我结合了前面的所有答案：

    public static string UrlPathCombine(string path1, string path2)
    {
        path1 = path1.TrimEnd('/') + "/";
        path2 = path2.TrimStart('/');

        return Path.Combine(path1, path2)
            .Replace(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
    }

    [TestMethod]
    public void TestUrl()
    {
        const string P1 = "http://msdn.microsoft.com/slash/library//";
        Assert.AreEqual("http://msdn.microsoft.com/slash/library/site.aspx", UrlPathCombine(P1, "//site.aspx"));

        var path = UrlPathCombine("Http://MyUrl.com/", "Images/Image.jpg");

        Assert.AreEqual(
            "Http://MyUrl.com/Images/Image.jpg",
            path);
    }

这里已经有了一些很好的答案。根据mdsharpe的建议，这里有一个扩展方法，当您想处理Uri实例时可以轻松使用：

using System;
using System.Linq;

public static class UriExtensions
{
    public static Uri Append(this Uri uri, params string[] paths)
    {
        return new Uri(paths.Aggregate(uri.AbsoluteUri, (current, path) => string.Format("{0}/{1}", current.TrimEnd('/'), path.TrimStart('/'))));
    }
}

使用示例：

var url = new Uri("http://example.com/subpath/").Append("/part1/", "part2").AbsoluteUri;

这将产生http://example.com/subpath/part1/part2

如果您希望使用字符串而不是Uris，那么以下操作也会产生相同的结果，只需根据您的需要进行调整即可：

public string JoinUriSegments(string uri, params string[] segments)
{
    if (string.IsNullOrWhiteSpace(uri))
        return null;

    if (segments == null || segments.Length == 0)
        return uri;

    return segments.Aggregate(uri, (current, segment) => $"{current.TrimEnd('/')}/{segment.TrimStart('/')}");
}

var uri = JoinUriSegements("http://example.com/subpath/", "/part1/", "part2");

这是Microsoft的（OfficeDev PnP）方法UrlUtility。组合：

    const char PATH_DELIMITER = '/';

    /// <summary>
    /// Combines a path and a relative path.
    /// </summary>
    /// <param name="path"></param>
    /// <param name="relative"></param>
    /// <returns></returns>
    public static string Combine(string path, string relative) 
    {
        if(relative == null)
            relative = String.Empty;

        if(path == null)
            path = String.Empty;

        if(relative.Length == 0 && path.Length == 0)
            return String.Empty;

        if(relative.Length == 0)
            return path;

        if(path.Length == 0)
            return relative;

        path = path.Replace('\\', PATH_DELIMITER);
        relative = relative.Replace('\\', PATH_DELIMITER);

        return path.TrimEnd(PATH_DELIMITER) + PATH_DELIMITER + relative.TrimStart(PATH_DELIMITER);
    }

来源：GitHub

如果您不想拥有像Flurl这样的依赖项，可以使用它的源代码：

    /// <summary>
    /// Basically a Path.Combine for URLs. Ensures exactly one '/' separates each segment,
    /// and exactly on '&amp;' separates each query parameter.
    /// URL-encodes illegal characters but not reserved characters.
    /// </summary>
    /// <param name="parts">URL parts to combine.</param>
    public static string Combine(params string[] parts) {
        if (parts == null)
            throw new ArgumentNullException(nameof(parts));

        string result = "";
        bool inQuery = false, inFragment = false;

        string CombineEnsureSingleSeparator(string a, string b, char separator) {
            if (string.IsNullOrEmpty(a)) return b;
            if (string.IsNullOrEmpty(b)) return a;
            return a.TrimEnd(separator) + separator + b.TrimStart(separator);
        }

        foreach (var part in parts) {
            if (string.IsNullOrEmpty(part))
                continue;

            if (result.EndsWith("?") || part.StartsWith("?"))
                result = CombineEnsureSingleSeparator(result, part, '?');
            else if (result.EndsWith("#") || part.StartsWith("#"))
                result = CombineEnsureSingleSeparator(result, part, '#');
            else if (inFragment)
                result += part;
            else if (inQuery)
                result = CombineEnsureSingleSeparator(result, part, '&');
            else
                result = CombineEnsureSingleSeparator(result, part, '/');

            if (part.Contains("#")) {
                inQuery = false;
                inFragment = true;
            }
            else if (!inFragment && part.Contains("?")) {
                inQuery = true;
            }
        }
        return EncodeIllegalCharacters(result);
    }

    /// <summary>
    /// URL-encodes characters in a string that are neither reserved nor unreserved. Avoids encoding reserved characters such as '/' and '?'. Avoids encoding '%' if it begins a %-hex-hex sequence (i.e. avoids double-encoding).
    /// </summary>
    /// <param name="s">The string to encode.</param>
    /// <param name="encodeSpaceAsPlus">If true, spaces will be encoded as + signs. Otherwise, they'll be encoded as %20.</param>
    /// <returns>The encoded URL.</returns>
    public static string EncodeIllegalCharacters(string s, bool encodeSpaceAsPlus = false) {
        if (string.IsNullOrEmpty(s))
            return s;

        if (encodeSpaceAsPlus)
            s = s.Replace(" ", "+");

        // Uri.EscapeUriString mostly does what we want - encodes illegal characters only - but it has a quirk
        // in that % isn't illegal if it's the start of a %-encoded sequence https://stackoverflow.com/a/47636037/62600

        // no % characters, so avoid the regex overhead
        if (!s.Contains("%"))
            return Uri.EscapeUriString(s);

        // pick out all %-hex-hex matches and avoid double-encoding 
        return Regex.Replace(s, "(.*?)((%[0-9A-Fa-f]{2})|$)", c => {
            var a = c.Groups[1].Value; // group 1 is a sequence with no %-encoding - encode illegal characters
            var b = c.Groups[2].Value; // group 2 is a valid 3-character %-encoded sequence - leave it alone!
            return Uri.EscapeUriString(a) + b;
        });
    }

所以我有另一种方法，类似于使用UriBuilder的每个人。

我不想拆分我的BaseUrl（它可以包含路径的一部分，例如。http://mybaseurl.com/dev/)正如javajavajavajava所做的那样。

下面的代码片段显示了代码+测试。

注意：此解决方案降低了主机并附加了一个端口。如果不需要这样做，可以通过例如利用UriBuilder的Uri属性来编写字符串表示。

  public class Tests
  {
         public static string CombineUrl (string baseUrl, string path)
         {
           var uriBuilder = new UriBuilder (baseUrl);
           uriBuilder.Path = Path.Combine (uriBuilder.Path, path);
           return uriBuilder.ToString();
         }

         [TestCase("http://MyUrl.com/", "/Images/Image.jpg", "http://myurl.com:80/Images/Image.jpg")]
         [TestCase("http://MyUrl.com/basePath", "/Images/Image.jpg", "http://myurl.com:80/Images/Image.jpg")]
         [TestCase("http://MyUrl.com/basePath", "Images/Image.jpg", "http://myurl.com:80/basePath/Images/Image.jpg")]
         [TestCase("http://MyUrl.com/basePath/", "Images/Image.jpg", "http://myurl.com:80/basePath/Images/Image.jpg")]
         public void Test1 (string baseUrl, string path, string expected)
         {
           var result = CombineUrl (baseUrl, path);

           Assert.That (result, Is.EqualTo (expected));
         }
  }

在Windows 10上使用.NET Core 2.1进行测试。

为什么这样做？

尽管Path.Combine将返回反斜杠（至少在Windows上），但UriBuilder在路径设置器中处理这种情况。

摘自https://github.com/dotnet/corefx/blob/master/src/System.Private.Uri/src/System/UriBuilder.cs（注意调用字符串。替换）

[AllowNull]
public string Path
{
      get
      {
          return _path;
      }
      set
      {
          if ((value == null) || (value.Length == 0))
          {
              value = "/";
          }
          _path = Uri.InternalEscapeString(value.Replace('\\', '/'));
          _changed = true;
      }
 }

这是最好的方法吗？

当然，这个解决方案非常自我描述（至少在我看来）。但是，您依赖的是.NETAPI中的未记录（至少我在快速谷歌搜索中没有发现任何内容）“功能”。这可能会随着将来的版本而改变，因此请将方法与测试结合起来。

中有测试https://github.com/dotnet/corefx/blob/master/src/System.Private.Uri/tests/FunctionalTests/UriBuilderTests.cs（Path_Get_Set），用于检查\是否正确转换。

附带说明：如果Uri将用于System.Uri构造函数，则也可以直接使用UriBuilder.Uri属性。

如果您不想在ASP.NET Core（也在Microsoft.Owin中提供）中添加第三方依赖项（如Flurl）或创建自定义扩展方法，则可以使用PathString来构建URI路径。然后，可以使用URI和UriBuilder的组合创建完整的URI。

在这种情况下，应为：

new Uri(new UriBuilder("http", "MyUrl.com").Uri, new PathString("/Images").Add("/Image.jpg").ToString())

这提供了所有组成部分，而无需在基URL中指定分隔符。不幸的是，PathString要求在每个字符串前面加上/，否则它实际上会引发ArgumentException！但至少您可以以易于单元测试的方式确定地构建URI。