How can i manage changed value text by highlighting the edited text only?

问题

My question might seems to be duplicate although it is not the same issue i had experienced before.

I have successfully added a richtexbox column to the datagridview as it has being my problem for a while. now you can add richtextbox to the datagridview.

Now am trying to highlight the edited text only but am not setting it work as after i edited the text it highlight the whole text.

for an example from what i wanna get

"Test" = Testing

from the example i above, i only want to highlight the only added one from the existing one.

the code below highlighted the whole text from the datagridview richtextbox cell which is not what i want.

code that am using for color change

 private void Gridview_Output_CellValueChanged(object sender, DataGridViewCellEventArgs e)
    {
        if (e.RowIndex >= 0)
        {
            Gridview_Output.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.ForeColor = Color.Red;
        }
    }

Richtextbox class

public partial class RichTextBoxControl : DataGridViewColumn
{
    public RichTextBoxControl(): base()
    {
        base.CellTemplate = new RichTextboxCell1();
    }

    public override DataGridViewCell CellTemplate
    {
        get
        {
            return base.CellTemplate;
        }
        set
        {
            if (!((value == null)) && !(value.GetType().IsAssignableFrom(typeof(RichTextboxCell1))))
            {
                throw new InvalidCastException("Must be a CalendarCell");
            }
            base.CellTemplate = value;
        }
    }
}
public class RichTextboxCell1 : DataGridViewTextBoxCell
{

    public RichTextboxCell1()
    {

    }
    public override void InitializeEditingControl(int rowIndex, object initialFormattedValue, DataGridViewCellStyle dataGridViewCellStyle)
    {


        base.InitializeEditingControl(rowIndex, initialFormattedValue, dataGridViewCellStyle);

        RichTextBoxEditingControl1 ctl = (RichTextBoxEditingControl1)DataGridView.EditingControl;
        if (this.RowIndex >= 0)
        {
            if ((!object.ReferenceEquals(this.Value, DBNull.Value)))
            {
                if (this.Value != null)
                {
                    if (this.Value != string.Empty)
                    {
                        try
                        {
                            ctl.Text   =this.Value.ToString();
                        }
                        catch (Exception ex)
                        {
                        }
                    }
                }
            }
        }
    }

    public override System.Type EditType
    {
        get
        {
            return typeof(RichTextBoxEditingControl1);
        }
    }

    public override System.Type ValueType
    {
        get
        {
            return typeof(String);
        }
    }

    public override object DefaultNewRowValue
    {
        get
        {
            return String.Empty;
        }
    }
}

class RichTextBoxEditingControl1 : RichTextBox, IDataGridViewEditingControl
{
    private DataGridView dataGridViewControl;
    private bool valueIsChanged = false;
    private int rowIndexNum;

    public RichTextBoxEditingControl1()
    {

    }

    public object EditingControlFormattedValue
    {
        get
        {
            return this.Text;
        }
        set
        {  
            if (value is string)
            {
                this.Text = value.ToString();
            }
        }
    }

    public object GetEditingControlFormattedValue(DataGridViewDataErrorContexts context)
    {
        return this.Text;
    }

    public void ApplyCellStyleToEditingControl(DataGridViewCellStyle dataGridViewCellStyle)
    {
        this.Font = dataGridViewCellStyle.Font;
        this.ForeColor = dataGridViewCellStyle.ForeColor;
        this.BackColor = dataGridViewCellStyle.BackColor;
    }

    public int EditingControlRowIndex
    {
        get
        {
            return rowIndexNum;
        }
        set
        {
            rowIndexNum = value;
        }
    }

    public bool EditingControlWantsInputKey(Keys key, bool dataGridViewWantsInputKey)
    {
        if (Keys.KeyCode == Keys.Left || Keys.KeyCode == Keys.Up || Keys.KeyCode == Keys.Down || Keys.KeyCode == Keys.Right || Keys.KeyCode == Keys.Home || Keys.KeyCode == Keys.End || Keys.KeyCode == Keys.PageDown || Keys.KeyCode == Keys.PageUp)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public void PrepareEditingControlForEdit(bool selectAll)
    {
    }

    public bool RepositionEditingControlOnValueChange
    {
        get
        {
            return false;
        }
    }

    public DataGridView EditingControlDataGridView
    {
        get
        {
            return dataGridViewControl;
        }
        set
        {
            dataGridViewControl = value;
        }
    }

    public bool EditingControlValueChanged
    {
        get
        {
            return valueIsChanged;
        }
        set
        {
            valueIsChanged = value;
        }
    }

    public Cursor EditingControlCursor
    {
        get
        {
            return base.Cursor;
        }
    }

    protected override void OnTextChanged(System.EventArgs eventargs)
    {
        valueIsChanged = true;
        this.EditingControlDataGridView.NotifyCurrentCellDirty(true);
        base.OnTextChanged(eventargs);
    }


    #region IDataGridViewEditingControl Members


    Cursor IDataGridViewEditingControl.EditingPanelCursor
    {
       get { return base.Cursor; }
    }

    #endregion
}

来源：https://stackoverflow.com/questions/29629892/how-can-i-manage-changed-value-text-by-highlighting-the-edited-text-only