C# のテキストボックスにプレースホルダーを追加する

// `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;
}