simple random alpha char string variable in C#

RandomAlphaString.cs

private const int _length = 15;
private static string RandomAlphaString {
	get {
		var ret = "";
		var random = new Random();
		while (ret.Length < _length)
			if (random.Next(1, 9) % 2 == 0)
				ret = ret + Convert.ToChar(random.Next(65, 90));
			else
				ret = ret + Convert.ToChar(random.Next(97, 122));
		return ret;
	}
}