Question

I have a div and I am trying to add a CSS class to it in code but I receive the following error when I try

Property or indexer 'System.Web.UI.HtmlControls.HtmlControl.Style' cannot be assigned to -- it is read only

I am using the following code:

protected void BTNEvent_Click(object sender, ImageClickEventArgs e)
{
    BtnventCss.Style= "hom_but_a";                 
}

Can anyone please help me?

Answer 1

What if:

 <asp:Button ID="Button1" runat="server" CssClass="test1 test3 test-test" />

To add or remove a class, instead of overwriting all classes with

   BtnventCss.CssClass = "hom_but_a"

keep the HTML correct:

    string classname = "TestClass";

    // Add a class
    BtnventCss.CssClass = String.Join(" ", Button1
               .CssClass
               .Split(' ')
               .Except(new string[]{"",classname})
               .Concat(new string[]{classname})
               .ToArray()
       );

     // Remove a class
     BtnventCss.CssClass = String.Join(" ", Button1
               .CssClass
               .Split(' ')
               .Except(new string[]{"",classname})
               .ToArray()
       );

This assures

• The original classnames remain.
• There are no double classnames
• There are no disturbing extra spaces

Especially when client-side development is using several classnames on one element.

In your example, use

   string classname = "TestClass";

    // Add a class
    Button1.Attributes.Add("class", String.Join(" ", Button1
               .Attributes["class"]
               .Split(' ')
               .Except(new string[]{"",classname})
               .Concat(new string[]{classname})
               .ToArray()
       ));

     // Remove a class
     Button1.Attributes.Add("class", String.Join(" ", Button1
               .Attributes["class"]
               .Split(' ')
               .Except(new string[]{"",classname})
               .ToArray()
       ));

You should wrap this in a method/property 😉