diff --git a/test/Microsoft.ML.CodeAnalyzer.Tests/Code/BestFriendOnPublicDeclarationTest.cs b/test/Microsoft.ML.CodeAnalyzer.Tests/Code/BestFriendOnPublicDeclarationTest.cs
new file mode 100644
index 0000000000..ddd8fb5d72
--- /dev/null
+++ b/test/Microsoft.ML.CodeAnalyzer.Tests/Code/BestFriendOnPublicDeclarationTest.cs
@@ -0,0 +1,63 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
+using System.Reflection;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.ML.CodeAnalyzer.Tests.Helpers;
+using Xunit;
+
+namespace Microsoft.ML.InternalCodeAnalyzer.Tests
+{
+    public sealed class BestFriendOnPublicDeclarationTest : DiagnosticVerifier<BestFriendOnPublicDeclarationsAnalyzer>
+    {
+        private readonly Lazy<string> SourceAttribute = TestUtils.LazySource("BestFriendAttribute.cs");
+        private readonly Lazy<string> SourceDeclaration = TestUtils.LazySource("BestFriendOnPublicDeclaration.cs");
+
+        [Fact]
+        public void BestFriendOnPublicDeclaration()
+        {
+            Solution solution = null;
+            var projA = CreateProject("ProjectA", ref solution, SourceDeclaration.Value, SourceAttribute.Value);
+
+            var analyzer = new BestFriendOnPublicDeclarationsAnalyzer();
+
+            var refs = new List<MetadataReference> {
+                RefFromType<object>(), RefFromType<Attribute>(),
+                MetadataReference.CreateFromFile(Assembly.Load("netstandard, Version=2.0.0.0").Location),
+                MetadataReference.CreateFromFile(Assembly.Load("System.Runtime, Version=0.0.0.0").Location)
+            };
+
+            var comp = projA.GetCompilationAsync().Result.WithReferences(refs.ToArray());
+            var compilationWithAnalyzers = comp.WithAnalyzers(ImmutableArray.Create((DiagnosticAnalyzer)analyzer));
+            var allDiags = compilationWithAnalyzers.GetAnalyzerDiagnosticsAsync().Result;
+
+            var projectTrees = new HashSet<SyntaxTree>(projA.Documents.Select(r => r.GetSyntaxTreeAsync().Result));
+            var diags = allDiags
+                .Where(d => d.Location == Location.None || d.Location.IsInMetadata || projectTrees.Contains(d.Location.SourceTree))
+                .OrderBy(d => d.Location.SourceSpan.Start).ToArray();
+
+            var diag = analyzer.SupportedDiagnostics[0];
+            var expected = new DiagnosticResult[] {
+                diag.CreateDiagnosticResult(8, 6, "PublicClass"),
+                diag.CreateDiagnosticResult(11, 10, "PublicField"),
+                diag.CreateDiagnosticResult(14, 10, "PublicProperty"),
+                diag.CreateDiagnosticResult(20, 10, "PublicMethod"),
+                diag.CreateDiagnosticResult(26, 10, "PublicDelegate"),
+                diag.CreateDiagnosticResult(29, 10, "PublicClass"),
+                diag.CreateDiagnosticResult(35, 6, "PublicStruct"),
+                diag.CreateDiagnosticResult(40, 6, "PublicEnum"),
+                diag.CreateDiagnosticResult(47, 6, "PublicInterface"),
+                diag.CreateDiagnosticResult(102, 10, "PublicMethod")
+            };
+
+            VerifyDiagnosticResults(diags, analyzer, expected);
+        }
+    }
+}
+
diff --git a/test/Microsoft.ML.CodeAnalyzer.Tests/Resources/BestFriendOnPublicDeclaration.cs b/test/Microsoft.ML.CodeAnalyzer.Tests/Resources/BestFriendOnPublicDeclaration.cs
new file mode 100644
index 0000000000..1515f21e1f
--- /dev/null
+++ b/test/Microsoft.ML.CodeAnalyzer.Tests/Resources/BestFriendOnPublicDeclaration.cs
@@ -0,0 +1,107 @@
+using System;
+using Microsoft.ML;
+
+namespace TestNamespace
+{
+    // all of the best friend declaration should fail the diagnostic
+
+    [BestFriend]
+    public class PublicClass
+    {
+        [BestFriend]
+        public int PublicField;
+
+        [BestFriend]
+        public string PublicProperty
+        {
+            get { return string.Empty; }
+        }
+
+        [BestFriend]
+        public bool PublicMethod()
+        {
+            return true;
+        }
+
+        [BestFriend]
+        public delegate string PublicDelegate();
+
+        [BestFriend]
+        public PublicClass()
+        {
+        }
+    }
+
+    [BestFriend]
+    public struct PublicStruct
+    {
+    }
+
+    [BestFriend]
+    public enum PublicEnum
+    {
+        EnumValue1,
+        EnumValue2
+    }
+
+    [BestFriend]
+    public interface PublicInterface
+    {
+    }
+
+    // these should work
+
+    [BestFriend]
+    internal class InternalClass
+    {
+        [BestFriend]
+        internal int InternalField;
+
+        [BestFriend]
+        internal string InternalProperty
+        {
+            get { return string.Empty; }
+        }
+
+        [BestFriend]
+        internal bool InternalMethod()
+        {
+            return true;
+        }
+
+        [BestFriend]
+        internal delegate string InternalDelegate();
+
+        [BestFriend]
+        internal InternalClass()
+        {
+        }
+    }
+
+    [BestFriend]
+    internal struct InternalStruct
+    {
+    }
+
+    [BestFriend]
+    internal enum InternalEnum
+    {
+        EnumValue1,
+        EnumValue2
+    }
+
+    [BestFriend]
+    internal interface InternalInterface
+    {
+    }
+
+    // this should fail the diagnostic
+    // a repro for https://github.com/dotnet/machinelearning/pull/2434#discussion_r254770946
+    internal class InternalClassWithPublicMember
+    {
+        [BestFriend]
+        public void PublicMethod()
+        {
+        }
+    }
+}
diff --git a/tools-local/Microsoft.ML.InternalCodeAnalyzer/BestFriendOnPublicDeclarationsAnalyzer.cs b/tools-local/Microsoft.ML.InternalCodeAnalyzer/BestFriendOnPublicDeclarationsAnalyzer.cs
new file mode 100644
index 0000000000..658e957c16
--- /dev/null
+++ b/tools-local/Microsoft.ML.InternalCodeAnalyzer/BestFriendOnPublicDeclarationsAnalyzer.cs
@@ -0,0 +1,70 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace Microsoft.ML.InternalCodeAnalyzer
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public sealed class BestFriendOnPublicDeclarationsAnalyzer : DiagnosticAnalyzer
+    {
+        private const string Category = "Access";
+        internal const string DiagnosticId = "MSML_BestFriendOnPublicDeclaration";
+
+        private const string Title = "Public declarations should not have " + AttributeName + " attribute.";
+        private const string Format = "The " + AttributeName + " should not be applied to publicly visible members.";
+
+        private const string Description =
+            "The " + AttributeName + " attribute is not valid on public identifiers.";
+
+        private static DiagnosticDescriptor Rule =
+            new DiagnosticDescriptor(DiagnosticId, Title, Format, Category,
+                DiagnosticSeverity.Warning, isEnabledByDefault: true, description: Description);
+
+        private const string AttributeName = "Microsoft.ML.BestFriendAttribute";
+
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics =>
+            ImmutableArray.Create(Rule);
+
+        public override void Initialize(AnalysisContext context)
+        {
+            context.EnableConcurrentExecution();
+            context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+
+            context.RegisterCompilationStartAction(CompilationStart);
+        }
+
+        private void CompilationStart(CompilationStartAnalysisContext context)
+        {
+            var list = new List<string> { AttributeName, "Microsoft.ML.Internal.CpuMath.Core.BestFriendAttribute" };
+
+            foreach (var attributeName in list)
+            {
+                var attribute = context.Compilation.GetTypeByMetadataName(attributeName);
+
+                if (attribute == null)
+                    continue;
+                
+                context.RegisterSymbolAction(c => AnalyzeCore(c, attribute), SymbolKind.NamedType, SymbolKind.Method, SymbolKind.Field, SymbolKind.Property);
+            }
+        }
+
+        private void AnalyzeCore(SymbolAnalysisContext context, INamedTypeSymbol attributeType)
+        {
+            if (context.Symbol.DeclaredAccessibility != Accessibility.Public)
+                return;
+
+            var attribute = context.Symbol.GetAttributes().FirstOrDefault(a => a.AttributeClass == attributeType);
+            if (attribute == null)
+                return;
+
+            var diagnostic = Diagnostic.Create(Rule, attribute.ApplicationSyntaxReference.GetSyntax().GetLocation(), context.Symbol.Name);
+            context.ReportDiagnostic(diagnostic);
+        }
+    }
+}
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