Flurl website document shows it has a configuration which allows to pass in Delegating Handlers.
https://flurl.dev/docs/configuration/
It would be at the end of the above link.

While the clientless pattern in static clients format works, the DI pattern doesnt.

FlurlHttp.Clients.WithDefaults(builder => builder
    .AddMiddleware(new MyDelegatingHandler()));

Above config works.

But this pattern below doesnt.

// DI pattern: 
services.AddSingleton<IFlurlClientCache>(sp => new FlurlClientCache()
    .WithDefaults(builder => builder
        .AddMiddleware(sp.GetService<IMyDelegatingHandler>())

I need the DI format, because my CustomDelegationHandler has other classes injected into it and it would have been easier if the DI version worked.

Since I dont have an interface, I used the below format. Wasnt sure about Interfaces for DelegatingHandlers.

Sample DelegationHandler being used:

public class MyDelegationHandler : DelegatingHandler
{
    private static string CorrelationId => "correlation-id";

    private readonly ICorrelationIdQuery _correlationIdQuery;

    public MyDelegationHandler(ICorrelationIdQuery correlationIdQuery)
    {
        _correlationIdQuery = correlationIdQuery ;
    }

    protected async override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        string correlationId = await _correlationIdQuery.Execute();

        if (!request.Headers.Contains(CorrelationId))
        {
            request.Headers.Add(CorrelationId, correlationId);
        }

        return await base.SendAsync(request, cancellationToken);
    }
}

So my code becomes:

FlurlHttp.Clients.WithDefaults(builder => builder
        .AddMiddleware(() => new MyDelegationHandler(serviceProvider.GetService<ICorrelationIdQuery>())));

Above works, but not sure calling service provider like that is good idea in the Program class.

What can i be doing wrong in the below DI version of the code.

services.AddSingleton(sp => new FlurlClientCache()
    .WithDefaults(builder => builder
        .AddMiddleware(() => sp.GetService<MyDelegationHandler>())
    ));