change css element value dynamically-Collection of common programming errors

There are several ways we do,   Simplest way

textBox1.Attributes["class"] = "class name";

  Another way is to use Web Extension,

static class WebControlsExtensions { public static void AddCssClass (this WebControl control, string cssClass) { control.CssClass += " " + cssClass; } public static void RemoveCssClass (this WebControl control, string cssClass) { control.CssClass = control.CssClass.replace(" " + cssClass, "");
     }
 }

ctl.AddCssClass(“ReadOnly”); ctl.RemoveCssClass(“ReadOnly”);   Hope this helps.

cheers