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Getting Absolute URL's using ASP.NET

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In MVC 5, I had the following extension methods to generate absolute URL's, instead of relative ones:

public static class UrlHelperExtensions
{
    public static string AbsoluteAction(
        this UrlHelper url,
        string actionName, 
        string controllerName, 
        object routeValues = null)
    {
        string scheme = url.RequestContext.HttpContext.Request.Url.Scheme;
        return url.Action(actionName, controllerName, routeValues, scheme);
    }

    public static string AbsoluteContent(
        this UrlHelper url,
        string contentPath)
    {
        return new Uri(url.RequestContext.HttpContext.Request.Url, url.Content(contentPath)).ToString();
    }

    public static string AbsoluteRouteUrl(
        this UrlHelper url,
        string routeName,
        object routeValues = null)
    {
        string scheme = url.RequestContext.HttpContext.Request.Url.Scheme;
        return url.RouteUrl(routeName, routeValues, scheme);
    }
}

What would the equivalent be in asp .net MVC ?

UrlHelper.RequestContext no longer exists.
You can't get hold of the HttpContext as there is no longer a static HttpContext.Current property.
As far as I can see, you would now require the HttpContext or HttpRequest objects to be passed in also. Am I right? Is there some way to get hold of the current request?

Am I even on the right track, should the domain now be an environment variable, which is simple appended to the relative URL? Would this be a better approach?

You could modify your extension class to use the IHttpContextAccessor interface to get the HttpContext. Once you have the context, then you can get the HttpRequest instance from HttpContext.Request and use its properties Scheme, Host, Protocol etc as in:

string scheme = HttpContextAccessor.HttpContext.Request.Scheme;

For example, you could require your class to be configured with an HttpContextAccessor:

public static class UrlHelperExtensions
{        
    private static IHttpContextAccessor HttpContextAccessor;
    public static void Configure(IHttpContextAccessor httpContextAccessor)
    {           
        HttpContextAccessor = httpContextAccessor;  
    }

    public static string AbsoluteAction(
        this IUrlHelper url,
        string actionName, 
        string controllerName, 
        object routeValues = null)
    {
        string scheme = HttpContextAccessor.HttpContext.Request.Scheme;
        return url.Action(actionName, controllerName, routeValues, scheme);
    }

    ....
}

Which is something you can do on your Startup class (Startup.cs file):

public void Configure(IApplicationBuilder app)
{
    ...

    var httpContextAccessor = app.ApplicationServices.GetRequiredService<IHttpContextAccessor>();
    UrlHelperExtensions.Configure(httpContextAccessor);

    ...
}

You could probably come up with different ways of getting the IHttpContextAccessor in your extension class, but if you want to keep your methods as extension methods in the end you will need to inject the IHttpContextAccessor into your static class. (Otherwise you will need the IHttpContext as an argument on each call)

How to get ASP.NET Core's local server URLs

https://swimburger.net/blog/dotnet/how-to-get-aspdotnet-core-server-urls

https://andrewlock.net/finding-the-urls-of-an-aspnetcore-app-from-a-hosted-service-in-dotnet-6/