fix!: throw exception on startup when using local evaluation without server key (#141)

* Throw exception when trying to instantiate client in local eval without server key

* Fix test muddle

* Update fixture api key

Author: matthewelwell

Flagsmith.Client.Test/Fixtures.cs

@@ -8,7 +8,7 @@ namespace Flagsmith.FlagsmithClientTest
 {
     internal class Fixtures
     {
-        public static string ApiKey => "test_key";
+        public static string ApiKey => "ser.test_key";
         public static string ApiUrl => "http://test_url/";
         public static AnalyticsProcessorTest GetAnalyticalProcessorTest() => new(new HttpClient(), ApiKey, ApiUrl);
         public static JObject JsonObject = JObject.Parse(@"{

Flagsmith.Client.Test/FlagsmithTest.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -523,6 +523,24 @@ public void TestCannotCreateFlagsmithClientInRemoteEvaluationWithoutAPIKey()
             Assert.Equal("ValueError: environmentKey is required", exception.Message);
         }
 
+        [Fact]
+        public void TestCannotCreateFlagsmithClientInLocalEvaluationWithoutServerAPIKey()
+        {
+            // When
+            Action createFlagsmith = () => new FlagsmithClient(
+                environmentKey: "foobar",
+                enableClientSideEvaluation: true
+            );
+
+            // Then
+            var exception = Assert.Throws<Exception>(() => createFlagsmith());
+            Assert.Equal
+            (
+                "ValueError: In order to use local evaluation, please generate a server key in the environment settings page.",
+                exception.Message
+            );
+        }
+
         [Fact]
         /// <summary>
         /// Test that analytics data is consistent with concurrent calls to get flags.
@@ -554,7 +572,7 @@ public async Task TestAnalyticsDataConsistencyWithConcurrentCallsToGetFlags()
                 featuresDictionary.TryAdd($"Feature_{i}", 0);
             }
 
-            // When 
+            // When
             var tasks = new Task[numberOfThreads];
 
             // Create numberOfThreads threads.
@@ -566,13 +584,13 @@ public async Task TestAnalyticsDataConsistencyWithConcurrentCallsToGetFlags()
                 // Prepare an array of feature names of length callsPerThread.
                 for (int j = 0; j < callsPerThread; j++)
                 {
-                    // The feature names are randomly selected from the featuresDictionary and added to the 
-                    // list of features, which represents the features that have been evaluated.  
+                    // The feature names are randomly selected from the featuresDictionary and added to the
+                    // list of features, which represents the features that have been evaluated.
                     string featureName = $"Feature_{new Random().Next(1, featuresDictionary.Count + 1)}";
                     features[j] = featureName;
 
                     // The relevant key in the featuresDictionary is incremented to simulate an evaluation
-                    // to track for that feature. 
+                    // to track for that feature.
                     featuresDictionary[featureName]++;
                 }
 

Flagsmith.FlagsmithClient/FlagsmithClient.cs

@@ -150,8 +150,8 @@ public FlagsmithClient(
                 {
                     if (!EnvironmentKey!.StartsWith("ser."))
                     {
-                        Console.WriteLine(
-                            "In order to use local evaluation, please generate a server key in the environment settings page."
+                        throw new Exception(
+                            "ValueError: In order to use local evaluation, please generate a server key in the environment settings page."
                         );
                     }
 
@@ -399,4 +399,4 @@ private IFlags GetIdentityFlagsFromDocument(string identifier, List<ITrait>? tra
 
         ~FlagsmithClient() => _pollingManager?.StopPoll();
     }
-}
+}