C#-Code

<Window x:Class="Chess960_WPF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Chess960_WPF"
        mc:Ignorable="d"
        Title="Chess969_WPF" Height="500" Width="500" WindowStartupLocation="CenterScreen">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <StackPanel Margin="5" Orientation="Horizontal">
            <Button x:Name="bt_normal" Content=" startpos normal " Click="bt_normal_Click"/>
            <Button x:Name="bt_random960" Content=" random960 " Width="75" Click="bt_random960_Click" Margin="5,0,0,0"/>
        </StackPanel>
        <TextBox x:Name="textBox" Margin="5" Grid.Row="1" TextWrapping="Wrap" FontFamily="Chess Berlin" FontSize="48" Background="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
    </Grid>
</Window>

C#-Code

using System;
using static System.Console;
using System.Collections.Generic;
using System.Text;
using System.Windows;

namespace Chess960_WPF
{
    public partial class MainWindow : Window
    {
        private Random random = new Random();
        private Chess960_Class.CHESS960 c960;
        public MainWindow()
        {
            InitializeComponent();

            c960 = new Chess960_Class.CHESS960(random);
            WriteLine(c960.string_position_CONSOLE);            
            this.textBox.Text = c960.string_position_CHESS_BERLIN_normalschach;
        }

        private void bt_random960_Click(object sender, RoutedEventArgs e)
        {
            c960 = new Chess960_Class.CHESS960(random);
            this.textBox.Text = c960.string_position_CHESS_BERLIN;
        }

        private void bt_normal_Click(object sender, RoutedEventArgs e)
        {
            this.textBox.Text = c960.string_position_CHESS_BERLIN_normalschach;
        }
    }
}

namespace Chess960_Class
{
    public class CHESS960
    {
        public int[] position_white_officers { get; private set; }
        public int[] position_white_officers_normalschach { get; private set; }

        public string string_position_CONSOLE;
        public string string_position_CHESS_BERLIN;
        public string string_position_CHESS_BERLIN_normalschach;

        public CHESS960(Random rand)    {
            position_white_officers = new int[8];
            position_white_officers_normalschach = new int[8];
            for (var i = 0; i < position_white_officers_normalschach.Length; i++)
                position_white_officers_normalschach[i] = i;
            List<int> list = null;

            bool pos_valid = false;            
            while (!pos_valid)  {
                list = new List<int>();
                for (var i = 0; i < position_white_officers.Length; i++)    {
                    bool bo_okay = false;
                    int next = -1;
                    while (!bo_okay)    {
                        next = rand.Next(8);
                        bo_okay = !list.Contains(next);                                                    
                    }
                    list.Add(next);
                }
                pos_valid = bo_valid(list);                
            }
            position_white_officers = list.ToArray();

            string_position_CONSOLE = position(position_white_officers);
            string_position_CHESS_BERLIN = position_CHESS_ALPHA(position_white_officers);
            string_position_CHESS_BERLIN_normalschach = position_CHESS_ALPHA(position_white_officers_normalschach);
        }

        private bool bo_valid(List<int> input)  {
            int king = 4, 
                pos_king = input.IndexOf(king);
            int rock1 = 0, rock2 = 7, 
                pos_rock1 = input.IndexOf(rock1), pos_rock2 = input.IndexOf(rock2);
            int bishop1 = 2, bishop2 = 5, 
                pos_bishop1 = input.IndexOf(bishop1), pos_bishop2 = input.IndexOf(bishop2);
            return 
                Math.Max(pos_rock1, pos_rock2) > pos_king && 
                Math.Min(pos_rock1, pos_rock2) < pos_king && 
                (pos_bishop1 + pos_bishop2) % 2 == 1;
        }

        private static string position(int[] L) {
            StringBuilder sb = new StringBuilder();
            string[][] officers = new string[][]
            {
                new string[] { "wT ", "wS ", "wL ", "wD ", "wK ", "wL ", "wS ", "wT " },
                new string[] { "bT ", "bS ", "bL ", "bD ", "bK ", "bL ", "bS ", "bT " }
            };
            string[] pawns_blank = new string[] { "wP ", "sP ", " . " };

            for (var j = 0; j < 8; j++)
                sb.Append(officers[1][L[j]]);
            sb.AppendLine();
            for (var j = 0; j < 8; j++)
                sb.Append(pawns_blank[1]);
            sb.AppendLine();
            for (var i = 0; i < 4; i++) {
                for (var j = 0; j < 8; j++)
                    sb.Append(pawns_blank[2]);
                sb.AppendLine();
            }
            for (var j = 0; j < 8; j++)
                sb.Append(pawns_blank[0]);
            sb.AppendLine();
            for (var j = 0; j < 8; j++)
                sb.Append(officers[0][L[j]]);

            return sb.ToString();
        }

        //http://www.enpassant.dk/chess/downl/berlin.zip

        private static string position_CHESS_ALPHA(int[] L)
        {
            StringBuilder sb = new StringBuilder();

            string[][] officers = new string[][]
            {
                new string[] { "R", "H", "B", "Q", "K", "B", "H", "R", "r", "h", "b", "q", "k", "b", "h", "r",},
                new string[] { "t", "j", "n", "w", "l", "n", "j", "t", "T", "J", "N", "W", "L", "N", "J", "T" }
            };
            string[] pawns_blank = new string[] { "P", "o", " ", "p", "O", "+" };

            for (var j = 0; j < 8; j++)
            {
                int jj;
                jj = j % 2 == 0 ? 0 : 8;
                sb.Append(officers[1][L[j] + jj]);
            }
            sb.AppendLine();

            for (var j = 0; j < 8; j++)
            {
                int jj;
                jj = j % 2 == 1 ? 1 : 4;
                sb.Append(pawns_blank[jj]);
            }
            sb.AppendLine();

            for (var i = 0; i < 4; i++)
            {
                for (var j = 0; j < 8; j++)
                {
                    int jj;
                    jj = (j + i) % 2 == 0 ? 2 : 2 + 3;
                    sb.Append(pawns_blank[jj]);
                }
                sb.AppendLine();
            }

            for (var j = 0; j < 8; j++)
            {
                int jj;
                jj = j % 2 == 1 ? 0 : 0 + 3;
                sb.Append(pawns_blank[jj]);
            }
            sb.AppendLine();

            for (var j = 0; j < 8; j++)
            {
                int jj;
                jj = j % 2 == 0 ? 0 : 8;
                sb.Append(officers[0][L[j] + jj]);
            }

            return sb.ToString();
        }
    }
}