        {
            int id = random.Next(digits.Length);
            ch = Convert.ToChar(digits[id]);
            digitAmount--;
        }
        else
        {
            bool isLower = random.Next(2) == 0 ? false : true;
            ch = Convert.ToChar(
                 Convert.ToInt32(
                 Math.Floor(26 * random.NextDouble() + ((isLower) ? 65 : 97))
                     )
                 );

        }
        builder.Append(ch);
    }

    return builder.ToString();
}