Passing per-request headers

src/GraphQL.Client.Http/GraphQLHttpClient.cs

@@ -103,7 +103,7 @@ public IObservable<GraphQLResponse<TResponse>> CreateSubscriptionStream<TRespons
 
 		private async Task<GraphQLResponse<TResponse>> SendHttpPostRequestAsync<TResponse>(GraphQLRequest request, CancellationToken cancellationToken = default) {
 			var preprocessedRequest = await Options.PreprocessRequest(request, this);
-			using var httpRequestMessage = this.GenerateHttpRequestMessage(preprocessedRequest.SerializeToJson(Options));
+			using var httpRequestMessage = this.GenerateHttpRequestMessage(preprocessedRequest);
 			using var httpResponseMessage = await this.HttpClient.SendAsync(httpRequestMessage, cancellationToken);
 			if (!httpResponseMessage.IsSuccessStatusCode) {
 				throw new GraphQLHttpException(httpResponseMessage);
@@ -113,10 +113,15 @@ private async Task<GraphQLResponse<TResponse>> SendHttpPostRequestAsync<TRespons
 			return await bodyStream.DeserializeFromJsonAsync<GraphQLHttpResponse<TResponse>>(Options, cancellationToken);
 		}
 
-		private HttpRequestMessage GenerateHttpRequestMessage(string requestString) {
-			return new HttpRequestMessage(HttpMethod.Post, this.Options.EndPoint) {
-				Content = new StringContent(requestString, Encoding.UTF8, "application/json")
+		private HttpRequestMessage GenerateHttpRequestMessage(GraphQLRequest request) {
+			var message = new HttpRequestMessage(HttpMethod.Post, this.Options.EndPoint) {
+				Content = new StringContent(request.SerializeToJson(Options), Encoding.UTF8, "application/json")
 			};
+
+			if (request is GraphQLHttpRequest httpRequest)
+				httpRequest.PreprocessHttpRequestMessage(message);
+
+			return message;
 		}
 
 		private Uri GetWebSocketUri() {

src/GraphQL.Client.Http/GraphQLHttpRequest.cs

@@ -1,5 +1,22 @@
+using System;
+using System.Net.Http;
+using System.Text.Json.Serialization;
+
 namespace GraphQL.Client.Http {
 
 	public class GraphQLHttpRequest : GraphQLRequest {
+		public GraphQLHttpRequest()
+		{
+		}
+
+		public GraphQLHttpRequest(string query, object? variables = null, string? operationName = null) : base(query, variables, operationName)
+		{
+		}
+
+		/// <summary>
+		/// Allows to preprocess a <see cref="HttpRequestMessage"/> before it is sent, i.e. add custom headers
+		/// </summary>
+		[JsonIgnore]
+		public Action<HttpRequestMessage> PreprocessHttpRequestMessage { get; set; } = message => { };
 	}
 }

tests/GraphQL.Integration.Tests/Helpers/WebHostHelpers.cs

@@ -50,7 +50,7 @@ public static TestServerSetup SetupTest<TStartup>(bool requestsViaWebsocket = fa
 	public class TestServerSetup : IDisposable
 	{
 		public IWebHost Server { get; set; }
-		public IGraphQLClient Client { get; set; }
+		public GraphQLHttpClient Client { get; set; }
 		public void Dispose()
 		{
 			Server?.Dispose();

tests/GraphQL.Integration.Tests/QueryAndMutationTests.cs

@@ -1,4 +1,6 @@
+using System.Net.Http;
 using GraphQL.Client;
+using GraphQL.Client.Http;
 using GraphQL.Integration.Tests.Helpers;
 using GraphQL.Integration.Tests.TestData;
 using IntegrationTestServer;
@@ -111,5 +113,24 @@ query Human($id: String!){
 				Assert.Equal("Han Solo", queryResponse.Data.Human.Name);
 			}
 		}
+
+		[Fact]
+		public async void PreprocessHttpRequestMessageIsCalled() {
+			var callbackTester = new CallbackTester<HttpRequestMessage>();
+			var graphQLRequest = new GraphQLHttpRequest($"{{ human(id: \"1\") {{ name }} }}") {
+				PreprocessHttpRequestMessage = callbackTester.Callback
+			};
+
+			using (var setup = SetupTest()) {
+				var defaultHeaders = setup.Client.HttpClient.DefaultRequestHeaders;
+				var response = await setup.Client.SendQueryAsync(graphQLRequest, () => new { Human = new { Name = string.Empty } })
+					.ConfigureAwait(false);
+				callbackTester.CallbackShouldHaveBeenInvoked(message => {
+					Assert.Equal(defaultHeaders, message.Headers);
+				});
+				Assert.Null(response.Errors);
+				Assert.Equal("Luke", response.Data.Human.Name);
+			}
+		}
 	}
 }