Get relative part of an Uri

UriExtensions.cs

public static string ToRelative(this Uri uri)
{
    if (uri == null)
    {
        throw new ArgumentNullException(nameof(uri));
    }

    return uri.IsAbsoluteUri ? uri.PathAndQuery : uri.OriginalString;
}

public static string ToAbsolute(this Uri uri, string baseUrl)
{
    if (uri == null)
    {
        throw new ArgumentNullException(nameof(uri));
    }
    if (baseUrl == null)
    {
        throw new ArgumentNullException(nameof(baseUrl));
    }

    var baseUri = new Uri(baseUrl);

    return uri.ToAbsolute(baseUri);
}

public static string ToAbsolute(this Uri uri, Uri baseUri)
{
    if (uri == null)
    {
        throw new ArgumentNullException(nameof(uri));
    }
    if (baseUri == null)
    {
        throw new ArgumentNullException(nameof(baseUri));
    }

    var relative = uri.ToRelative();

    if (Uri.TryCreate(baseUri, relative, out var absolute))
    {
        return absolute.ToString();
    }

    return uri.IsAbsoluteUri ? uri.ToString() : null;
}