A polyfill for Refit 2.0+ in scripting environments (e.g. LINQPad). Warning: This won't work on all platforms Refit 2.0+ supports (e.g. iOS, maybe WinRT?).

ProxyRestService.cs

using System.Net.Http;
using Castle.DynamicProxy; // From Castle.Core
using Refit;

public class ProxyRestService
{
    static readonly ProxyGenerator Generator = new ProxyGenerator();
    
    public static T For<T>(HttpClient client)
        where T : class
    {
        if(!typeof(T).IsInterface) {
            throw new InvalidOperationException("T must be an interface.");
        }
        
        var interceptor = new RestMethodInterceptor<T>(client);
        return Generator.CreateInterfaceProxyWithoutTarget<T>(interceptor);
    }

    public static T For<T>(string hostUrl)
        where T : class
    {
        var client = new HttpClient() { BaseAddress = new Uri(hostUrl) };
        return For<T>(client);
    }
    
    class RestMethodInterceptor<T> : IInterceptor
    {
        static readonly Dictionary<string, Func<HttpClient, object[], object>> methodImpls 
            = RequestBuilder.ForType<T>().InterfaceHttpMethods
                .ToDictionary(k => k, v => RequestBuilder.ForType<T>().BuildRestResultFuncForMethod(v));
    
        readonly HttpClient client;
        
        public RestMethodInterceptor(HttpClient client)
        {
            this.client = client;
        }
        
        public void Intercept(IInvocation invocation)
        {
            if (!methodImpls.ContainsKey(invocation.Method.Name)) {
            throw new NotImplementedException();
            }   
            invocation.ReturnValue = methodImpls[invocation.Method.Name](client, invocation.Arguments);
        }
    }
}

just use the following to create the dictionary:

	static readonly Dictionary<string, Func<HttpClient, object[], object>> methodImpls
		= typeof(T).GetMethods().Select(m => m.Name)
			.ToDictionary(k => k, v => RequestBuilder.ForType<T>().BuildRestResultFuncForMethod(v));