IsNullOrEmpty' does not exist in the current context : <p>I have this code:</p>

<pre><code>public static FormattedString AddParagraph(this FormattedString formattedString, string text)
    {
        if (IsNullOrEmpty(text))
            return formattedString;
        else
        {
            formattedString.Spans.Add(new Span { Text = text + Environment.NewLine + Environment.NewLine, ForegroundColor = Color.FromHex("555555") });
            return formattedString;
        }
    }
</code></pre>

<p>But it's telling me: The name 'IsNullOrEmpty' does not exist in the current context </p>

<p>Can anyone give me advice on how I can check if this string is "" or it has something other than ""?</p>
