Per Partition Automatic Failover: Adds requirement to enable application region or application preferred regions for PPAF

Microsoft.Azure.Cosmos/src/CosmosClientOptions.cs

@@ -1136,11 +1136,12 @@ private void ValidateLimitToEndpointSettings()
         }
 
         private void ValidatePartitionLevelFailoverSettings()
-        {
-            if (this.EnablePartitionLevelFailover
-                && (this.ApplicationPreferredRegions == null || this.ApplicationPreferredRegions.Count == 0))
-            {
-                throw new ArgumentException($"{nameof(this.ApplicationPreferredRegions)} is required when {nameof(this.EnablePartitionLevelFailover)} is enabled.");
+        {
+            if (this.EnablePartitionLevelFailover
+                && string.IsNullOrEmpty(this.ApplicationRegion)
+                && (this.ApplicationPreferredRegions is null || this.ApplicationPreferredRegions.Count == 0))
+            {
+                throw new ArgumentException($"{nameof(this.ApplicationPreferredRegions)} or {nameof(this.ApplicationRegion)} is required when {nameof(this.EnablePartitionLevelFailover)} is enabled.");
             }
         }
 

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosClientOptionsUnitTests.cs

@@ -1134,6 +1134,47 @@ public void TestServerCertificatesValidationWithDisableSSLFlagTrue(string connSt
 #nullable disable
         }
 
+        [TestMethod]
+        public void PPAFClientApplicationRegionCreationTest()
+        {
+            CosmosClientOptions cosmosClientOptions = new CosmosClientOptions
+            {
+                ApplicationRegion = Regions.WestUS2,
+                EnablePartitionLevelFailover = true
+            };
+
+            CosmosClient cosmosClient = new CosmosClient(ConnectionString, cosmosClientOptions);
+            Assert.AreEqual(Regions.WestUS2, cosmosClient.ClientOptions.ApplicationRegion);
+            Assert.IsTrue(cosmosClient.ClientOptions.EnablePartitionLevelFailover);
+        }
+
+        [TestMethod]
+        public void PPAFClientApplicationPreferredRegionCreationTest()
+        {
+            CosmosClientOptions cosmosClientOptions = new CosmosClientOptions
+            {
+                ApplicationPreferredRegions = new List<string> { Regions.WestUS2, Regions.EastUS2 },
+                EnablePartitionLevelFailover = true
+            };
+
+            CosmosClient cosmosClient = new CosmosClient(ConnectionString, cosmosClientOptions);
+            Assert.AreEqual(Regions.WestUS2, cosmosClient.ClientOptions.ApplicationPreferredRegions[0]);
+            Assert.AreEqual(Regions.EastUS2, cosmosClient.ClientOptions.ApplicationPreferredRegions[1]);
+            Assert.IsTrue(cosmosClient.ClientOptions.EnablePartitionLevelFailover);
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(ArgumentException))]
+        public void PPAFClientNoRegionsTest()
+        {
+            CosmosClientOptions cosmosClientOptions = new CosmosClientOptions
+            {
+                EnablePartitionLevelFailover = true
+            };
+
+            _ = new CosmosClient(ConnectionString, cosmosClientOptions);
+        }
+
         private class TestWebProxy : IWebProxy
         {
             public ICredentials Credentials { get; set; }