微軟套裝的ComboBox本身就提供了AutoCompete功能，只要設定AutoCompleteMode和AutoCompleteSource屬性即可，而且功能還是很強大的。但是…還是滿足不了我的要求。
1. AutoComplete時下拉框只顯示近似匹配的項，不是顯示全部項並自動定位到匹配項。
2. 當輸入的內容找不到匹配項時，還允許使用者錄入，並且SelectedIndex將為-1， 同時SelectedValue為null。
但是在多數情況下，我們是不允許使用者輸入選項以外的內容，輸入只是進行lookup。而且SelectedIndex變為-1也不是所期盼的。因此，擴充套件ComboBox做了一個自定義控制元件。
1. 當獲取焦點時全部文字預設是選中的，這時使用者錄入的內容就會代替全部內容去lookup。
2. 如果找到匹配項，就定位到匹配項，並且將文字的選中區域從使用者錄入內容向後選中；沒有匹配項則錄入無效。
3. 當使用者退格或Del時，如果沒有找到匹配項則只是將文字的選中區域擴大。
4. 當使用者刪除了內容時，如果沒有找到匹配項則刪除無效。如果使用者要把內容全部刪掉，就會跟據LimitedToList 屬性來決定是否清空文字，當然也就是是否將SelectedIndex設定為-1。
5. 不允許Ctrl+X，最主要的原因是我在OnKeyPress事件中停止事件繼續冒泡，這樣剪下的內容並沒有複製到剪下板。
6. 不允許ESC，原因是當下拉選單開啟時ESC會將SelectedIndex恢復，但是使用者錄入匹配的文字不會恢復。
public class LookupComboBox : System.Windows.Forms.ComboBox
{
#region Variables
/// <summary>
/// A value indicating whether open drop down list when click comboBox.
/// </summary>
private bool dropOnEntry = true;
/// <summary>
/// A value indicating whether auto size drop down list.
/// </summary>
private bool allowAutoSize = true;
/// <summary>
/// A value indicating whether this control can be modified.
/// </summary>
private bool readOnly;
/// <summary>
/// Hold the value of BackColor property, used for restore BackColor property when change ReadOnly to false.
/// </summary>
private System.Drawing.Color tBackColor;
/// <summary>
/// Hold the value of ForeColor property, used for restore ForeColor property when change ReadOnly to false.
/// </summary>
private System.Drawing.Color tForeColor;
/// <summary>
/// Hold the value of DropDownStyle property, used for restore DropDownStyle property when change ReadOnly to false.
/// </summary>
private ComboBoxStyle dropDownStyle = ComboBoxStyle.DropDown;
/// <summary>
/// The back color value for ReadOnly
/// </summary>
private System.Drawing.Color readOnlyBackColor = SystemColors.Control;
/// <summary>
/// The fore color value for ReadOnly
/// </summary>
private System.Drawing.Color readOnlyForeColor = SystemColors.WindowText;
/// <summary>
/// The value indicated whether user pressed DEL.
/// </summary>
private bool _pressedDel = false;
/// <summary>
/// A value indicate whether Selected an item in the list.
/// </summary>
private bool _limitedToList = true;
#endregion
#region Public Properties
/// <summary>
/// Gets a value indicating whether open drop down list when click comboBox.
/// </summary>
public bool DropOnEntry
{
set
{
dropOnEntry = value;
}
get
{
return dropOnEntry;
}
}
/// <summary>
/// Gets a value indicating whether auto size drop down list.
/// </summary>
public bool AllowAutoSize
{
set
{
allowAutoSize = value;
}
get
{
return allowAutoSize;
}
}
/// <summary>
/// Gets or sets the back color value for ReadOnly
/// </summary>
public System.Drawing.Color ReadOnlyBackColor
{
get
{
return readOnlyBackColor;
}
set
{
readOnlyBackColor = value;
}
}
/// <summary>
/// Gets or sets the fore color value for ReadOnly
/// </summary>
public System.Drawing.Color ReadOnlyForeColor
{
get
{
return readOnlyForeColor;
}
set
{
readOnlyForeColor = value;
}
}
/// <summary>
/// Gets a value indicating whether this control can be modified.
/// </summary>
public bool ReadOnly
{
get
{
return readOnly;
}
set
{
if (readOnly != value)
{
readOnly = value;
if (value)
{
base.DropDownStyle = ComboBoxStyle.Simple;
base.Height = 21;
this.tBackColor = this.BackColor;
this.tForeColor = this.ForeColor;
base.BackColor = this.ReadOnlyBackColor;
base.ForeColor = this.ReadOnlyForeColor;
}
else
{
this.ContextMenu = null;
base.DropDownStyle = dropDownStyle;
base.BackColor = tBackColor;
base.ForeColor = tForeColor;
}
}
}
}
/// <summary>
/// Gets a value indicating whether Selected an item in the list.
/// If LimitedToList is true. SelectedIndex will be 0 when user deleted all text and leave the focus.
/// If LimitedToList is false. SelectedIndex will be -1 when user delete all text and leave the focus.
/// Default value is true.
/// </summary>
[Description("If LimitedToList is true SelectedIndex can`t be -1 except no items in list. If can`t match any item in list when OnValidaing, SelectedIndex will be 0. Otherwise can be -1. Default value is true")]
[DefaultValue(true)]
public bool LimitedToList
{
get
{
return this._limitedToList;
}
set
{
this._limitedToList = value;
}
}
#endregion
#region Event Handlers
/// <summary>
/// Raises the System.Windows.Forms.Control.MouseDown event.
/// </summary>
/// <param name="e">A System.Windows.Forms.MouseEventArgs that contains the event data.</param>
protected override void OnMouseDown(System.Windows.Forms.MouseEventArgs e)
{
if (this.ReadOnly)
{
return;
}
if ((dropOnEntry) && (this.DroppedDown != true))
this.DroppedDown = true;
base.OnMouseDown(e);
}
/// <summary>
/// Raises the System.Windows.Forms.ComboBox.DropDown event.
/// </summary>
/// <param name="e">An System.EventArgs that contains the event data.</param>
protected override void OnDropDown(System.EventArgs e)
{
base.OnDropDown(e);
if (!this.allowAutoSize)
return;
int width = this.DropDownWidth;
Graphics g = this.CreateGraphics();
Font font = this.Font;
int vertScrollBarWidth = (this.Items.Count > this.MaxDropDownItems) ? SystemInformation.VerticalScrollBarWidth : 0;
int newWidth;
foreach (object o in this.Items)
{
string s = o.ToString();
newWidth = (int)g.MeasureString(s, font).Width + vertScrollBarWidth;
if (width < newWidth)
{
width = newWidth;
}
}
this.DropDownWidth = width;
}
/// <summary>
/// Raises the System.Windows.Forms.Control.KeyPress event.
/// </summary>
/// <param name="e">A System.Windows.Forms.KeyPressEventArgs that contains the event data.</param>
protected override void OnKeyPress(KeyPressEventArgs e)
{
base.OnKeyPress(e);
string findString = string.Empty;
if (e.KeyChar == (char)Keys.Back && this._pressedDel) // DEL
{
// Special treat for decreasing Change event trigger times.
if (this.SelectionStart == 0 && this.SelectionLength > 0 && this.SelectionLength == this.Text.Length - 1)
{
this.SelectAll();
e.Handled = true;
return;
}
if (this.SelectionStart < this.Text.Length)
{
this.SelectionLength += 1;
}
// delete the selected text
findString = this.ReplaceSelectedString(string.Empty);
}
else if (e.KeyChar == (char)Keys.Back && !this._pressedDel) //Back
{
// Special treat for decreasing Change event trigger times.
if (this.SelectionStart == 1 && this.SelectionLength > 0 && this.SelectionLength == this.Text.Length - 1)
{
this.SelectAll();
e.Handled = true;
return;
}
if (this.SelectionStart > 0)
{
this.SelectionStart -= 1;
this.SelectionLength += 1;
}
// delete the selected text
findString = this.ReplaceSelectedString(string.Empty);
}
else if (e.KeyChar == (char)22) //CTRL+V
{
IDataObject data = Clipboard.GetDataObject();
string copiedText = data.GetData(DataFormats.Text) as String;
findString = this.ReplaceSelectedString(copiedText);
}
else if (char.IsControl(e.KeyChar))
{
return;
}
else
{
// if user input a normal character
findString = this.ReplaceSelectedString(e.KeyChar.ToString());
}
// if user delete all text set indext to -1
if (string.IsNullOrEmpty(findString))
{
if (this.LimitedToList && this.Items != null && this.Items.Count > 0)
{
this.SelectedIndex = 0;
this.SelectAll();
}
else
{
this.SelectedIndex = -1;
}
e.Handled = true;
return;
}
int matchedIndex = this.FindStringExact(findString);
if (matchedIndex == -1)
{
matchedIndex = this.FindString(findString);
}
if (matchedIndex == -1)
{
e.Handled = true;
return;
}
this.SelectedIndex = matchedIndex;
this.SelectionStart = findString.Length;
this.SelectionLength = this.Text.Length - this.SelectionStart;
e.Handled = true;
}
/// <summary>
/// Raises the System.Windows.Forms.Control.KeyDown event.
/// </summary>
/// <param name="e">A System.Windows.Forms.KeyEventArgs that contains the event data.</param>
protected override void OnKeyDown(KeyEventArgs e)
{
base.OnKeyDown(e);
if (e.KeyCode == Keys.Delete)
{
this._pressedDel = true;
this.OnKeyPress(new KeyPressEventArgs((char)Keys.Back));
this._pressedDel = false;
e.Handled = true;
}
}
/// <summary>
/// Override the event that the character was processed by the control
/// </summary>
/// <param name="msg">A System.Windows.Forms.Message, passed by reference, that represents the window message to process.</param>
/// <param name="keyData">One of the System.Windows.Forms.Keys values that represents the key to process.</param>
/// <returns>True if the character was processed by the control; otherwise, false.</returns>
protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
// blocked Esc
if (keyData == Keys.Escape || keyData == (Keys.Control | Keys.X))
{
return true;
}
return base.ProcessCmdKey(ref msg, keyData);
}
/// <summary>
/// Raises the System.Windows.Forms.Control.Validating event.
/// </summary>
/// <param name="e">A System.ComponentModel.CancelEventArgs that contains the event data.</param>
protected override void OnValidating(System.ComponentModel.CancelEventArgs e)
{
int exactMatchedIndex;
// For some reason, the index is reset when the drop down closes
if (this.DroppedDown)
{
this.DroppedDown = false;
}
exactMatchedIndex = this.FindStringExact(this.Text);
// if not matching anything and limited to list set selectedIndex to 0
if (exactMatchedIndex == -1 && this.LimitedToList && this.Items != null && this.Items.Count > 0)
{
exactMatchedIndex = 0;
}
this.SelectedIndex = exactMatchedIndex;
base.OnValidating(e);
}
#endregion
#region Methods
/// <summary>
/// Replace selected string with specific text
/// </summary>
/// <param name="replaceText">The text want to replace with</param>
/// <returns>Replace result string</returns>
private string ReplaceSelectedString(string replaceText)
{
return String.Format("{0}{1}{2}", this.Text.Substring(0, this.SelectionStart), replaceText, this.Text.Substring(this.SelectionStart + this.SelectionLength));
}
#endregion
}
可能你注意到OnValidating方法。與OnLeave方法不同OnValidating方法不僅在失去焦點時會被呼叫，在使用者使用快捷鍵觸發按鈕事件時也會被呼叫。後者對我來說更重要，因為使用者都通過按鈕的快捷鍵來完成儲存操作，而快捷鍵並不會使ComboBox命運焦點，即不會呼叫OnLeave方法。
做這個控制元件還發生了一個波折，最初我是在OnTextChanged方法中完成匹配選中的，並且沒有新增OnValidating方法，在我的本機(Windows 7)測試通過(當然除了使用者按快捷鍵的情況)。但是使用者使用的環境是XP和Windows 2003，Lookup匹配完成後ComboBox的SelectedIndex競然是-1。
當然如果加上了OnValidatin方法也可以解決，但是使用overrid OnTextChanged的解決方案會引發多次TextChanged事件，最終還是改用了OnKeyPress方法。
順便說一下，如果要在LookupComboBox的SelectedIndexChanged事件中想彈出一個確認對話方塊，讓使用者選是否或取消的那種，在彈框前要呼叫ComboBox.DroppedDown = false，這樣ComboBox會接受lookup的最終結果。
如果你有更好的解決方案，請指教。