1. Create a new ASP.NET Web Application, Windows Forms Control Library or Class Library project
  2. Create a new class and inherit from System.Web.UI.WebControls.WebControl or from a specific control (like TextBox)
  3. Override the Render method and call writer.write() to write raw HTML to the page. ASP.Net server tags won’t work
  4. Use WriteBeginTag(), WriteEndTag(), RenderBeginTag(), RenderEndTag(), AddAttribute(), AddStyleAttribute() and WriteAttribute() to create HTML tags and attributes
  5. Add design-time attributes to properties, like:
    [Description(“The text to be shown in the control”)]
  6. Define a tagprefix attribute:
    [assembly: System.Web.UI.TagPrefix(“Anthoro.WebControls”, “awc”)]
    namespace Anthoro.WebControls
    { …
  7. Add a toolbox icon:
    1. Create a 16×16 pixel bitmap (BMP 16 colors) with the same name as the control.
    2. Add it to the root of your project
    3. Set its Build Action to “Embedded Resource”
  8. Add additional resources (stylesheet, images, text files, …) and mark them as “Embedded Resource” also
  9. Add an Assembly attribute for each resource to your AssemblyInfo.cs:
    [assembly: WebResource(“Anthoro.Controls.style.css”, “text/css”, PerformSubstitution = true)]
    When the PerformSubstitution parameter is set to true, the resource will be processed and other webresource URL’s inside it will be parsed and replaced (works only for text-based resources:
    background-image: url(‘<%= WebResource("Anthoro.Controls.back.png")%>‘);
  10. Pay attention to the full namespace and folder structure where your resources are stored. The resource name is build as: Namespace + Subfolders + filename
  11. At design-time, the control is displayed by running the rendering logic. To override this, use the GetDesignTimeHtml(), GetEmptyDesignTimeHtml() and GetErrorDesignTimeHtml() methods

Update: Unlike in C#, in VB.Net the subfolders are not taken into account with the namespace. So the example above in point 10 will be: Namespace + filename