public RadForm1()
{
    InitializeComponent();
    this.radGridView1.EditorRequired += this.RadGridView1_EditorRequired;

}

private void RadGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(RadTextBoxEditor))
    {
        e.Editor = new MyEditor();
    }
}
public class MyEditor: RadTextBoxControlEditor
{
    protected override RadElement CreateEditorElement()
    {
        return new MyRadControlEditorElement();
    }
}
internal class MyRadControlEditorElement : RadTextBoxControlElement
{
    protected override void CreateChildElements()
    {
        base.CreateChildElements();

        this.Navigator = new MyTextBoxNavigator(this);

    }

    protected override TextBoxViewElement CreateViewElement()
    {
        return new MyTextBoxViewElement();
    }
}

internal class MyTextBoxNavigator : TextBoxNavigator
{
    public MyTextBoxNavigator(RadTextBoxControlElement textBoxElement) : base(textBoxElement)
    {
    }

    public override bool Select(TextPosition start, TextPosition end)
    {
        if (start != null && start.TextBlock == null)
        {
            return false;
        }

        return base.Select(start, end);
    }

    public override TextPosition GetPositionFromOffset(int offset)
    {
        if (offset < 0)
        {
            throw new ArgumentOutOfRangeException("offset");
        }

        if (offset > this.TextBoxElement.TextLength)
        {
            offset = this.TextBoxElement.TextLength;
        }

        TextBoxViewElement layoutPanel = this.TextBoxElement.ViewElement;
        LineInfo line = layoutPanel.Lines.BinarySearchByOffset(offset);

        if (line == null)
        {
            return null;
        }

        ITextBlock textBlock = layoutPanel.BinarySearchTextBlockByOffset(line, offset);
        int characterPosition = offset;
        if (textBlock != null)
        {
            characterPosition -= textBlock.Offset;
        }
        TextPosition position = new TextPosition(line, textBlock, characterPosition);
        return position;
    }

}
internal class MyTextBoxViewElement : TextBoxViewElement
{
    protected override ITextBlock BinarySearch(LineInfo line, IComparer comparer)
    {
        if (this.Children.Count == 0)
        {
            return null;

        }
        
        return base.BinarySearch(line, comparer);
    }
}