显示 C# winforms 编辑控件的提示

Question

我正在开发 C# winforms 应用程序（VS.NET 2008、.NET 3.5 sp 1）。我在表单上有一个搜索字段，而不是在搜索字段旁边有一个标签，我想在搜索字段本身的背景中显示一些灰色文本（例如“搜索词”）。当用户开始在搜索字段中输入文本时，文本应该会消失。我怎样才能做到这一点？

Answer 1

您需要使用一些 P/Inovke 互操作代码来执行此操作。查找 Win32 API SendMessage 函数和 EM_SETCUEBANNER 消息。

Answer 2

最好发布代码而不是链接。我从here 发布这个

            //Copyright (c) 2008 Jason Kemp

            //Permission is hereby granted, free of charge, to any person obtaining a copy
            //of this software and associated documentation files (the "Software"), to deal
            //in the Software without restriction, including without limitation the rights
            //to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
            //copies of the Software, and to permit persons to whom the Software is
            //furnished to do so, subject to the following conditions:

            //The above copyright notice and this permission notice shall be included in
            //all copies or substantial portions of the Software.

            //THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
            //IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
            //FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
            //AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
            //LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
            //OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
            //THE SOFTWARE.

            using System;
            using System.Runtime.InteropServices;
            using System.Windows.Forms;
            using System.Text;


            public static class Win32Utility
            {
                [DllImport("user32.dll", CharSet = CharSet.Auto)]
                private static extern Int32 SendMessage(IntPtr hWnd, int msg,
                    int wParam, [MarshalAs(UnmanagedType.LPWStr)] string lParam);

                [DllImport("user32.dll")]
                private static extern bool SendMessage(IntPtr hwnd, int msg, int wParam, StringBuilder lParam);

                [DllImport("user32.dll")]
                private static extern bool GetComboBoxInfo(IntPtr hwnd, ref COMBOBOXINFO pcbi);

                [StructLayout(LayoutKind.Sequential)]
                private struct COMBOBOXINFO
                {
                    public int cbSize;
                    public RECT rcItem;
                    public RECT rcButton;
                    public IntPtr stateButton;
                    public IntPtr hwndCombo;
                    public IntPtr hwndItem;
                    public IntPtr hwndList;
                }

                [StructLayout(LayoutKind.Sequential)]
                private struct RECT
                {
                    public int left;
                    public int top;
                    public int right;
                    public int bottom;
                }

                private const int EM_SETCUEBANNER = 0x1501;
                private const int EM_GETCUEBANNER = 0x1502;

                public static void SetCueText(Control control, string text)
                {
                    if (control is ComboBox)
                    {
                        COMBOBOXINFO info = GetComboBoxInfo(control);
                        SendMessage(info.hwndItem, EM_SETCUEBANNER, 0, text);
                    }
                    else
                    {
                        SendMessage(control.Handle, EM_SETCUEBANNER, 0, text);
                    }
                }

                private static COMBOBOXINFO GetComboBoxInfo(Control control)
                {
                    COMBOBOXINFO info = new COMBOBOXINFO();
                    //a combobox is made up of three controls, a button, a list and textbox;
                    //we want the textbox
                    info.cbSize = Marshal.SizeOf(info);
                    GetComboBoxInfo(control.Handle, ref info);
                    return info;
                }

                public static string GetCueText(Control control)
                {
                    StringBuilder builder = new StringBuilder();
                    if (control is ComboBox)
                    {
                        COMBOBOXINFO info = new COMBOBOXINFO();
                        //a combobox is made up of two controls, a list and textbox;
                        //we want the textbox
                        info.cbSize = Marshal.SizeOf(info);
                        GetComboBoxInfo(control.Handle, ref info);
                        SendMessage(info.hwndItem, EM_GETCUEBANNER, 0, builder);
                    }
                    else
                    {
                        SendMessage(control.Handle, EM_GETCUEBANNER, 0, builder);
                    }
                    return builder.ToString();
                }
            }

Answer 3

我认为通常这样做的方法是将文本的颜色设置为灰色并用提示文本预填充它。

然后，为文本字段的焦点事件编写处理程序，根据获得和失去焦点的时间修改字段内容和颜色。这是一些伪代码（抱歉，它完全不是 C# 代码。我现在脑子里有 Actionscript）：

TextInput myInput;
boolean showingHint = true;

myInput.text = "Search Terms";
myInput.color = 0xcccccc;

myInput.onFocusGained = function() {
  if(showingHint) {
    myInput.text = "";
    myInput.color = 0x000000;
    showingHint = false;
  }
}

myInput.onFocusLost = function() {
  if(!showingHint && myInput.text.length == 0) {
    myInput.text = "Search Terms";
    myInput.color = 0xcccccc;
    showingHint = true;
  }
}

请记住，如果用户没有手动更改文本，您只想更改失去焦点的文本。使用单独的布尔值来跟踪您是否显示提示，以便您可以区分用户手动输入“提示”文本作为实际内容。同样，您只想在显示提示时清除输入框的内容，以免意外删除用户输入。

Answer 4

文本框控件中有内置功能 -- AutoCompleteMode 和 AutoCompleteSource。在您进入自定义或第 3 方控件之前，它们可能值得一试。