Vault of Thoughts

2006-06-20

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Validation controls as images - workaround

I have been trying to make the Validation controls show an image instead of plain text in case of an error. In doing so I have used Reflector a lot.


My investigation had lead me to some conclusions and even more questions. The main problem here is that for some creepy reason, the BaseValidator inherits directly from a Label control!!! Now I have always been aware of this fact but it was until not long age when it struck me how bad decision it was on Microsoft part. Keeping the parent class in mind it should be obvious that making a Label display as an image was not going to be an easy task.


There are few possible ways in which you can make the validator control show an image instead of text. One of them is providing a img tag as a Text property for the control and it works, but is very unellegant. I thought that it is time to get to know the new feature of ASP.NET 2.0 - the Control Adapters.


After a rather long time it came out that it is not possible to use the control adapters for the validation control since some of the important logic that renders the control is in the Render method of a BaseValidator class and it cannot be easily replaced. I have left the adapter path but having learned a lot nonetheless.


Afer yet more time I have realized that the only real solutions are to either create my own classes implementing the IValidator interface - good there is such a thing or use some third party controls. I had however no spare time for writing my own fully blown controls and choosing the right toolset of third party controls is also time consuming I have decided to take a dirty shortcut.


Below is a listing of all the things I have done in my inherited validator control. Baasicaly I'm changing the TagKey to img, I'm providing a property for users to specify the ImageUrl. The rest is to trick the BaseValidator control to render image instead of a label. The important thing is that the Text property has to be something different than a null, empty or white-space string - otherwise it is possible that ErrorMessage will be rendered inside the img tags which is invalid.



protected override System.Web.UI.HtmlTextWriterTag TagKey
{
get
{
return System.Web.UI.HtmlTextWriterTag.Img;
}
}
public string ImageUrl
{
get {
return (string)(ViewState["ImageUrl"] ?? String.Empty);
}
set { ViewState["ImageUrl"] = value; }
}
protected override void
AddAttributesToRender(System.Web.UI.HtmlTextWriter writer)
{
writer.AddAttribute(
System.Web.UI.HtmlTextWriterAttribute.Src,
ResolveClientUrl(ImageUrl));
if (!string.IsNullOrEmpty(Text))
{
writer.AddAttribute(
System.Web.UI.HtmlTextWriterAttribute.Title, Text);
writer.AddAttribute(
System.Web.UI.HtmlTextWriterAttribute.Alt, Text);
}
base.AddAttributesToRender(writer);
}
protected override void
RenderContents(System.Web.UI.HtmlTextWriter writer)
{
// to prevent rendering
}
[DefaultValue("Error")]
public override string Text
{
get
{
return (string.IsNullOrEmpty(base.Text) ? "Error" : base.Text);
}
set
{
base.Text = value;
}
}

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