Use DecodeValue to check attribute values (#17605)

src/SourceBuild/content/eng/tools/tasks/Microsoft.DotNet.SourceBuild.Tasks.LeakDetection/CheckForPoison.cs

@@ -152,8 +152,6 @@ public class CheckForPoison : Task
 
         private const string SbrpAttributeType = "System.Reflection.AssemblyMetadataAttribute";
 
-        private const string SbrpAttributeValuePattern = "source\\s?source\\-build\\-reference\\-packages";
-
         private record CandidateFileEntry(string ExtractedPath, string DisplayPath);
 
         public override bool Execute()
@@ -370,11 +368,17 @@ private static bool IsAttributeSbrp(MetadataReader reader, CustomAttribute attr)
 
             if (attributeType == SbrpAttributeType)
             {
-                BlobReader blobReader = reader.GetBlobReader(attr.Value);
-                string attributeValue = Encoding.UTF8.GetString(blobReader.ReadBytes(blobReader.Length));
-                attributeValue = Regex.Replace(attributeValue, @"\p{C}+", string.Empty);
-                return Regex.IsMatch(attributeValue, SbrpAttributeValuePattern);
+                var decodedValue = attr.DecodeValue(DummyAttributeTypeProvider.Instance);
+                try
+                {
+                    return decodedValue.FixedArguments[0].Value.ToString() == "source" && decodedValue.FixedArguments[1].Value.ToString() == "source-build-reference-packages";
+                }
+                catch
+                {
+                    throw new InvalidOperationException($"{SbrpAttributeType} is not formatted properly with a key, value pair.");
+                }
             }
+
             return false;
         }
 

src/SourceBuild/content/eng/tools/tasks/Microsoft.DotNet.SourceBuild.Tasks.LeakDetection/DummyAttributeTypeProvider.cs

@@ -0,0 +1,34 @@
+// 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.Reflection;
+using System.Reflection.Metadata;
+using System.Reflection.Metadata.Ecma335;
+
+namespace Microsoft.DotNet.SourceBuild.Tasks.LeakDetection
+{
+
+    // An empty ICustomAttributeTypeProvider implementation is necessary to read metadata attribute values.
+    internal class DummyAttributeTypeProvider : ICustomAttributeTypeProvider<Type>
+    {
+        public static readonly DummyAttributeTypeProvider Instance = new();
+
+        public Type GetPrimitiveType(PrimitiveTypeCode typeCode) => default(Type);
+
+        public Type GetSystemType() => default(Type);
+
+        public Type GetSZArrayType(Type elementType) => default(Type);
+
+        public Type GetTypeFromDefinition(MetadataReader reader, TypeDefinitionHandle handle, byte rawTypeKind) => default(Type);
+
+        public Type GetTypeFromReference(MetadataReader reader, TypeReferenceHandle handle, byte rawTypeKind) => default(Type);
+
+        public Type GetTypeFromSerializedName(string name) => default(Type);
+
+        public PrimitiveTypeCode GetUnderlyingEnumType(Type type) => default(PrimitiveTypeCode);
+
+        public bool IsSystemType(Type type) => default(bool);
+    }
+}