HttpClientFactory.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
using System.Web.Http;

namespace System.Net.Http
{
    public static class HttpClientFactory
    {
        /// <summary>
        /// Creates a new <see cref="HttpClient"/> instance configured with the handlers provided and with an
        /// <see cref="HttpClientHandler"/> as the innermost handler.
        /// </summary>
        /// <param name="handlers">An ordered list of <see cref="DelegatingHandler"/> instances to be invoked as an 
        /// <see cref="HttpRequestMessage"/> travels from the <see cref="HttpClient"/> to the network and an 
        /// <see cref="HttpResponseMessage"/> travels from the network back to <see cref="HttpClient"/>.
        /// The handlers are invoked in a top-down fashion. That is, the first entry is invoked first for 
        /// an outbound request message but last for an inbound response message.</param>
        /// <returns>An <see cref="HttpClient"/> instance with the configured handlers.</returns>
        [SuppressMessage("Microsoft.Reliability", "CA2000:Dispose objects before losing scope", Justification = "Handler is disposed with HttpClient")]
        public static HttpClient Create(params DelegatingHandler[] handlers)
        {
            return Create(new HttpClientHandler(), handlers);
        }

        /// <summary>
        /// Creates a new <see cref="HttpClient"/> instance configured with the handlers provided and with the
        /// provided <paramref name="innerHandler"/> as the innermost handler.
        /// </summary>
        /// <param name="innerHandler">The inner handler represents the destination of the HTTP message channel.</param>
        /// <param name="handlers">An ordered list of <see cref="DelegatingHandler"/> instances to be invoked as an 
        /// <see cref="HttpRequestMessage"/> travels from the <see cref="HttpClient"/> to the network and an 
        /// <see cref="HttpResponseMessage"/> travels from the network back to <see cref="HttpClient"/>.
        /// The handlers are invoked in a top-down fashion. That is, the first entry is invoked first for 
        /// an outbound request message but last for an inbound response message.</param>
        /// <returns>An <see cref="HttpClient"/> instance with the configured handlers.</returns>
        [SuppressMessage("Microsoft.Reliability", "CA2000:Dispose objects before losing scope", Justification = "Handler is disposed with HttpClient")]
        public static HttpClient Create(HttpMessageHandler innerHandler, params DelegatingHandler[] handlers)
        {
            HttpMessageHandler pipeline = CreatePipeline(innerHandler, handlers);
            return new HttpClient(pipeline);
        }

        /// <summary>
        /// Creates an instance of an <see cref="HttpMessageHandler"/> using the <see cref="DelegatingHandler"/> instances
        /// provided by <paramref name="handlers"/>. The resulting pipeline can be used to manually create <see cref="HttpClient"/>
        /// or <see cref="HttpMessageInvoker"/> instances with customized message handlers.
        /// </summary>
        /// <param name="innerHandler">The inner handler represents the destination of the HTTP message channel.</param>
        /// <param name="handlers">An ordered list of <see cref="DelegatingHandler"/> instances to be invoked as part 
        /// of sending an <see cref="HttpRequestMessage"/> and receiving an <see cref="HttpResponseMessage"/>.
        /// The handlers are invoked in a top-down fashion. That is, the first entry is invoked first for 
        /// an outbound request message but last for an inbound response message.</param>
        /// <returns>The HTTP message channel.</returns>
        public static HttpMessageHandler CreatePipeline(HttpMessageHandler innerHandler, IEnumerable<DelegatingHandler> handlers)
        {
            if (innerHandler == null)
            {
                throw Error.ArgumentNull("innerHandler");
            }

            if (handlers == null)
            {
                return innerHandler;
            }

            // Wire handlers up in reverse order starting with the inner handler
            HttpMessageHandler pipeline = innerHandler;
            IEnumerable<DelegatingHandler> reversedHandlers = handlers.Reverse();
            foreach (DelegatingHandler handler in reversedHandlers)
            {
                if (handler == null)
                {
                    throw Error.Argument("handlers", Properties.Resources.DelegatingHandlerArrayContainsNullItem, typeof(DelegatingHandler).Name);
                }

                if (handler.InnerHandler != null)
                {
                    throw Error.Argument("handlers", Properties.Resources.DelegatingHandlerArrayHasNonNullInnerHandler, typeof(DelegatingHandler).Name, "InnerHandler", handler.GetType().Name);
                }

                handler.InnerHandler = pipeline;
                pipeline = handler;
            }

            return pipeline;
        }
    }
}