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

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

[SteveSandersonMS]

I’ve investigated, and it is a Mono issue. Filed https://github.com/mono/mono/issues/14630 and closing as external.

Thanks to everyone who posted workaround suggestions.

[codemonkey85]

I’m seeing the same issue following along with the workshop. According to the docs in https://github.com/dotnet-presentations/blazor-workshop/blob/master/docs/03-show-order-status.md#polling-for-order-details I’m supposed to use:

orderWithStatus = await HttpClient.GetJsonAsync<OrderWithStatus>($"/orders/{OrderId}");

Turns out, removing the first ‘/’ on the route (in front of ‘orders’) fixes the issue:

orderWithStatus = await HttpClient.GetJsonAsync<OrderWithStatus>($"orders/{OrderId}");