Generate Random Strings with High Performance with a SQL CLR function

using System;
using System.Data;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
using System.Security.Cryptography;
using System.Text;

public partial class UserDefinedFunctions
{
    [Microsoft.SqlServer.Server.SqlFunction]
    public static SqlString fn_random_string(SqlInt32 minLen, SqlInt32 maxLen, SqlInt32 seed)
    {
        int min_i = (int)minLen;
        int max_i = (int)maxLen;

        int i = 0;
        if (min_i <= 0 || min_i > max_i)
        { return new SqlString(string.Empty); }
        else
        {
            int sd = (int)seed;
            Random r = new Random();
            if (sd != 0)
            {
                r = new Random(sd);
            }

            i = r.Next(min_i, max_i + 1);
            byte[] rnd = new byte[i];
            using (var rng = new RNGCryptoServiceProvider())
            {
                rng.GetNonZeroBytes(rnd);
                string rs = Convert.ToBase64String(rnd);
                rs = rs.Substring(0, i);
                return new SqlString(rs);
            }
        }
    } //fn_random_string


    public static SqlString fn_random_pattern(SqlString pat, SqlInt32 seed)
    {
        string pattern = pat.ToString();
        if (pattern == string.Empty)
        { return new SqlString(string.Empty); }
        else
        {
            string CharList = "abcdefghijklmnopqrstvvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string NumList = "0123456789";
            char[] cl_a = CharList.ToCharArray();
            char[] nl_a = NumList.ToCharArray();
            int sd = (int)seed;
            Random rnd = new Random();
            if (sd != 0)
            {
                rnd = new Random(sd);
            }

            StringBuilder sb = new StringBuilder(pattern.Length);

            char[] a = pattern.ToCharArray();
            for (int i = 0; i < a.Length; i++)
            {
                switch (a[i])
                {
                    case '@':
                        sb.Append(cl_a[rnd.Next(0, CharList.Length)]);
                        break;
                    case '!':
                        sb.Append(nl_a[rnd.Next(0, NumList.Length)]);
                        break;
                    default:
                        sb.Append(a[i]);
                        break;
                }
            }//for
            return new SqlString(sb.ToString());
        }//else
    } // fn_random_pattern
} //UserDefinedFunctions

use MSSQLTips -- this is my test database
alter database MSSQLTips set trustworthy on;
exec sp_configure 'clr enabled', 1;
reconfigure with override
go

create assembly clr_random_string
from 'C:\mssqltips\Random_String\bin\CLR_Rand_String.dll'
with permission_set = safe
go

create function dbo.ucf_random_string (@minLen int, @maxLen int, @seed int =0)
returns nvarchar(max) with execute as caller
as 
external name [clr_random_string].[UserDefinedFunctions].fn_random_string;
go

/*
@pattern: @ means one letter from a to z (both lower and upper cases), ! means one digit number, i.e. 0 to 9. Anything else will not change.
so if we have a @pattern='abc !! def', then we may have strings like 'abc 12 def' or 'abc 87 def' generated.
For canadian post code the pattern can be '@!@ !@!' (the middle blank space will be kept as it is in the generated string, like 'V1A 2P5'
*/
create function dbo.ucf_random_pattern (@pattern nvarchar(max), @seed int=0 )
returns nvarchar(max) with execute as caller
as 
external name [clr_random_string].[UserDefinedFunctions].fn_random_pattern;
go

-- test performance between CLR and T-SQL
Use MSSQLTips;
set nocount on;
declare @i int=1, @start_time datetime = getdate(); 
declare @str varchar(8000), @seed int;
declare @t_tsql table (run_count int, duration_ms int); -- for tsql execution stats
declare @t_clr table (run_count int, duration_ms int); -- for clr execution stats
-- run tsql solution 20,000 times
while @i <= 20000
begin
set @seed = @i;
exec dbo.usp_generateIdentifier
@minLen = 2000
, @maxLen = 4000
, @seed = @seed output
, @string = @str output;

if (@i % 2000 = 0)
  insert into @t_tsql (run_count, duration_ms)
  select @i, datediff(ms, @start_time, getdate());
  set @i = @i+1;
end

select @i = 1, @start_time = getdate(); -- reinitialize variable
-- run clr solution 20,000 times
while @i <= 20000
begin
set @seed = @i;
select @str = dbo.ucf_random_string(2000, 4000, @seed)
if (@i % 2000 = 0)
  insert into @t_clr (run_count, duration_ms)
  select @i, datediff(ms, @start_time, getdate());
set @i = @i+1;
end

select t1.run_count, tsql_duration=t1.duration_ms, clr_duration=t2.duration_ms
from @t_tsql t1
inner join @t_clr t2
on t1.run_count = t2.run_count;