Changing the default landing page in ASP.NET Core Razor page?

Question

I tried:

services.AddMvc().AddRazorPagesOptions(options =>
{
    options.Conventions.AddPageRoute("/Index", "old");
    options.Conventions.AddPageRoute("/NewIndex", "");
}).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

This exception is thrown:

AmbiguousMatchException: The request matched multiple endpoints. Matches:

Page: /Index

Page: /NewIndex

I found this, that suggests renaming the Index page, but it obviously, if not a good reason presented, is a workaround. Can't I just change the default page without renaming the /Index page?

EDIT

The suggested SO threads don't cover the problem I explained, which is overriding the default route without having to rename the default Index page. The accepted answer solved the problem.

Answer 1

Default pages in Razor Pages are those that have an empty string route template generated for them. You can use a custom PageRouteModelConvention to remove the empty string route template that gets generated for the Index.cshtml page, and add it instead to whichever page you want as your default page:

public class HomePageRouteModelConvention : IPageRouteModelConvention
{
    public void Apply(PageRouteModel model)
    {
        if(model.RelativePath == "/Pages/Index.cshtml")
        {
            var currentHomePage = model.Selectors.Single(s => s.AttributeRouteModel.Template == string.Empty);
            model.Selectors.Remove(currentHomePage);
        }

        if (model.RelativePath == "/Pages/NewIndex.cshtml")
        {
            model.Selectors.Add(new SelectorModel()
            {
                AttributeRouteModel = new AttributeRouteModel
                {
                    Template = string.Empty
                }
            });
        }
    }
}

You register the convention in ConfigureServices:

services.AddMvc().AddRazorPagesOptions(options =>
{
    options.Conventions.Add(new HomePageRouteModelConvention());
}).SetCompatibilityVersion(CompatibilityVersion.Latest);

You can read more about custom page route model conventions here: https://www.learnrazorpages.com/advanced/custom-route-conventions