[chore] Demonstrate how users can provide their own proxy in custom HttpClient

EasyPost.Tests/ClientTest.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Linq;
+using System.Net;
 using System.Net.Http;
 using System.Threading;
 using System.Threading.Tasks;
@@ -109,6 +111,51 @@ public void TestHttpClientOverride()
             Assert.Equal(httpClient, overrideClient.CustomHttpClient);
         }
 
+        [Fact]
+        public async void TestHttpClientCustomProxy()
+        {
+            const string proxyAddress = "localhost:8888";
+
+            // Define a custom proxy in a custom HttpClientHandler in a custom HttpClient
+            HttpClientHandler handler = new()
+            {
+                UseProxy = true,
+                Proxy = new WebProxy($"http://{proxyAddress}"),
+            };
+            HttpClient httpClient = new(handler: handler);
+
+            Client client = new(new ClientConfiguration(FakeApikey)
+            {
+                CustomHttpClient = httpClient,
+            });
+
+            Assert.Equal(httpClient, client.CustomHttpClient);
+
+            // Assert that the proxy is set in the HttpClient by attempting to make a request (should fail due to invalid proxy address)
+            try
+            {
+                await client.Address.Create(new Parameters.Address.Create());
+                Assert.Fail("Expected HttpRequestException");
+            }
+            catch (HttpRequestException e)
+            {
+                // GitHub runner will reject the connection attempt, this is considered a pass
+                if (e.Message.Contains("No connection could be made because the target machine actively refused it"))
+                {
+                    Assert.True(true);
+                }
+                else // Evaluate the error message
+                {
+#if NET5_0_OR_GREATER
+                    Assert.Equal($"Connection refused ({proxyAddress})", e.Message);
+#else
+                    // Message is inconsistent in .NET Framework, so just assert that the exception was thrown
+                    Assert.True(true);
+#endif
+                }
+            }
+        }
+
         [Fact]
         public async Task TestRequestHooks()
         {

README.md

@@ -218,6 +218,28 @@ client.Hooks.OnRequestExecuting += (sender, args) => { /* ... */ };
 client.Hooks.OnRequestExecuting -= OnRequestExecutingHandler;
 ```
 
+### Proxies
+
+If you need to use a proxy to make requests to the EasyPost API, you can define a custom `HttpClientHandler` on a custom `HttpClient` passed to the `ClientConfiguration` constructor.
+
+```csharp
+// Define a custom HttpClientHandler with details about the proxy
+HttpClientHandler handler = new()
+{
+        UseProxy = true,
+        Proxy = new WebProxy($"http://localhost:8888"),
+};
+
+// Define a custom HttpClient with the custom handler
+HttpClient httpClient = new(handler: handler);
+
+// Pass the custom HttpClient to the ClientConfiguration constructor when creating a new EasyPost Client
+Client client = new(new ClientConfiguration(FakeApikey)
+{
+    CustomHttpClient = httpClient,
+});
+```
+
 ## Documentation
 
 API documentation can be found at: <https://easypost.com/docs/api>.