Introduction to Developing a Custom Web Control.

Maharishi Bhatia

3.90/5 (7 votes)

Feb 15, 2005

CPOL

2 min read

54715

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This article explains the development of a custom web control using C#.

Introduction

Every organization has got its own controls for faster development of software. My organization has a standard way for displaying information, errors and warnings on the Web application. So I thought why not make it into a control so that different projects can use the same control and have full control on the look and feel to make it gel into the project UI. Though I have been working on C# components for the past two years this was the first opportunity to develop a custom control for the UI developers. I was totally amazed by the power of custom web controls and the design time support of the Visual Studio IDE.

Points of interest

Editor at Design Time

InfoImage property is used to set the image URL for the Information. The special attribute of this code was the Editor property which enables a window for navigating the local file images and selecting those images that we require:

[Bindable(true), Category("Information Details"), DefaultValue(""),
Editor("System.Web.UI.Design.ImageUrlEditor, System.Design, 
Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", 
typeof(UITypeEditor))]
public string InfoImage{
    get{
        return this._InfoImage;
    }
    set{
        this._InfoImage = value;
    }
}

Hiding inherited properties at design time

Since the custom control is inherited from the System.Web.UI.WebControls.WebControl class it has got some default properties like the forecolor, backcolor etc.. AccessKey is one of them which is shown at design time. I didn't want this property to be shown to the user at design time. So there is a special attribute called Browsable which if set to false, does not show the property at design time.
```
[Browsable(false), Bindable(false)]
public override string AccessKey {
    get {
        return base.AccessKey;
    }
    set {
        base.AccessKey = value;
    }
}
```
Hiding of detailed messages

If the IsDebug property of the control is set to true then the detailed message is shown. This can be useful if you are debugging some page, and if you want to see the detailed error message then the IsDebug property of the control can be set to true:
```
[Bindable(true),Category("Behavior"),DefaultValue(false)]
public bool IsDebug{
    get{
        return this._IsDebug;
    }
    set{
        this._IsDebug = value;
    }
}
```

Exception Support

This control directly takes an object of the System.Exception class and sets the message and stack trace to the control:

[Browsable(false),Bindable(true),DefaultValue(null)]
public Exception Exception{
    set{
        this._Exception = value;
        Text = this._Exception.Message;
        Details = 
          this._Exception.StackTrace+"\n\n"+this._Exception.TargetSite;
        MsgType = MessageType.ERROR;
    }
}

Rendering the Control

The control is rendered at design time which is very much similar to rendering at runtime. So the control is more like WYSIWYG, but much more easier and convenient:

/// <summary>
/// Render this control to the output parameter specified.
/// </summary>
protected override void Render(HtmlTextWriter output)
{
    if(Enabled){
        output.AddAttribute(HtmlTextWriterAttribute.Title,
                                             ToolTip,false);
        output.AddAttribute(HtmlTextWriterAttribute.Cellpadding,
                                        this.Cellpadding,false);
        output.AddAttribute(HtmlTextWriterAttribute.Cellspacing,
                                        this.Cellspacing,false);
        output.AddAttribute(HtmlTextWriterAttribute.Border, 
                            BorderWidth.Value.ToString(),false);
        output.AddAttribute(HtmlTextWriterAttribute.Bordercolor, 
                                        BorderColor.Name,false);
        output.AddStyleAttribute(HtmlTextWriterStyle.BorderStyle, 
                                         BorderStyle.ToString());
        output.AddAttribute(HtmlTextWriterAttribute.Bgcolor, 
                                           BackColor.Name,false);
        output.AddAttribute(HtmlTextWriterAttribute.Width,
             (Width.Value.ToString()+GetUnit(Width.Type)),false);
        output.AddAttribute(HtmlTextWriterAttribute.Height,
            (Height.Value.ToString()+GetUnit(Height.Type)),false);
        output.AddAttribute(HtmlTextWriterAttribute.Class,
                                                  CssClass,false);
        output.RenderBeginTag(HtmlTextWriterTag.Table);
    
        RenderHeader(output);
        RenderInfo(output);
        if(IsDebug && Details.Length > 0){
            RenderDetails(output);
        }
        output.RenderEndTag();
    }
}

private void RenderHeader(HtmlTextWriter output){
    output.RenderBeginTag(HtmlTextWriterTag.Tr);
        output.AddAttribute(HtmlTextWriterAttribute.Bgcolor, 
                                         TdBackColor.Name,false);
        output.AddAttribute(HtmlTextWriterAttribute.Rowspan,"2", 
                                                          false);
        output.AddAttribute(HtmlTextWriterAttribute.Width,"0%", 
                                                          false);
        output.AddAttribute(HtmlTextWriterAttribute.Height,"0%", 
                                                          false);
        output.RenderBeginTag(HtmlTextWriterTag.Td);
            switch(MsgType){
                case MessageType.ERROR:
                    output.AddAttribute(HtmlTextWriterAttribute.Src, 
                                                   ErrorImage,false);
                    break;
                case MessageType.WARNING:
                    output.AddAttribute(HtmlTextWriterAttribute.Src, 
                                                 WarningImage,false);
                    break;
                default:
                    output.AddAttribute(HtmlTextWriterAttribute.Src,
                                                    InfoImage,false);
                    break;
            }
            output.RenderBeginTag(HtmlTextWriterTag.Img);
            output.RenderEndTag();
        output.RenderEndTag();

        output.AddAttribute(HtmlTextWriterAttribute.Bgcolor, 
                                                TdBackColor.Name,false);
        output.AddAttribute(HtmlTextWriterAttribute.Width,"100%",false);
        output.AddAttribute(HtmlTextWriterAttribute.Height,"100%",false);
        switch(MsgType){
            case MessageType.ERROR:
                output.AddAttribute(HtmlTextWriterAttribute.Class, 
                                               ErrorHeaderTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(ErrorHeaderText);
                output.RenderEndTag();
                break;
            case MessageType.WARNING:
                output.AddAttribute(HtmlTextWriterAttribute.Class, 
                                             WarningHeaderTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(WarningHeaderText);
                output.RenderEndTag();
                break;
            default:
                output.AddAttribute(HtmlTextWriterAttribute.Class,
                                                InfoHeaderTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(InfoHeaderText);
                output.RenderEndTag();
                break;
        }
    output.RenderEndTag();
}

private void RenderInfo(HtmlTextWriter output){
    output.RenderBeginTag(HtmlTextWriterTag.Tr);
        output.AddAttribute(HtmlTextWriterAttribute.Bgcolor,
                                            TdBackColor.Name,false);
        switch(MsgType){
            case MessageType.ERROR:
                output.AddAttribute(HtmlTextWriterAttribute.Class,
                                           ErrorBreifTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(Text);
                output.RenderEndTag();
                break;
            case MessageType.WARNING:
                output.AddAttribute(HtmlTextWriterAttribute.Class, 
                                         WarningBreifTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(Text);
                output.RenderEndTag();
                break;
            default:
                output.AddAttribute(HtmlTextWriterAttribute.Class, 
                                            InfoBreifTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(Text);
                output.RenderEndTag();
                break;
        }
    output.RenderEndTag();
}

private void RenderDetails(HtmlTextWriter output){
    output.RenderBeginTag(HtmlTextWriterTag.Tr);
        output.AddAttribute(HtmlTextWriterAttribute.Bgcolor,
                                                TdBackColor.Name,false);
        output.AddAttribute(HtmlTextWriterAttribute.Colspan,"2",false);
        switch(MsgType){
            case MessageType.ERROR:
                output.AddAttribute(HtmlTextWriterAttribute.Class,
                                             ErrorDetailsTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(Details);
                output.RenderEndTag();
                break;
            case MessageType.WARNING:
                output.AddAttribute(HtmlTextWriterAttribute.Class, 
                                           WarningDetailsTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(Details);
                output.RenderEndTag();
                break;
            default:
                output.AddAttribute(HtmlTextWriterAttribute.Class,
                                              InfoDetailsTextCss,false);
                output.RenderBeginTag(HtmlTextWriterTag.Td);
                    output.Write(Details);
                output.RenderEndTag();
                break;
        }
    output.RenderEndTag();
}

Further works

Working on Web controls which can fire events, manage client side data and post back the data.