ComboBox in a DataGrid column

This is one combo box column that I initially took from www.codeproject.com and then fixed some stuff in it. It works fine but I don't know if it will work with your filtering (which I did not understand quite):

using System;
using System.Windows.Forms;
using System.Drawing;
using System.Data;
using System.Collections;
using System.Diagnostics;
using System.Reflection;

namespace AdvancedControls
{
      /// <summary>
      /// Implementation of a ComboBox as a column in a DataGrid
      /// </summary>
      public class DataGridDataBoundComboBoxColumn : DataGridTextBoxColumn
      {
            private ComboBox internalComboBox = new ComboBox();
            private bool currentlyInEdit = false;
            private CurrencyManager source = null;
            private int rowNum = -1;
            private AdvancedDataGrid dataGrid = null;

            #region Properties
            private object nullValue = DBNull.Value;
            /// <summary>
            /// Gives a value that will be considered null and will be replaced for showing by NullValue
            /// </summary>
            public object NullValue
            {
                  set {this.nullValue = value;}
                  get {return this.nullValue;}
            }

            private object nullCorrespondingValue = null;
            /// <summary>
            /// Gives a value that will replace the null values (given by NullValue) for showing
            /// </summary>
            public object NullCorrespondingValue
            {
                  set {this.nullCorrespondingValue = value;}
                  get {return this.nullCorrespondingValue;}
            }

            public object DataSource
            {
                  set
                  {
                        // If the display member or value member is wrong,
                        // this line will clear it
                        this.internalComboBox.DataSource = value;
                        SetComboBoxDropDownWidth();
                  }
                  get { return this.internalComboBox.DataSource; }
            }
            public string DisplayMember
            {
                  set
                  {
                        this.internalComboBox.DisplayMember = value;
                        SetComboBoxDropDownWidth();
                  }
                  get { return this.internalComboBox.DisplayMember; }
            }
            public string ValueMember
            {
                  set {this.internalComboBox.ValueMember = value; }
                  get { return this.internalComboBox.ValueMember; }
            }
            #endregion
            
            #region Construction
            public DataGridDataBoundComboBoxColumn () : base()
            {
                  this.internalComboBox.Visible = false;
                  this.internalComboBox.DropDownStyle = ComboBoxStyle.DropDownList;
                  // Some events that guarantee correct working
                  this.internalComboBox.LostFocus += new EventHandler (OnInternalComboBoxLostFocus);
            }
            #endregion
            
            #region Event handlers for the combo box
            private void OnInternalComboBoxLostFocus (object sender, EventArgs e)
            {
                  if ((sender is DataGrid && !this.internalComboBox.Focused) ||
                        sender == this.internalComboBox)
                  {
                        if (this.currentlyInEdit)
                              Commit (this.source, this.rowNum);
                  }
            }
            #endregion

            #region Key management
            /// <summary>
            /// Handles opening and closing of the ComboBox with the keyboard
            /// For this function to work, the data grid should be of type AdvancedDataGrid
            /// </summary>
            /// <param name="msg"></param>
            /// <param name="keyData"></param>
            /// <returns></returns>
            public bool ProcessCmdKey(ref Message msg, Keys keyData)
            {
                  if (!this.currentlyInEdit)
                        return false;

                  if (keyData == Keys.Down)
                  {
                        // Try to select next or the first item
                        if (this.internalComboBox.SelectedIndex < this.internalComboBox.Items.Count - 1)
                              this.internalComboBox.SelectedIndex++;
                        else if (this.internalComboBox.Items.Count > 0)
                              this.internalComboBox.SelectedIndex = 0;
                        return true;
                  }
                  else if (keyData == Keys.Up)
                  {
                        // Try to select previous or the last item
                        if (this.internalComboBox.SelectedIndex > 0)
                              this.internalComboBox.SelectedIndex--;
                        else if (this.internalComboBox.Items.Count > 0)
                              this.internalComboBox.SelectedIndex = this.internalComboBox.Items.Count - 1;
                        return true;
                  }
                  else if ((keyData == (Keys.Up | Keys.Alt) ||
                        keyData == (Keys.Down | Keys.Alt)) &&
                        !this.internalComboBox.DroppedDown)
                  {
                        this.internalComboBox.DroppedDown = true;
                        return true;
                  }
                        // Keys.Down | Keys.Alt work automatically if DropedDown
                  else if ((keyData == Keys.Enter || keyData == (Keys.Up | Keys.Alt)) &&
                        this.internalComboBox.DroppedDown)
                  {
                        HideDropDown();
                        return true;
                  }
                  else if (keyData == Keys.Escape)
                  {
                        Abort (this.rowNum);
                        return true;
                  }

                  return false;
            }
            #endregion

            #region Overriden methods
            
            protected override object GetColumnValueAtRow (CurrencyManager source, int rowNum)
            {
                  object val = base.GetColumnValueAtRow (source, rowNum);
                  // Check for the null value
                  if (Object.Equals (val, this.NullValue) && this.NullCorrespondingValue != null)
                        val = this.NullCorrespondingValue;

                  foreach (object o in this.internalComboBox.Items)
                  {
                        object oVal = GetValueMember (o);
                        if ((oVal == null && val == null) ||
                              (oVal != null && oVal.Equals (val)))
                              return GetDisplayMember (o);
                  }
                  return DBNull.Value;
            }

            protected override void Abort(int rowNum)
            {
                  // If in read only mode, just follow the basic methods
                  if (this.DataGridTableStyle.DataGrid.ReadOnly || this.ReadOnly ||
                        (this.DataGridTableStyle.DataGrid.DataSource is DataView &&
                        !((DataView)this.DataGridTableStyle.DataGrid.DataSource).AllowEdit))
                  {
                        base.Abort (rowNum);
                        return;
                  }

                  if(this.currentlyInEdit)
                  {
                        this.currentlyInEdit = false;
                        this.source = null;
                        this.rowNum = -1;
                        this.internalComboBox.Hide();
                        if (this.internalComboBox.DroppedDown)
                              HideDropDown();
                  }
            }

            protected override bool Commit(CurrencyManager dataSource,int rowNum)
            {
                  // If in read only mode, just follow the basic methods
                  if (this.DataGridTableStyle.DataGrid.ReadOnly || this.ReadOnly ||
                        (this.DataGridTableStyle.DataGrid.DataSource is DataView &&
                        !((DataView)this.DataGridTableStyle.DataGrid.DataSource).AllowEdit))
                        return base.Commit(dataSource, rowNum);

                  if(this.currentlyInEdit)
                  {
                        object selValue = this.internalComboBox.SelectedValue;
                        if (selValue == null || Object.Equals (selValue, this.NullCorrespondingValue))
                              selValue = this.NullValue;
                        SetColumnValueAtRow(dataSource, rowNum, selValue);

                        this.currentlyInEdit = false;
                        this.source = null;
                        this.rowNum = -1;
                        this.internalComboBox.Hide();
                        if (this.internalComboBox.DroppedDown)
                              HideDropDown();
                  }

                  return true;
            }
            
            protected override void Edit(CurrencyManager source, int rowNum, Rectangle bounds, bool readOnly, string instantText, bool cellIsVisible)
            {
                  // If in read only mode, just follow the basic methods
                  if (this.DataGridTableStyle.DataGrid.ReadOnly || this.ReadOnly ||
                        (this.DataGridTableStyle.DataGrid.DataSource is DataView &&
                        !((DataView)this.DataGridTableStyle.DataGrid.DataSource).AllowEdit))
                  {
                        base.Edit(source, rowNum, bounds, readOnly, instantText, cellIsVisible);
                        return;
                  }

                  if (this.currentlyInEdit)
                        return;

                  if(!cellIsVisible)
                        return;

                  // Important: show it before setting the selected value
                  // otherwise in some cases the value from the previous edit is returned
                  // (when the drop down is open with the mouse, a new value is pointed
                  // and Tab is pressed)
                  this.internalComboBox.Bounds = bounds;
                  this.internalComboBox.Show();
                  
                  // Get the underlying value
                  // The current implementation of GetColumnValueAtRow will return the DisplayMember
                  object val = base.GetColumnValueAtRow (source, rowNum);
                  // Check for the null value
                  if (Object.Equals (val, this.NullValue) && this.NullCorrespondingValue != null)
                        val = this.NullCorrespondingValue;

                  if (val == null || val == DBNull.Value)
                        if (this.internalComboBox.Items.Count > 0)
                              this.internalComboBox.SelectedIndex = 0;
                        else
                              this.internalComboBox.SelectedIndex = -1;
                  else
                        this.internalComboBox.SelectedValue = val;
                  
                  this.currentlyInEdit = true;
                  this.source = source;
                  this.rowNum = rowNum;

                  ColumnStartedEditing (this.internalComboBox);
            }

            protected override int GetMinimumHeight()
            {
                  return this.internalComboBox.PreferredHeight + 1;
            }

            protected override void SetDataGridInColumn(DataGrid value)
            {
                  base.SetDataGridInColumn (value);

                  if (value == this.dataGrid)
                        return;

                  Debug.Assert (value != null);
                  Debug.Assert (value is AdvancedDataGrid,"The used data grid should be of type AdvancedDataGrid");

                  if (this.dataGrid != null)
                  {
                        this.dataGrid.Controls.Remove (this.internalComboBox);
                        this.dataGrid.LostFocus -= new EventHandler (OnInternalComboBoxLostFocus);
                  }

                  this.dataGrid = (AdvancedDataGrid)value;

                  this.dataGrid.Controls.Add(this.internalComboBox);
                  // Some events that guarantee correct working
                  this.dataGrid.LostFocus += new EventHandler (OnInternalComboBoxLostFocus);
            }
            #endregion

            #region Helper functions
            private void HideDropDown()
            {
                  // Unknown why, the value is lost when DroppedDown is set to false
                  object val = this.internalComboBox.SelectedValue;
                  this.internalComboBox.DroppedDown = false;
                  this.internalComboBox.SelectedValue = val;
            }
            private object GetMember (object o, string member)
            {
                  DataRow row = null;
                  if (o is DataRowView)
                        row = ((DataRowView)o).Row;
                  if (o is DataRow)
                        row = (DataRow)o;
                  if (row != null)
                  {
                        object ret = row[member];
                        if (ret == null)
                              return DBNull.Value;
                        else
                              return ret;
                  }
                  else
                  {
                        PropertyInfo pi = o.GetType().GetProperty (member);
                        Debug.Assert (pi != null);
                        return pi.GetValue (o, null);
                  }
            }
            
            private string GetDisplayMember (object o)
            {
                  if (o == null || o == DBNull.Value)
                        return this.NullText;
                  object val = GetMember (o, this.internalComboBox.DisplayMember);
                  if (val == null || val == DBNull.Value)
                        return this.NullText;
                  else
                        return val.ToString();
            }

            private object GetValueMember (object o)
            {
                  return GetMember (o, this.internalComboBox.ValueMember);
            }

            /// <summary>
            /// Sets the DropDownWidth property of the combo box if all needed
            /// properties are already set
            /// </summary>
            private void SetComboBoxDropDownWidth()
            {
                  if (this.internalComboBox.DataSource == null ||
                        this.internalComboBox.DisplayMember == null ||
                        this.internalComboBox.DisplayMember == String.Empty)
                        return;

                  Graphics g = this.internalComboBox.CreateGraphics ();
                  float w = 0;
                  foreach (object o in this.internalComboBox.Items)
                        w = Math.Max (w, g.MeasureString (GetDisplayMember (o),
                              this.internalComboBox.Font).Width);
                  if (w > 0)
                        this.internalComboBox.DropDownWidth = (int)(w + 0.5);
            }
            #endregion
      }
}