C#

<br />
System.NullReferenceException was unhandled<br />
Message="Object reference not set to an instance of an object."<br />

using System;
using System.ComponentModel;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace TestApp
{
	/// <summary>
	/// Summary description for ShapeControl.
	/// </summary>
	public class ShapeControl : Control
	{
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private Container components = null;

		private Color shapeColor;
		private ShapeStyle shapeStyle;
		private int shapeSides;

		public ShapeControl()
		{
			// Enable Double-Buffering and other painting parameters
			base.SetStyle(ControlStyles.DoubleBuffer | ControlStyles.UserPaint |
				ControlStyles.AllPaintingInWmPaint | ControlStyles.ResizeRedraw |
				ControlStyles.SupportsTransparentBackColor, true);
			this.shapeColor = Color.Red;
			this.shapeStyle = ShapeStyle.Circle;
			this.shapeSides = 4;
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose(bool disposing)
		{
			if (disposing)
			{
				if (components != null)
					components.Dispose();
			}
			base.Dispose(disposing);
		}

		protected override void OnPaint(PaintEventArgs pe)
		{
			base.OnPaint(pe);
			pe.Graphics.SmoothingMode = SmoothingMode.AntiAlias;

			// Find minimum side
			int minSide = Math.Min(pe.ClipRectangle.Width, pe.ClipRectangle.Height);
			// Create Rectangle on center of control
			PointF center = new PointF((pe.ClipRectangle.X + pe.ClipRectangle.Right - minSide)/2f, (pe.ClipRectangle.Y + pe.ClipRectangle.Bottom - minSide)/2f);
			RectangleF rect = new RectangleF(center, new Size(minSide, minSide));
			if (this.shapeStyle == ShapeStyle.Circle)
			{
				pe.Graphics.FillEllipse(new SolidBrush(this.shapeColor), rect);
			}
			else
			{
				PointF[] points = new PointF[this.shapeSides];
				for (int i = 0; i < this.shapeSides; i++)
				{
					points[i].X = (float) (minSide/2f*Math.Cos(i/((float) this.shapeSides)*Math.PI*2)) + center.X+minSide/2f;
					points[i].Y = (float) (minSide/2f*Math.Sin(i/((float) this.shapeSides)*Math.PI*2)) + center.Y+minSide/2f;
				}
				pe.Graphics.FillPolygon(new SolidBrush(this.shapeColor), points);
			}
		}

		#region Component Designer generated code

		/// <summary>
		/// Required method for Designer support - do not modify 
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			components = new System.ComponentModel.Container();
		}

		#endregion

		protected override Size DefaultSize
		{
			get { return new Size(200, 200); }
		}

		[Category("Appearance")]
		[DefaultValue(typeof (Color), "Red")]
		[Description("The shape color.")]
		public Color ShapeColor
		{
			get { return this.shapeColor; }
			set
			{
				this.shapeColor = value;
				this.Invalidate();
			}
		}

		[Category("Appearance")]
		[DefaultValue(typeof (ShapeStyle), "Circle")]
		[Description("The shape style.")]
		[RefreshProperties(RefreshProperties.All)]
		public ShapeStyle ShapeStyle
		{
			get { return this.shapeStyle; }
			set
			{
				this.shapeStyle = value;
				if (this.shapeStyle == ShapeStyle.Circle)
				{
					// I want to set ShapeSides to be a read only, or non-browsable property
					// at design time
				}
				this.Invalidate();
			}
		}

		[Category("Appearance")]
		[DefaultValue(4)]
		[Description("The shape number of sides.")]
		[Browsable(true)]  // How can I change this attribute depending on the condition above?
		public int ShapeSides
		{
			get { return this.shapeSides; }
			set
			{
				if (value > 2)
				{
					this.shapeSides = value;
					this.Invalidate();
				}
			}
		}
	}

	/// <summary>Style of Shape to display.</summary>
	public enum ShapeStyle
	{
		// Fields
		Circle,
		Polygon
	}
}