C#: Convert Dictionary to NameValueCollection

Why not use a simple foreach loop?

foreach(var kvp in dict)
{
    nameValueCollection.Add(kvp.Key.ToString(), kvp.Value.ToString());
}

This could be embedded into an extension method:

public static NameValueCollection ToNameValueCollection<TKey, TValue>(
    this IDictionary<TKey, TValue> dict)
{
    var nameValueCollection = new NameValueCollection();

    foreach(var kvp in dict)
    {
        string value = null;
        if(kvp.Value != null)
            value = kvp.Value.ToString();

        nameValueCollection.Add(kvp.Key.ToString(), value);
    }

    return nameValueCollection;
}

You could then call it like this:

var nameValueCollection = dict.ToNameValueCollection();