Fix #760

src/SendGrid/SendGridClient.cs

@@ -4,6 +4,7 @@
 // </copyright>
 
 using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 using SendGrid.Helpers.Mail;
 using SendGrid.Helpers.Reliability;
 using System;
@@ -214,6 +215,23 @@ public async Task<Response> RequestAsync(
         {
             var endpoint = this.client.BaseAddress + this.BuildUrl(urlPath, queryParams);
 
+            // Additional validation if the request is a mail/send request
+            if (urlPath.ToLower() == "mail/send" && !string.IsNullOrWhiteSpace(requestBody))
+            {
+                var body = JObject.Parse(requestBody);
+                var template_id = (string)body["template_id"];
+
+                // If a template ID is provided and the account has GET access to tokens, check that the token exists
+                if (!string.IsNullOrWhiteSpace(template_id))
+                {
+                    var response = await this.RequestAsync(Method.GET, urlPath: $"templates/{template_id}");
+                    if (response.StatusCode == HttpStatusCode.NotFound)
+                    {
+                        throw new Exception("The provided template_id does not exist for this account.");
+                    }
+                }
+            }
+
             // Build the request body
             StringContent content = null;
             if (requestBody != null)

tests/SendGrid.Tests/Integration.cs

@@ -6125,6 +6125,22 @@ public async Task TestRetryBehaviourSucceedsOnSecondAttempt()
 
             Assert.Equal(HttpStatusCode.OK, result.StatusCode);
         }
+
+        [Fact]
+        public async Task TestUsingInvalidTemplateIdThrowsException()
+        {
+            var headers = new Dictionary<string, string> { { "X-Mock", "404" } };
+            var sg = new SendGridClient(fixture.apiKey, fixture.host, headers);
+            var msg = new SendGridMessage();
+            msg.SetFrom(new EmailAddress("[email protected]"));
+            msg.AddTo(new EmailAddress("[email protected]"));
+            msg.SetSubject("Hello World from the SendGrid CSharp Library");
+            msg.SetTemplateId("1000");
+
+            var exception = await Assert.ThrowsAsync<Exception>(() => sg.SendEmailAsync(msg));
+
+            Assert.NotNull(exception);
+        }
     }
 
     public class FakeWebProxy : IWebProxy