C# URI Concatenation

The Uri class in C# has a constructor that takes a Uri baseUri and string relativeUri. However, you may not get the expected, concatenated Uri, unless you know how certain situations are handled (I was recently tripped-up by this 😉).

Quiz

Given the following code:

Uri baseUri = new Uri("https://www.example.com/api");
Uri result = new Uri(baseUri, "v1/helloworld"); // https://www.example.com/api/v1/helloworld

What’s the value of result? If you said "https://www.example.com/api/v1/helloworld", then you are… not correct 😲! While it may seem a bit counterintuitive, relativeUri gets applied “relative” to the canonical-version of baseUri. RFC 3986 has a lot of info. on URI syntax – short-version for us specific to this issue is to know that a URI-path needs to end with a trailing-slash.

How to get the expected output

Add trailing-slash to baseUri

Uri baseUri = new Uri("https://www.example.com/api/");
Uri result = new Uri(baseUri, "v1/helloworld");

⚠ Remember, though, if relativeUri has a leading-slash, then we’re opting for an “absolute path” and will concatenate relative to baseUri.Host. So, the following has the same result as our original code:

Uri baseUri = new Uri("https://www.example.com/api/"); // `result` is the same regardless of trailing-slash.
Uri result = new Uri(baseUri, "/v1/helloworld"); // https://www.example.com/v1/helloworld

What about string.Join?

If you’re looking for something without needing to verify leading or trailing slashes, then this approach could work. But, some servers may not like redundant slashes, so test appropriately. Given this helper-method:

Uri ConcatenateUris(Uri baseUri, Uri relativeUri) => new Uri(string.Join("/", [ baseUri.ToString(), relativeUri.ToString() ]));

and this setup:

Uri baseUri = new Uri(baseInput);
Uri relativeUri = new Uri(relativeInput, UriKind.Relative);

Uri result = ConcatenateUris(baseUri, relativeUri);

Then, here’s a breakout of result:

Recommendations?

If you have hard-coded values, then I’d opt for adding the trailing-slash to baseUri. If you are using app. settings and you want resiliency, then something like ConcatendateUris could work.