<Window x:Class="dotnetthoughts.net.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="clr-namespace:System;assembly=System"
Title="WPF SpellCheck Demo" Height="350" Width="525" Loaded="Window_Loaded">
<DockPanel>
<TextBox Name="txtEdit" SpellCheck.IsEnabled="True"
                AcceptsReturn="True"
ContextMenuOpening="txtEdit_ContextMenuOpening"
                VerticalScrollBarVisibility="Visible">
<TextBox.ContextMenu>
<ContextMenu Name="ctxMenu" />
</TextBox.ContextMenu>
</TextBox>
</DockPanel>
</Window>

And here is the code behind

namespace dotnetthoughts.net
{
    using System;
    using System.IO;
using System.Linq;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Documents;
    using System.Windows.Input;

public partial class MainWindow : Window
    {
public MainWindow()
        {
InitializeComponent();
        }

private void txtEdit_ContextMenuOpening(object sender, ContextMenuEventArgs e)
        {
            int index = 0;
this.txtEdit.ContextMenu.Items.Clear(); //Clearing the existing items
            //Getting the spellcheck suggestions.
SpellingError spellingError = this.txtEdit.GetSpellingError(this.txtEdit.CaretIndex);
if (spellingError != null && spellingError.Suggestions.Count() >= 1)
            {
                //Creating the suggestions menu items.
foreach (string suggestion in spellingError.Suggestions)
                {
MenuItem menuItem = new MenuItem();
                    menuItem.Header = suggestion;
menuItem.FontWeight = FontWeights.Bold;
menuItem.Command = EditingCommands.CorrectSpellingError;
                    menuItem.CommandParameter = suggestion;
menuItem.CommandTarget = this.txtEdit;
this.txtEdit.ContextMenu.Items.Insert(index, menuItem);
                    index++;
                }
Separator seperator = new Separator();
this.txtEdit.ContextMenu.Items.Insert(index, seperator);
                index++;
//Adding the IgnoreAll menu item
MenuItem IgnoreAllMenuItem = new MenuItem();
                IgnoreAllMenuItem.Header = "Ignore All";
IgnoreAllMenuItem.Command = EditingCommands.IgnoreSpellingError;
IgnoreAllMenuItem.CommandTarget = this.txtEdit;
                this.txtEdit.ContextMenu.Items.Insert(index, IgnoreAllMenuItem);
                index++;
            }
            else
            {
//No Suggestions found, add a disabled NoSuggestions menuitem.
                MenuItem menuItem = new MenuItem();
menuItem.Header = "No Suggestions";
                menuItem.IsEnabled = false;
this.txtEdit.ContextMenu.Items.Insert(index, menuItem);
                index++;
            }
//.Net 4.0 Supports CustomDictionaries, Option for Adding to dictionary.
int selectionStart = this.txtEdit.GetSpellingErrorStart(this.txtEdit.CaretIndex);
            if (selectionStart >= 0)
            {
                Separator seperator1 = new Separator();
this.txtEdit.ContextMenu.Items.Insert(index, seperator1);
                index++;
MenuItem AddToDictionary = new MenuItem();
                AddToDictionary.Header = "Add to Dictionary";
                //Getting the word to add
this.txtEdit.SelectionStart = selectionStart;
this.txtEdit.SelectionLength = this.txtEdit.GetSpellingErrorLength(this.txtEdit.CaretIndex);
                //Ignoring the added word.
AddToDictionary.Command = EditingCommands.IgnoreSpellingError;
AddToDictionary.CommandTarget = this.txtEdit;
AddToDictionary.Click += (object o, RoutedEventArgs rea) =>
                {
this.AddToDictionary(this.txtEdit.SelectedText);
                };
this.txtEdit.ContextMenu.Items.Insert(index, AddToDictionary);
                index++;
            }

//Common Edit MenuItems.
            Separator seperator2 = new Separator();
this.txtEdit.ContextMenu.Items.Insert(index, seperator2);
            index++;
            //Cut
MenuItem cutMenuItem = new MenuItem();
cutMenuItem.Command = ApplicationCommands.Cut;
this.txtEdit.ContextMenu.Items.Insert(index, cutMenuItem);
            index++;
            //Copy
MenuItem copyMenuItem = new MenuItem();
copyMenuItem.Command = ApplicationCommands.Copy;
this.txtEdit.ContextMenu.Items.Insert(index, copyMenuItem);
            index++;
            //Paste
MenuItem pasteMenuItem = new MenuItem();
pasteMenuItem.Command = ApplicationCommands.Paste;
this.txtEdit.ContextMenu.Items.Insert(index, pasteMenuItem);
            index++;
            Separator seperator3 = new Separator();
this.txtEdit.ContextMenu.Items.Insert(index, seperator3);
            index++;
            //Delete
MenuItem deleteMenuItem = new MenuItem();
deleteMenuItem.Command = ApplicationCommands.Delete;
            this.txtEdit.ContextMenu.Items.Insert(index, deleteMenuItem);
            index++;
            Separator seperator4 = new Separator();
this.txtEdit.ContextMenu.Items.Insert(index, seperator4);
            index++;
            //Select All
MenuItem selectAllMenuItem = new MenuItem();
selectAllMenuItem.Command = ApplicationCommands.SelectAll;
            this.txtEdit.ContextMenu.Items.Insert(index, selectAllMenuItem);
            index++;
        }
        //Method to Add text to Dictionary
private void AddToDictionary(string entry)
        {
using (StreamWriter streamWriter = new StreamWriter(@"D:\WPF\MyCustomDictionary.lex", true))
            {
streamWriter.WriteLine(entry);
            }
        }

private void Window_Loaded(object sender, RoutedEventArgs e)
        {
//Assigning custom Dictionary to TextBox
this.txtEdit.SpellCheck.CustomDictionaries.Add(new Uri(@"D:\WPF\MyCustomDictionary.lex"));
        }
    }
}

The .Net Framework 4.0 supports CustomDictionaries, which helps to create your own Dictionary.

//.Net 4.0 Supports CustomDictionaries, Option for Adding to dictionary.
int selectionStart = this.txtEdit.GetSpellingErrorStart(this.txtEdit.CaretIndex);
if (selectionStart >= 0)
{
    Separator seperator1 = new Separator();
this.txtEdit.ContextMenu.Items.Insert(index, seperator1);
    index++;
MenuItem AddToDictionary = new MenuItem();
    AddToDictionary.Header = "Add to Dictionary";
    //Getting the word to add
this.txtEdit.SelectionStart = selectionStart;
this.txtEdit.SelectionLength = this.txtEdit.GetSpellingErrorLength(this.txtEdit.CaretIndex);
    //Ignoring the added word.
AddToDictionary.Command = EditingCommands.IgnoreSpellingError;
AddToDictionary.CommandTarget = this.txtEdit;
AddToDictionary.Click += (object o, RoutedEventArgs rea) =>
    {
this.AddToDictionary(this.txtEdit.SelectedText);
    };
this.txtEdit.ContextMenu.Items.Insert(index, AddToDictionary);
    index++;
}

Here is the screenshot of Demo Application.

WPF Spell Check Demo - Screenshot