[Blazor] HttpClient error when url starts with '/'

[stavroskasidis]

Describe the bug

Attempting to make an http call in blazor client-side, using a url that starts with "/", (e.g. "/api/GetValues") results in:
System.ArgumentException: Only 'http' and 'https' schemes are allowed.

To Reproduce

Steps to reproduce the behavior:

1. Using ASP.NET Core 3.0 preview 4
2. Create a new blazorhosted project
3. Go to FetchData.razor and change the url from api/SampleData/WeatherForecasts to /api/SampleData/WeatherForecasts
4. Run the app and navigate to /fetchdata

Screenshots

[danroth27]

@SteveSandersonMS Is potentially an issue with our underlying HttpClientHandler? We're still not using the one from the Mono folks yet, correct?

[SteveSandersonMS]

Yes. We probably should fix this independently of moving to the Mono handler.

[danroth27]

@Eilon hit this as well when updating Hubbup to Preview4.

[Eilon]

I was migrating from Blazor 0.7.0 to ASP.NET Core 3.0 Preview 4 and hit this. I had to hard-code my fully qualified URL to get it to work:

var result = await Http.GetJsonAsync<IssueMoveData>($"https://localhost:44347/api/getmovedata/{fromOwner}/{fromRepo}/{fromIssueNumber}");

Otherwise the BaseAddress was the "current URL" in the browser, which had some pre-existing path segment (e.g. https://localhost:44347/Mover/), which would cause URLs to be resolved incorrectly.

[CRodriguez25]

Is there a workaround for this (besides just hardcoding the URL)?

[Eilon]

I set the URL explicitly on the call to HttpClient. Slight;y annoying but it was only in a few places, so not too bad.

[CRodriguez25]

I was actually just able to inject a URIHelper and use _uriHelper.GetBaseUri() to construct the API calls.

public async Task<Something> Get()
{
    var client = new HttpClient();
    var baseUri = _uriHelper.GetBaseUri();
    if (!baseUri.EndsWith("/")) baseUrl += '/';
    var result = await client.GetJsonAsync<Something>($"{baseUri}api/somethings");
    return result;
}