ManagedIdentityClient uses AZURE_CLIENT_ID when available

sdk/identity/Azure.Identity/src/DefaultAzureCredentialOptions.cs

@@ -15,7 +15,7 @@ public class DefaultAzureCredentialOptions : TokenCredentialOptions
         /// <see cref="InteractiveBrowserCredential"/>. The default is null and will authenticate users to their default tenant.
         /// The value can also be set by setting the environment variable AZURE_TENANT_ID.
         /// </summary>
-        public string InteractiveBrowserTenantId { get; set; } = GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
+        public string InteractiveBrowserTenantId { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
 
         /// <summary>
         /// Specifies the tenant id of the preferred authentication account, to be retrieved from the shared token cache for single sign on authentication with
@@ -25,21 +25,21 @@ public class DefaultAzureCredentialOptions : TokenCredentialOptions
         /// If multiple accounts are found in the shared token cache and no value is specified, or the specified value matches no accounts in
         /// the cache the SharedTokenCacheCredential will not be used for authentication.
         /// </remarks>
-        public string SharedTokenCacheTenantId { get; set; } = GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
+        public string SharedTokenCacheTenantId { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
 
         /// <summary>
         /// The tenant id of the user to authenticate, in the case the <see cref="DefaultAzureCredential"/> authenticates through, the
         /// <see cref="VisualStudioCredential"/>. The default is null and will authenticate users to their default tenant.
         /// The value can also be set by setting the environment variable AZURE_TENANT_ID.
         /// </summary>
-        public string VisualStudioTenantId { get; set; } = GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
+        public string VisualStudioTenantId { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
 
         /// <summary>
         /// The tenant id of the user to authenticate, in the case the <see cref="DefaultAzureCredential"/> authenticates through, the
         /// <see cref="VisualStudioCodeCredential"/>. The default is null and will authenticate users to their default tenant.
         /// The value can also be set by setting the environment variable AZURE_TENANT_ID.
         /// </summary>
-        public string VisualStudioCodeTenantId { get; set; } = GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
+        public string VisualStudioCodeTenantId { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.TenantId);
 
         /// <summary>
         /// Specifies the preferred authentication account to be retrieved from the shared token cache for single sign on authentication with
@@ -49,12 +49,12 @@ public class DefaultAzureCredentialOptions : TokenCredentialOptions
         /// If multiple accounts are found in the shared token cache and no value is specified, or the specified value matches no accounts in
         /// the cache the SharedTokenCacheCredential will not be used for authentication.
         /// </remarks>
-        public string SharedTokenCacheUsername { get; set; } = GetNonEmptyStringOrNull(EnvironmentVariables.Username);
+        public string SharedTokenCacheUsername { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.Username);
 
         /// <summary>
         /// Specifies the client id of the azure ManagedIdentity in the case of user assigned identity.
         /// </summary>
-        public string ManagedIdentityClientId { get; set; } = GetNonEmptyStringOrNull(EnvironmentVariables.ClientId);
+        public string ManagedIdentityClientId { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.ClientId);
 
         /// <summary>
         /// Specifies whether the <see cref="EnvironmentCredential"/> will be excluded from the authentication flow. Setting to true disables reading
@@ -94,10 +94,5 @@ public class DefaultAzureCredentialOptions : TokenCredentialOptions
         /// Specifies whether the <see cref="VisualStudioCodeCredential"/> will be excluded from the <see cref="DefaultAzureCredential"/> authentication flow.
         /// </summary>
         public bool ExcludeVisualStudioCodeCredential { get; set; }
-
-        private static string GetNonEmptyStringOrNull(string str)
-        {
-            return !string.IsNullOrEmpty(str) ? str : null;
-        }
     }
 }

sdk/identity/Azure.Identity/src/EnvironmentVariables.cs

@@ -26,5 +26,10 @@ internal class EnvironmentVariables
         public static string ProgramFilesX86 => Environment.GetEnvironmentVariable("ProgramFiles(x86)");
         public static string ProgramFiles => Environment.GetEnvironmentVariable("ProgramFiles");
         public static string AuthorityHost => Environment.GetEnvironmentVariable("AZURE_AUTHORITY_HOST");
+
+         public static string GetNonEmptyStringOrNull(string str)
+        {
+            return !string.IsNullOrEmpty(str) ? str : null;
+        }
     }
 }

sdk/identity/Azure.Identity/src/ManagedIdentityClient.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
-using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
 using Azure.Core;
@@ -27,13 +26,13 @@ public ManagedIdentityClient(CredentialPipeline pipeline, string clientId = null
         public ManagedIdentityClient(ManagedIdentityClientOptions options)
         {
             _options = options;
-            ClientId = options.ClientId;
+            ClientId = options.ClientId ?? EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.ClientId);
             Pipeline = options.Pipeline;
         }
 
         internal CredentialPipeline Pipeline { get; }
 
-        protected string ClientId { get; }
+        protected internal string ClientId { get; }
 
         public virtual async ValueTask<AccessToken> AuthenticateAsync(bool async, TokenRequestContext context, CancellationToken cancellationToken)
         {

sdk/identity/Azure.Identity/src/ManagedIdentityClientOptions.cs

@@ -7,7 +7,7 @@ internal class ManagedIdentityClientOptions
     {
         public TokenCredentialOptions Options { get; set; }
 
-        public string ClientId { get; set; }
+        public string ClientId { get; set; } = EnvironmentVariables.GetNonEmptyStringOrNull(EnvironmentVariables.ClientId);
 
         public bool PreserveTransport { get; set; }
 

sdk/identity/Azure.Identity/tests/ManagedIdentityClientOptionsTests.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Azure.Identity.Tests;
+using NUnit.Framework;
+
+namespace Azure.Identity.Tests
+{
+    public class ManagedIdentityClientOptionsTests
+    {
+        public const string ExplicitClientId = "1234";
+        public const string ClientIdFromEnvironment = "4567";
+
+        [Test]
+        public void ClientIdIsReadFromEnvironmentVariableWhenAvailable(
+            [Values(null, ExplicitClientId)] string explicitClientId,
+            [Values(null, ClientIdFromEnvironment)] string clientIdFromEnvironment)
+        {
+            using (new TestEnvVar(new ()
+            {
+                { "AZURE_CLIENT_ID", clientIdFromEnvironment }
+            }))
+            {
+                var options = new ManagedIdentityClientOptions();
+                if (explicitClientId != null)
+                {
+                    options.ClientId = explicitClientId;
+                }
+
+                string expectedClientId = explicitClientId switch
+                {
+                    null when clientIdFromEnvironment is null => null,
+                    null when clientIdFromEnvironment is not null => clientIdFromEnvironment,
+                    _ => explicitClientId,
+                };
+
+                Assert.That(options.ClientId, Is.EqualTo(expectedClientId));
+            }
+        }
+    }
+}

sdk/identity/Azure.Identity/tests/ManagedIdentityClientTests.cs

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Azure.Identity.Tests;
+using NUnit.Framework;
+
+namespace Azure.Identity.Tests
+{
+    public class ManagedIdentityClientTests
+    {
+        public const string ClientIdFromCtor = "1234";
+        public const string ClientIdFromEnvironment = "4567";
+
+        [Test]
+        public void ClientIdIsReadFromEnvironmentVariableWhenAvailable(
+            [Values(null, ClientIdFromCtor)] string clientIdFromCtor,
+            [Values(null, ClientIdFromEnvironment)] string clientIdFromEnvironment)
+        {
+            using (new TestEnvVar(new ()
+            {
+                { "AZURE_CLIENT_ID", clientIdFromEnvironment }}))
+            {
+                var client = new ManagedIdentityClient(default, clientIdFromCtor);
+
+                string expectedClientId = clientIdFromCtor switch
+                {
+                    null when clientIdFromEnvironment is null => null,
+                    null when clientIdFromEnvironment is not null => clientIdFromEnvironment,
+                    _ => clientIdFromCtor,
+                };
+
+                Assert.That(client.ClientId, Is.EqualTo(expectedClientId));
+            }
+        }
+    }
+}