在 C# 中将占位符添加到文本框

// `Form1` as your C# project form
// `Form1.Designer.cs` Code to create a `PlaceholderTxtB` textbox

namespace TextboxPlaceholder
{
    partial class Form1
    {

        private System.ComponentModel.IContainer components = null;

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        private void InitializeComponent()
        {
            this.PlaceholderTxtB = new System.Windows.Forms.TextBox();
            this.button1 = new System.Windows.Forms.Button();
            this.SuspendLayout();
            //
            // PlaceholderTxtB
            //
            this.PlaceholderTxtB.Location = new System.Drawing.Point(12, 12);
            this.PlaceholderTxtB.Name = "PlaceholderTxtB";
            this.PlaceholderTxtB.Size = new System.Drawing.Size(188, 20);
            this.PlaceholderTxtB.TabIndex = 0;
            //
            // Create a `button1` button to lose focus from the `PlaceholderTxtB` textbox
            //
            this.button1.Location = new System.Drawing.Point(57, 39);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75, 23);
            this.button1.TabIndex = 1;
            this.button1.Text = "Focus";
            this.button1.UseVisualStyleBackColor = true;
            //
            // Form1
            //
            this.ClientSize = new System.Drawing.Size(284, 261);
            this.Controls.Add(this.button1);
            this.Controls.Add(this.PlaceholderTxtB);
            this.Name = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.TextBox PlaceholderTxtB;
        private System.Windows.Forms.Button button1;
    }
}

// `Form1.cs` code to add a placeholder text to `PlaceholderTxtB` textbox

using System;
using System.Drawing;
using System.Windows.Forms;


namespace TextboxPlaceholder
{

    public partial class Form1 : Form
    {
        string ph = "Type your user name here...";

        public Form1()
        {

            InitializeComponent();
            PlaceholderTxtB.GotFocus += RemoveText;
            PlaceholderTxtB.LostFocus += AddText;

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
        public void RemoveText(object sender, EventArgs e)
        {

            if (PlaceholderTxtB.Text == ph)
                PlaceholderTxtB.Text = "";
        }
        public void AddText(object sender, EventArgs e)
        {
            if (String.IsNullOrWhiteSpace(PlaceholderTxtB.Text))
            {
                PlaceholderTxtB.ForeColor = Color.Gray;
                PlaceholderTxtB.Text = ph;
            }
        }
    }
}

// `Form1.cs` code for `Enter Focus` and `Leave Focus` events to add a placeholder text to a `textBox1` textbox

string phd = "Password@123";

private void Enter_Focus(object sender, EventArgs e)
{
    //textBox1.ForeColor = Color.Gray;
    if (textBox1.Text == phd)
    textBox1.Text = "";
}

private void Leave_Focus(object sender, EventArgs e)
{
    textBox1.ForeColor = Color.Gray;
    if (textBox1.Text == "")
    textBox1.Text = phd;
}