How to add an additional css class from code behind using ASP.NET?

Question

How to add an additional css class from code behind using ASP.NET?

CURRENT TEXTBOX

<asp:TextBox ID="txt" CssClass="MyClass" runat="Server" />

DESIRED OUTPUT

<asp:TextBox ID="txt" CssClass="MyClass Error" runat="Server" />

---

Testing

txt.CssClass = "Error"

This replaces current css class.

txt.CssClass = "MyClass Error"

This works but but is greatly inefficient having to specify class.

txt.Attributes.Add("class", "Error")

This only works if no initial class is set.

txt.Attributes("class") += " Error"

This does not work for me.

Answer 1

Add additional CssClass like this:

txt.CssClass = txt.CssClass + " Error"

The above can also be abbraviated as:

txt.CssClass += " Error"

Answer 2

I know that you were looking for a quick one-liner. However, this previous answer may prove useful going forward:

How to add more than 1 class to an element in ASP.NET?