[DotNetDevelopment] Problem with a namespace...

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I cannot find the correct reference to one piece of code I have. This
is some sample code I found, and I need to build on it. What reference
do I need to add to make this work??

Here is a code snippet:

namespace TestSignature
{
    using System;
    using System.Diagnostics;
    using System.Drawing;
    using System.IO;
    using System.Reflection;
    using System.Security.Cryptography;
    using System.Security.Cryptography.X509Certificates;
    using System.Security.Cryptography.Xml;
    using System.Text;
    using System.Windows.Forms;
    using System.Xml;

    using Extensibility;
    using Microsoft.Office.Core;
    using stdole;

    using TestSignature.Utils;

    using XmlDataObject = System.Security.Cryptography.Xml.DataObject;

I get the error at using TestSignature.Utils;
If I comment the line out, some of the code losses functionality.
What needs to be added?