Generation of random alphanumeric string in C#

The following code snippet shows how to generate random alphanumeric string in C#.

using System;
using System.Text;

public string RandomString (Random r, int len)
{
string str
= "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";
StringBuilder sb = new StringBuilder();

while ((len--)> 0)
sb.Append(str[(int)(r.NextDouble() * str.Length)]);

return sb.ToString();
}