diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedAsyncStepInfo.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedAsyncStepInfo.cs
index 2d49f9ba4c..0e5d8f6dc7 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedAsyncStepInfo.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedAsyncStepInfo.cs
@@ -18,7 +18,7 @@ public SymUnmanagedAsyncStepInfo(int yieldOffset, int resumeOffset, int resumeMe
             ResumeMethod = resumeMethod;
         }
 
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             return obj is SymUnmanagedAsyncStepInfo && Equals((SymUnmanagedAsyncStepInfo)obj);
         }
diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Constant.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Constant.cs
index 774ce09df9..a909038285 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Constant.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Constant.cs
@@ -18,7 +18,7 @@ public static string GetName(this ISymUnmanagedConstant constant)
             }
 
             return BufferToString(GetItems(constant,
-                (ISymUnmanagedConstant a, int b, out int c, char[] d) => a.GetName(b, out c, d)));
+                (ISymUnmanagedConstant a, int b, out int c, char[] d) => a.GetName(b, out c, d))!);
         }
 
         public static object GetValue(this ISymUnmanagedConstant constant)
diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Document.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Document.cs
index d54df712ad..bcb9c1ef08 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Document.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Document.cs
@@ -20,7 +20,7 @@ public static string GetName(this ISymUnmanagedDocument document)
             }
 
             return BufferToString(GetItems(document,
-                (ISymUnmanagedDocument a, int b, out int c, char[] d) => a.GetUrl(b, out c, d)));
+                (ISymUnmanagedDocument a, int b, out int c, char[] d) => a.GetUrl(b, out c, d))!);
         }
 
         public static byte[] GetChecksum(this ISymUnmanagedDocument document)
diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Method.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Method.cs
index 4a81d5b485..e398e4d9b9 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Method.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Method.cs
@@ -104,7 +104,7 @@ public static IEnumerable<SymUnmanagedSequencePoint> GetSequencePoints(this ISym
             }
         }
 
-        public static ISymUnmanagedAsyncMethod AsAsyncMethod(this ISymUnmanagedMethod method)
+        public static ISymUnmanagedAsyncMethod? AsAsyncMethod(this ISymUnmanagedMethod method)
         {
             var asyncMethod = method as ISymUnmanagedAsyncMethod;
             if (asyncMethod == null)
diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Namespace.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Namespace.cs
index 84bc686aad..4dfdb6e80b 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Namespace.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Namespace.cs
@@ -18,7 +18,7 @@ public static string GetName(this ISymUnmanagedNamespace @namespace)
             }
 
             return BufferToString(GetItems(@namespace,
-                (ISymUnmanagedNamespace a, int b, out int c, char[] d) => a.GetName(b, out c, d)));
+                (ISymUnmanagedNamespace a, int b, out int c, char[] d) => a.GetName(b, out c, d))!);
         }
     }
 }
diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Reader.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Reader.cs
index 65e24a5d39..dfe1bcba5c 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Reader.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Reader.cs
@@ -16,7 +16,7 @@ partial class SymUnmanagedExtensions
         // MSCUSTOMDEBUGINFO in Dev10.
         private const string CdiAttributeName = "MD2";
 
-        public static void UpdateSymbolStore(this ISymUnmanagedReader reader, Stream stream, string fileName = null)
+        public static void UpdateSymbolStore(this ISymUnmanagedReader reader, Stream stream, string? fileName = null)
         {
             if (reader == null)
             {
@@ -26,7 +26,7 @@ public static void UpdateSymbolStore(this ISymUnmanagedReader reader, Stream str
             ThrowExceptionForHR(reader.UpdateSymbolStore(fileName, SymUnmanagedStreamFactory.CreateStream(stream)));
         }
 
-        public static void Initialize(this ISymUnmanagedReader3 reader, Stream stream, object metadataImporter, string fileName = null, string searchPath = null)
+        public static void Initialize(this ISymUnmanagedReader3 reader, Stream stream, object metadataImporter, string? fileName = null, string? searchPath = null)
         {
             if (reader == null)
             {
@@ -39,7 +39,7 @@ public static void Initialize(this ISymUnmanagedReader3 reader, Stream stream, o
         /// <summary>
         /// Get the blob of binary custom debug info for a given method.
         /// </summary>
-        public static byte[] GetCustomDebugInfo(this ISymUnmanagedReader3 reader, int methodToken, int methodVersion)
+        public static byte[]? GetCustomDebugInfo(this ISymUnmanagedReader3 reader, int methodToken, int methodVersion)
         {
             if (reader == null)
             {
@@ -113,7 +113,7 @@ public static ISymUnmanagedMethod[] GetMethodsInDocument(this ISymUnmanagedReade
                 (ISymUnmanagedReader2 a, ISymUnmanagedDocument b, int c, out int d, ISymUnmanagedMethod[] e) => a.GetMethodsInDocument(b, c, out d, e)));
         }
 
-        public static ISymUnmanagedMethod GetMethod(this ISymUnmanagedReader reader, int methodToken)
+        public static ISymUnmanagedMethod? GetMethod(this ISymUnmanagedReader reader, int methodToken)
         {
             if (reader == null)
             {
@@ -137,7 +137,7 @@ public static ISymUnmanagedMethod GetMethod(this ISymUnmanagedReader reader, int
             return method;
         }
 
-        public static ISymUnmanagedMethod GetMethodByVersion(this ISymUnmanagedReader reader, int methodToken, int methodVersion)
+        public static ISymUnmanagedMethod? GetMethodByVersion(this ISymUnmanagedReader reader, int methodToken, int methodVersion)
         {
             if (reader == null)
             {
@@ -176,7 +176,7 @@ public static int GetMethodVersion(this ISymUnmanagedReader reader, ISymUnmanage
         /// <summary>
         /// Returns compiler version number and name.
         /// </summary>
-        public static bool TryGetCompilerInfo(this ISymUnmanagedCompilerInfoReader reader, out Version version, out string name)
+        public static bool TryGetCompilerInfo(this ISymUnmanagedCompilerInfoReader reader, out Version? version, out string? name)
         {
             if (reader == null)
             {
diff --git a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Variable.cs b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Variable.cs
index 861ab6adae..d8d20c8b02 100644
--- a/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Variable.cs
+++ b/src/Microsoft.DiaSymReader/Extensions/SymUnmanagedExtensions.Variable.cs
@@ -42,7 +42,7 @@ public static string GetName(this ISymUnmanagedVariable local)
             }
 
             return BufferToString(GetItems(local,
-                (ISymUnmanagedVariable a, int b, out int c, char[] d) => a.GetName(b, out c, d)));
+                (ISymUnmanagedVariable a, int b, out int c, char[] d) => a.GetName(b, out c, d))!);
         }
     }
 }
diff --git a/src/Microsoft.DiaSymReader/Metadata/SymReaderMetadataAdapter.cs b/src/Microsoft.DiaSymReader/Metadata/SymReaderMetadataAdapter.cs
index ea23c5491b..dcbe850a14 100644
--- a/src/Microsoft.DiaSymReader/Metadata/SymReaderMetadataAdapter.cs
+++ b/src/Microsoft.DiaSymReader/Metadata/SymReaderMetadataAdapter.cs
@@ -24,7 +24,7 @@ internal unsafe sealed partial class SymReaderMetadataAdapter : MetadataAdapterB
         public SymReaderMetadataAdapter(ISymReaderMetadataProvider metadataProvider)
         {
             Debug.Assert(metadataProvider != null);
-            _metadataProvider = metadataProvider;
+            _metadataProvider = metadataProvider!;
         }
 
         public override int GetSigFromToken(
diff --git a/src/Microsoft.DiaSymReader/Metadata/SymWriterMetadataAdapter.cs b/src/Microsoft.DiaSymReader/Metadata/SymWriterMetadataAdapter.cs
index e6270cbc4d..cd578ba125 100644
--- a/src/Microsoft.DiaSymReader/Metadata/SymWriterMetadataAdapter.cs
+++ b/src/Microsoft.DiaSymReader/Metadata/SymWriterMetadataAdapter.cs
@@ -25,7 +25,7 @@ internal unsafe sealed partial class SymWriterMetadataAdapter : MetadataAdapterB
         public SymWriterMetadataAdapter(ISymWriterMetadataProvider metadataProvider)
         {
             Debug.Assert(metadataProvider != null);
-            _metadataProvider = metadataProvider;
+            _metadataProvider = metadataProvider!;
         }
 
         public override int GetTokenFromSig(byte* voidPointerSig, int byteCountSig)
diff --git a/src/Microsoft.DiaSymReader/Microsoft.DiaSymReader.csproj b/src/Microsoft.DiaSymReader/Microsoft.DiaSymReader.csproj
index f36bfd699f..587ade6ba2 100644
--- a/src/Microsoft.DiaSymReader/Microsoft.DiaSymReader.csproj
+++ b/src/Microsoft.DiaSymReader/Microsoft.DiaSymReader.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFrameworks>$(NetCurrent);netstandard2.0</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <Nullable>enable</Nullable>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <IsPackable>true</IsPackable>
     <Description>Microsoft DiaSymReader interop interfaces and utilities.</Description>
diff --git a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader.cs b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader.cs
index 1cc97b9a6a..0d413ce59d 100644
--- a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader.cs
+++ b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader.cs
@@ -79,15 +79,15 @@ int GetNamespaces(
         [PreserveSig]
         int Initialize(
             [MarshalAs(UnmanagedType.Interface)] object metadataImporter,
-            [MarshalAs(UnmanagedType.LPWStr)] string fileName,
-            [MarshalAs(UnmanagedType.LPWStr)] string searchPath,
+            [MarshalAs(UnmanagedType.LPWStr)] string? fileName,
+            [MarshalAs(UnmanagedType.LPWStr)] string? searchPath,
 #if NET9_0_OR_GREATER
             [MarshalUsing(typeof(ComStreamWrapper.Marshaller))]
 #endif
             System.Runtime.InteropServices.ComTypes.IStream stream);
 
         [PreserveSig]
-        int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName,
+        int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string? fileName,
 #if NET9_0_OR_GREATER
             [MarshalUsing(typeof(ComStreamWrapper.Marshaller))]
 #endif
diff --git a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader2.cs b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader2.cs
index dcba7e8470..8b3ad89bda 100644
--- a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader2.cs
+++ b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader2.cs
@@ -85,12 +85,12 @@ public partial interface ISymUnmanagedReader2 : ISymUnmanagedReader
         [PreserveSig]
         new int Initialize(
             [MarshalAs(UnmanagedType.Interface)] object metadataImporter,
-            [MarshalAs(UnmanagedType.LPWStr)] string fileName,
-            [MarshalAs(UnmanagedType.LPWStr)] string searchPath,
+            [MarshalAs(UnmanagedType.LPWStr)] string? fileName,
+            [MarshalAs(UnmanagedType.LPWStr)] string? searchPath,
             IStream stream);
 
         [PreserveSig]
-        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
+        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string? fileName, IStream stream);
 
         [PreserveSig]
         new int ReplaceSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
diff --git a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader3.cs b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader3.cs
index adc5b94714..8736d23911 100644
--- a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader3.cs
+++ b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader3.cs
@@ -85,12 +85,12 @@ public partial interface ISymUnmanagedReader3 : ISymUnmanagedReader2
         [PreserveSig]
         new int Initialize(
             [MarshalAs(UnmanagedType.Interface)] object metadataImporter,
-            [MarshalAs(UnmanagedType.LPWStr)] string fileName,
-            [MarshalAs(UnmanagedType.LPWStr)] string searchPath,
+            [MarshalAs(UnmanagedType.LPWStr)] string? fileName,
+            [MarshalAs(UnmanagedType.LPWStr)] string? searchPath,
             IStream stream);
 
         [PreserveSig]
-        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
+        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string? fileName, IStream stream);
 
         [PreserveSig]
         new int ReplaceSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
diff --git a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader4.cs b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader4.cs
index eda7a251fa..b2cd8cf74c 100644
--- a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader4.cs
+++ b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader4.cs
@@ -85,12 +85,12 @@ public partial interface ISymUnmanagedReader4 : ISymUnmanagedReader3
         [PreserveSig]
         new int Initialize(
             [MarshalAs(UnmanagedType.Interface)] object metadataImporter,
-            [MarshalAs(UnmanagedType.LPWStr)] string fileName,
-            [MarshalAs(UnmanagedType.LPWStr)] string searchPath,
+            [MarshalAs(UnmanagedType.LPWStr)] string? fileName,
+            [MarshalAs(UnmanagedType.LPWStr)] string? searchPath,
             IStream stream);
 
         [PreserveSig]
-        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
+        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string? fileName, IStream stream);
 
         [PreserveSig]
         new int ReplaceSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
diff --git a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader5.cs b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader5.cs
index 0721ee7f71..2ef6164720 100644
--- a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader5.cs
+++ b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader5.cs
@@ -85,12 +85,12 @@ public unsafe partial interface ISymUnmanagedReader5 : ISymUnmanagedReader4
         [PreserveSig]
         new int Initialize(
             [MarshalAs(UnmanagedType.Interface)] object metadataImporter,
-            [MarshalAs(UnmanagedType.LPWStr)] string fileName,
-            [MarshalAs(UnmanagedType.LPWStr)] string searchPath,
+            [MarshalAs(UnmanagedType.LPWStr)] string? fileName,
+            [MarshalAs(UnmanagedType.LPWStr)] string? searchPath,
             IStream stream);
 
         [PreserveSig]
-        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
+        new int UpdateSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string? fileName, IStream stream);
 
         [PreserveSig]
         new int ReplaceSymbolStore([MarshalAs(UnmanagedType.LPWStr)] string fileName, IStream stream);
diff --git a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader6.cs b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader6.cs
index 6500f8ef16..3b674e2531 100644
--- a/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader6.cs
+++ b/src/Microsoft.DiaSymReader/Reader/ISymUnmanagedReader6.cs
@@ -24,6 +24,6 @@ int GetCompilerInfo(
             out ushort revision,
             int bufferLength,
             out int count,
-            [In, Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)] char[] name);
+            [In, Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)] char[]? name);
     }
 }
diff --git a/src/Microsoft.DiaSymReader/Reader/SymUnmanagedReaderFactory.cs b/src/Microsoft.DiaSymReader/Reader/SymUnmanagedReaderFactory.cs
index fda72897cc..e7af55c058 100644
--- a/src/Microsoft.DiaSymReader/Reader/SymUnmanagedReaderFactory.cs
+++ b/src/Microsoft.DiaSymReader/Reader/SymUnmanagedReaderFactory.cs
@@ -49,7 +49,7 @@ public static TSymUnmanagedReader CreateReaderWithMetadataImport<TSymUnmanagedRe
                 throw new ArgumentNullException(nameof(metadataImport));
             }
 
-            object symReader = SymUnmanagedFactory.CreateObject(
+            object? symReader = SymUnmanagedFactory.CreateObject(
                 createReader: true, 
                 useAlternativeLoadPath: (options & SymUnmanagedReaderCreationOptions.UseAlternativeLoadPath) != 0,
                 useComRegistry: (options & SymUnmanagedReaderCreationOptions.UseComRegistry) != 0,
@@ -65,7 +65,7 @@ public static TSymUnmanagedReader CreateReaderWithMetadataImport<TSymUnmanagedRe
                     throw loadException;
                 }
 
-                throw new DllNotFoundException(loadException.Message, loadException);
+                throw new DllNotFoundException(loadException!.Message, loadException);
             }
 
             if (!(symReader is TSymUnmanagedReader symReader3))
diff --git a/src/Microsoft.DiaSymReader/SymUnmanagedFactory.cs b/src/Microsoft.DiaSymReader/SymUnmanagedFactory.cs
index c8604636d6..1922837f01 100644
--- a/src/Microsoft.DiaSymReader/SymUnmanagedFactory.cs
+++ b/src/Microsoft.DiaSymReader/SymUnmanagedFactory.cs
@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// 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.
 
@@ -46,7 +46,7 @@ internal static partial class SymUnmanagedFactory
 #if NET
         private const string IUnknownIid = "00000000-0000-0000-C000-000000000046";
 #else
-        private static Type s_lazySymReaderComType, s_lazySymWriterComType;
+        private static Type? s_lazySymReaderComType, s_lazySymWriterComType;
 #endif
 
         internal static string DiaSymReaderModuleName
@@ -112,7 +112,7 @@ internal static string DiaSymReaderModuleName
 #endif
 
         // internal for testing
-        internal static string GetEnvironmentVariable(string name)
+        internal static string? GetEnvironmentVariable(string name)
         {
             try
             {
@@ -124,7 +124,7 @@ internal static string GetEnvironmentVariable(string name)
             }
         }
 
-        private static unsafe object TryLoadFromAlternativePath(Guid clsid, bool createReader)
+        private static unsafe object? TryLoadFromAlternativePath(Guid clsid, bool createReader)
         {
             var dir = GetEnvironmentVariable(AlternativeLoadPathEnvironmentVariableName);
             if (string.IsNullOrEmpty(dir))
@@ -138,7 +138,7 @@ private static unsafe object TryLoadFromAlternativePath(Guid clsid, bool createR
                 Marshal.ThrowExceptionForHR(Marshal.GetHRForLastWin32Error());
             }
 
-            object instance = null;
+            object? instance = null;
             try
             {
                 string factoryName = createReader ? CreateSymReaderFactoryName : CreateSymWriterFactoryName;
@@ -179,7 +179,7 @@ private static unsafe object ActivateClass(Guid clsid)
             return instance;
         }
 #else
-        private static object ActivateClass(ref Type lazyType, Guid clsid)
+        private static object ActivateClass(ref Type? lazyType, Guid clsid)
         {
             if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
             {
@@ -190,9 +190,9 @@ private static object ActivateClass(ref Type lazyType, Guid clsid)
         }
 #endif
 
-        internal static unsafe object CreateObject(bool createReader, bool useAlternativeLoadPath, bool useComRegistry, out string moduleName, out Exception loadException)
+        internal static unsafe object? CreateObject(bool createReader, bool useAlternativeLoadPath, bool useComRegistry, out string? moduleName, out Exception? loadException)
         {
-            object instance = null;
+            object? instance = null;
             loadException = null;
             moduleName = null;
 
@@ -200,7 +200,7 @@ internal static unsafe object CreateObject(bool createReader, bool useAlternativ
 
             try
             {
-                DllNotFoundException loadExceptionCandidate = null;
+                DllNotFoundException? loadExceptionCandidate = null;
 
                 try
                 {
diff --git a/src/Microsoft.DiaSymReader/Utilities/ComStreamWrapper.cs b/src/Microsoft.DiaSymReader/Utilities/ComStreamWrapper.cs
index 002dadddbf..8af8955c50 100644
--- a/src/Microsoft.DiaSymReader/Utilities/ComStreamWrapper.cs
+++ b/src/Microsoft.DiaSymReader/Utilities/ComStreamWrapper.cs
@@ -28,7 +28,7 @@ internal unsafe sealed partial class ComStreamWrapper : IUnsafeComStream, System
         internal ComStreamWrapper(Stream stream)
         {
             Debug.Assert(stream != null);
-            Debug.Assert(stream.CanSeek);
+            Debug.Assert(stream!.CanSeek);
 
             _stream = stream;
         }
@@ -211,7 +211,7 @@ public static IntPtr ConvertToUnmanaged(System.Runtime.InteropServices.ComTypes.
 
             public static System.Runtime.InteropServices.ComTypes.IStream ConvertToManaged(IntPtr native)
             {
-                IUnsafeComStream marshalledStream = ComInterfaceMarshaller<IUnsafeComStream>.ConvertToManaged((void*)native);
+                IUnsafeComStream? marshalledStream = ComInterfaceMarshaller<IUnsafeComStream>.ConvertToManaged((void*)native);
                 if (marshalledStream is null)
                 {
                     throw new NotSupportedException("IStream cannot be marshalled to managed");
diff --git a/src/Microsoft.DiaSymReader/Utilities/IUnsafeComStream.cs b/src/Microsoft.DiaSymReader/Utilities/IUnsafeComStream.cs
index aa4e751efd..6b94977282 100644
--- a/src/Microsoft.DiaSymReader/Utilities/IUnsafeComStream.cs
+++ b/src/Microsoft.DiaSymReader/Utilities/IUnsafeComStream.cs
@@ -45,7 +45,7 @@ internal unsafe partial interface IUnsafeComStream
     [NativeMarshalling(typeof(STATSTGMarshaller))]
     public struct STATSTG
     {
-        public string pwcsName;
+        public string? pwcsName;
         public int type;
         public long cbSize;
         public FILETIME mtime;
@@ -79,7 +79,7 @@ public struct Native
 
         public static STATSTG ConvertToManaged(Native n)
         {
-            string name = null;
+            string? name = null;
             if (n.pwcsName != null)
             {
                 name = Utf16StringMarshaller.ConvertToManaged(n.pwcsName);
diff --git a/src/Microsoft.DiaSymReader/Utilities/InteropUtilities.cs b/src/Microsoft.DiaSymReader/Utilities/InteropUtilities.cs
index ddbcac9bf6..b137abf547 100644
--- a/src/Microsoft.DiaSymReader/Utilities/InteropUtilities.cs
+++ b/src/Microsoft.DiaSymReader/Utilities/InteropUtilities.cs
@@ -12,7 +12,7 @@ internal class InteropUtilities
     {
         private static readonly IntPtr s_ignoreIErrorInfo = new IntPtr(-1);
 
-        internal static T[] NullToEmpty<T>(T[] items) => items == null ? [] : items;
+        internal static T[] NullToEmpty<T>(T[]? items) => items == null ? [] : items;
 
         internal static void ThrowExceptionForHR(int hr)
         {
@@ -24,14 +24,9 @@ internal static void ThrowExceptionForHR(int hr)
             }
         }
 
-        internal unsafe static void CopyQualifiedTypeName(char* qualifiedName, int qualifiedNameBufferLength, int* qualifiedNameLength, string namespaceStr, string nameStr)
+        internal unsafe static void CopyQualifiedTypeName(char* qualifiedName, int qualifiedNameBufferLength, int* qualifiedNameLength, string? namespaceStr, string nameStr)
         {
-            Debug.Assert(nameStr != null);
-
-            if (namespaceStr == null)
-            {
-                namespaceStr = string.Empty;
-            }
+            namespaceStr ??= string.Empty;
 
             if (qualifiedNameLength != null)
             {
@@ -102,7 +97,7 @@ internal static void ValidateItems(int actualCount, int bufferLength)
             }
         }
 
-        internal static TItem[] GetItems<TEntity, TItem>(TEntity entity, CountGetter<TEntity> countGetter, ItemsGetter<TEntity, TItem> itemsGetter)
+        internal static TItem[]? GetItems<TEntity, TItem>(TEntity entity, CountGetter<TEntity> countGetter, ItemsGetter<TEntity, TItem> itemsGetter)
         {
             int count;
             ThrowExceptionForHR(countGetter(entity, out count));
@@ -117,10 +112,10 @@ internal static TItem[] GetItems<TEntity, TItem>(TEntity entity, CountGetter<TEn
             return result;
         }
 
-        internal static TItem[] GetItems<TEntity, TItem>(TEntity entity, ItemsGetter<TEntity, TItem> getter)
+        internal static TItem[]? GetItems<TEntity, TItem>(TEntity entity, ItemsGetter<TEntity, TItem> getter)
         {
             int count;
-            ThrowExceptionForHR(getter(entity, 0, out count, null));
+            ThrowExceptionForHR(getter(entity, 0, out count, null!));
             if (count == 0)
             {
                 return null;
@@ -132,10 +127,10 @@ internal static TItem[] GetItems<TEntity, TItem>(TEntity entity, ItemsGetter<TEn
             return result;
         }
 
-        internal static TItem[] GetItems<TEntity, TArg1, TItem>(TEntity entity, TArg1 arg1, ItemsGetter<TEntity, TArg1, TItem> getter)
+        internal static TItem[]? GetItems<TEntity, TArg1, TItem>(TEntity entity, TArg1 arg1, ItemsGetter<TEntity, TArg1, TItem> getter)
         {
             int count;
-            ThrowExceptionForHR(getter(entity, arg1, 0, out count, null));
+            ThrowExceptionForHR(getter(entity, arg1, 0, out count, null!));
             if (count == 0)
             {
                 return null;
@@ -147,10 +142,10 @@ internal static TItem[] GetItems<TEntity, TArg1, TItem>(TEntity entity, TArg1 ar
             return result;
         }
 
-        internal static TItem[] GetItems<TEntity, TArg1, TArg2, TItem>(TEntity entity, TArg1 arg1, TArg2 arg2, ItemsGetter<TEntity, TArg1, TArg2, TItem> getter)
+        internal static TItem[]? GetItems<TEntity, TArg1, TArg2, TItem>(TEntity entity, TArg1 arg1, TArg2 arg2, ItemsGetter<TEntity, TArg1, TArg2, TItem> getter)
         {
             int count;
-            ThrowExceptionForHR(getter(entity, arg1, arg2, 0, out count, null));
+            ThrowExceptionForHR(getter(entity, arg1, arg2, 0, out count, null!));
             if (count == 0)
             {
                 return null;
diff --git a/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterException.cs b/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterException.cs
index 159fe5aaf6..a529fbb3a1 100644
--- a/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterException.cs
+++ b/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterException.cs
@@ -14,7 +14,7 @@ public sealed class SymUnmanagedWriterException : Exception
         /// <summary>
         /// The name of the module that implements the underlying PDB writer (e.g. diasymreader.dll), or null if not available.
         /// </summary>
-        public string ImplementationModuleName { get; }
+        public string? ImplementationModuleName { get; }
 
         public SymUnmanagedWriterException()
         {
@@ -29,13 +29,13 @@ public SymUnmanagedWriterException(string message, Exception innerException)
         {
         }
 
-        public SymUnmanagedWriterException(string message, Exception innerException, string implementationModuleName)
+        public SymUnmanagedWriterException(string message, Exception innerException, string? implementationModuleName)
             : base(message, innerException)
         {
             ImplementationModuleName = implementationModuleName;
         }
 
-        internal SymUnmanagedWriterException(Exception innerException, string implementationModuleName)
+        internal SymUnmanagedWriterException(Exception innerException, string? implementationModuleName)
             : this(innerException.Message, innerException, implementationModuleName)
         {
         }
diff --git a/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterFactory.cs b/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterFactory.cs
index b69b3b987b..d9eb4b072d 100644
--- a/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterFactory.cs
+++ b/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterFactory.cs
@@ -55,7 +55,7 @@ public static SymUnmanagedWriter CreateWriter(
                     throw loadException;
                 }
 
-                throw new DllNotFoundException(loadException.Message, loadException);
+                throw new DllNotFoundException(loadException!.Message, loadException);
             }
 
             if (!(symWriter is ISymUnmanagedWriter5 symWriter5))
@@ -91,7 +91,7 @@ public static SymUnmanagedWriter CreateWriter(
                 throw new SymUnmanagedWriterException(e, implModuleName);
             }
 
-            return new SymUnmanagedWriterImpl(pdbStream, symWriter5, implModuleName);
+            return new SymUnmanagedWriterImpl(pdbStream, symWriter5, implModuleName!);
         }
     }
 }
diff --git a/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterImpl.cs b/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterImpl.cs
index 669c3d871d..2cc9d2e0a1 100644
--- a/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterImpl.cs
+++ b/src/Microsoft.DiaSymReader/Writer/SymUnmanagedWriterImpl.cs
@@ -20,7 +20,7 @@ internal sealed class SymUnmanagedWriterImpl : SymUnmanagedWriter
     {
         private static object s_zeroInt32 = 0;
 
-        private ISymUnmanagedWriter5 _symWriter;
+        private ISymUnmanagedWriter5? _symWriter;
         private readonly ComMemoryStream _pdbStream;
         private readonly List<ISymUnmanagedDocumentWriter> _documentWriters;
         private readonly string _symWriterModuleName;
@@ -32,10 +32,10 @@ internal SymUnmanagedWriterImpl(ComMemoryStream pdbStream, ISymUnmanagedWriter5
             Debug.Assert(symWriter != null);
             Debug.Assert(symWriterModuleName != null);
 
-            _pdbStream = pdbStream;
-            _symWriter = symWriter;
+            _pdbStream = pdbStream!;
+            _symWriter = symWriter!;
             _documentWriters = new List<ISymUnmanagedDocumentWriter>();
-            _symWriterModuleName = symWriterModuleName;
+            _symWriterModuleName = symWriterModuleName!;
         }
 
         private ISymUnmanagedWriter5 GetSymWriter()
@@ -365,8 +365,6 @@ private unsafe void DefineLocalConstantImpl(ISymUnmanagedWriter5 symWriter, stri
 
         private bool DefineLocalStringConstant(ISymUnmanagedWriter5 symWriter, string name, string value, int constantSignatureToken)
         {
-            Debug.Assert(value != null);
-
             int encodedLength;
 
             // ISymUnmanagedWriter2 doesn't handle unicode strings with unmatched unicode surrogates.