Create a button properties for windows properties and in the form dll file

Assuming you want to create a re-usable Windows Forms Component, like a custom Button:

1. start a new C# Desktop Project of Type 'Class Library.

2. add references to the Project for 'System.Windows.Forms and System.Drawing

3. modify the Class Library code to look like this:

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

namespace Dec29_FancyButton.cs
{
    public class FancyButton: Button
    {
        private static Size defaultSize = new Size(100,30);
        private static Color defaultBackColor = Color.White;
        private static Color defaultForeColor = Color.Black;
        private const string defaultText = "FancyButton";

        // use default values if the "vanilla" ctor is called ...
        public FancyButton()
        {
            Size = defaultSize;
            BackColor = defaultBackColor;
            ForeColor = defaultForeColor;
            Text = defaultText;
        }

        // alternative 'ctor for specifying initial visual state
        // note that since 'Size.Empty and 'Color.Empty are not compile-time constants
        // they cannot be used as optional parameter values in a 'ctor
        public FancyButton(string text, Size bSize, Color backColor, Color foreColor)
        {
            this.Text = (text == String.Empty) ? defaultText : text;

            this.Size = (bSize == Size.Empty) ? defaultSize : bSize;

            this.BackColor = (backColor == Color.Empty) ? defaultBackColor:  backColor;
            this.ForeColor = (foreColor == Color.Empty) ? defaultForeColor:  foreColor;
        }
    }
}

4. compile this to create the 'dll.

5. use the compiled dll like this:

a. start a new Project

b. add a reference to the compiled dll for FancyButton

c. put a 'using statement on the Form where you want to use the Component, then create and use instances of the component:

// required
using Dec29_FancyButton.cs;

private void Form1_Load(object sender, EventArgs e)
{
    FancyButton fb1 = new FancyButton();
    this.Controls.Add(fb1);
    fb1.Text = "number one";
    fb1.Location = new Point(100, 100);

    FancyButton fb2 = new FancyButton("number two", 
        Size.Empty, Color.Empty, Color.Empty);   
    this.Controls.Add(fb2);
    fb2.Location = new Point(200,200);
}

6. You can mount the compiled 'dll in your ToolBox by:

a. create new Tab in the ToolBox ... or re-use one you've already created

b. select 'Choose Items'

c. add a reference to the 'dll