UriAppend.cs

using System;
using System.Linq;
using FluentAssertions;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace UriAppend
{
    public static class UriExtensions
    {
        public static Uri Append(this Uri baseUri, string relativeUrl) => new Uri(baseUri, relativeUrl);
        public static Uri Append(this Uri baseUri, params string[] relativeUrls) => relativeUrls.Aggregate(baseUri, (current, rel) => current?.Append(rel));
    }

    [TestClass]
    public class Tests
    {
        [TestMethod]
        public void Test()
        {
            new Uri("http://loclahost/")
                .Append("x/", "y/", "./z", "?p=1").ShouldBe("http://loclahost/x/y/z?p=1")
                .Append("#hash").ShouldBe("http://loclahost/x/y/z?p=1#hash")
                .Append("/moo").ShouldBe("http://loclahost/moo")
                .Append("//host/", "dir/", "file", "?par=1", "#h").ShouldBe("http://host/dir/file?par=1#h")
                .Invoking(u => u.Append("///")).ShouldThrow<UriFormatException>();

            new Uri("http://test.com/mydirectory/").Append("helloworld.aspx")
                .ShouldBe("http://test.com/mydirectory/helloworld.aspx");

            new Uri("https://xx.yy").Append("//localhost/", "d1/", "../d2/../d3/").ShouldBe("https://localhost/d3/")
                .Append(".").ShouldBe("https://localhost/d3/")
                .Append("./x").ShouldBe("https://localhost/d3/x")
                .Append("//h/", "s/../x", "?x=1", "?y=2").ShouldBe("https://h/x?y=2");
        }
    }

    public static class TestHelperExt
    {
        public static Uri ShouldBe(this Uri t, string what)
        {
            t.ShouldBeEquivalentTo(what);
            return t;
        }
    }
}