diff --git a/sandbox/ConsoleApp/ConsoleApp.csproj b/sandbox/ConsoleApp/ConsoleApp.csproj
index e225b75..c6fa777 100644
--- a/sandbox/ConsoleApp/ConsoleApp.csproj
+++ b/sandbox/ConsoleApp/ConsoleApp.csproj
@@ -2,15 +2,15 @@
 
 	<PropertyGroup>
 		<OutputType>Exe</OutputType>
-		<TargetFramework>net7.0</TargetFramework>
+		<TargetFramework>net9.0</TargetFramework>
 		<SignAssembly>true</SignAssembly>
 		<AssemblyOriginatorKeyFile>../../opensource.snk</AssemblyOriginatorKeyFile>
 		<IsPackable>false</IsPackable>
 	</PropertyGroup>
 
 	<ItemGroup>
-		<PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
-		<PackageReference Include="System.Text.Json" Version="8.0.4" />
+		<PackageReference Include="BenchmarkDotNet" Version="0.15.0" />
+		<PackageReference Include="System.Text.Json" Version="9.0.5" />
 		<ProjectReference Include="..\..\src\ZString\ZString.csproj">
 			<SetTargetFramework>TargetFramework=netstandard2.1</SetTargetFramework>
 		</ProjectReference>
diff --git a/sandbox/PerfBenchmark/PerfBenchmark.csproj b/sandbox/PerfBenchmark/PerfBenchmark.csproj
index 1c3b0ed..1f65918 100644
--- a/sandbox/PerfBenchmark/PerfBenchmark.csproj
+++ b/sandbox/PerfBenchmark/PerfBenchmark.csproj
@@ -8,7 +8,7 @@
 	</PropertyGroup>
 
 	<ItemGroup>
-		<PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
+		<PackageReference Include="BenchmarkDotNet" Version="0.15.0" />
 		<PackageReference Include="StringFormatter" Version="1.0.0.13" />
 	</ItemGroup>
 
diff --git a/src/ZString/EnumUtil.cs b/src/ZString/EnumUtil.cs
index 98601e3..cbdaa36 100644
--- a/src/ZString/EnumUtil.cs
+++ b/src/ZString/EnumUtil.cs
@@ -1,12 +1,11 @@
-using System;
+using System;
 using System.Buffers;
 using System.Collections.Generic;
 using System.Text;
 
 namespace Cysharp.Text
 {
-    internal static class EnumUtil<T>
-        // where T : Enum
+    internal static class EnumUtil<T> where T : notnull
     {
         const string InvalidName = "$";
 
@@ -45,7 +44,7 @@ public static bool TryFormatUtf16(T value, Span<char> dest, out int written, Rea
         {
             if (!names.TryGetValue(value, out var v) || v == InvalidName)
             {
-                v = value!.ToString(); // T is Enum, not null always
+                v = value!.ToString()!;
             }
 
             written = v.Length;
@@ -56,7 +55,7 @@ public static bool TryFormatUtf8(T value, Span<byte> dest, out int written, Stan
         {
             if (!utf8names.TryGetValue(value, out var v) || v.Length == 0)
             {
-                v = Encoding.UTF8.GetBytes(value!.ToString());
+                v = Encoding.UTF8.GetBytes(value!.ToString()!);
             }
 
             written = v.Length;
diff --git a/src/ZString/FormatHelper.cs b/src/ZString/FormatHelper.cs
index de9846c..82804aa 100644
--- a/src/ZString/FormatHelper.cs
+++ b/src/ZString/FormatHelper.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Text;
 using System.Buffers;
 using System.Runtime.CompilerServices;
@@ -47,7 +47,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
         {
             if (typeof(T) == typeof(string))
             {
-                var s = Unsafe.As<string>(arg);
+                var s = Unsafe.As<string>(arg)!;
                 int padding = width - s.Length;
                 if (padding > 0)
                 {
@@ -82,7 +82,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }
@@ -130,7 +130,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
         {
             if (typeof(T) == typeof(string))
             {
-                var s = Unsafe.As<string>(arg);
+                var s = Unsafe.As<string>(arg)!;
                 int padding = width - s.Length;
                 if (padding > 0)
                 {
@@ -163,10 +163,10 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }
     }
 
-}
\ No newline at end of file
+}
diff --git a/src/ZString/Utf16ValueStringBuilder.cs b/src/ZString/Utf16ValueStringBuilder.cs
index df6ec3b..8ca5dda 100644
--- a/src/ZString/Utf16ValueStringBuilder.cs
+++ b/src/ZString/Utf16ValueStringBuilder.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Buffers;
 using System.IO;
 using System.Runtime.CompilerServices;
@@ -47,11 +47,11 @@ static Utf16ValueStringBuilder()
         /// <summary>Length of written buffer.</summary>
         public int Length => index;
         /// <summary>Get the written buffer data.</summary>
-        public ReadOnlySpan<char> AsSpan() => buffer.AsSpan(0, index);
+        public ReadOnlySpan<char> AsSpan() => buffer!.AsSpan(0, index);
         /// <summary>Get the written buffer data.</summary>
-        public ReadOnlyMemory<char> AsMemory() => buffer.AsMemory(0, index);
+        public ReadOnlyMemory<char> AsMemory() => buffer!.AsMemory(0, index);
         /// <summary>Get the written buffer data.</summary>
-        public ArraySegment<char> AsArraySegment() => new ArraySegment<char>(buffer, 0, index);
+        public ArraySegment<char> AsArraySegment() => new ArraySegment<char>(buffer!, 0, index);
 
         /// <summary>
         /// Initializes a new instance
@@ -263,7 +263,7 @@ public void AppendLine(ReadOnlySpan<char> value)
         /// <summary>Appends the string representation of a specified value to this instance.</summary>
         public void Append<T>(T value)
         {
-            if (!FormatterCache<T>.TryFormatDelegate(value, buffer.AsSpan(index), out var written, default))
+            if (!FormatterCache<T>.TryFormatDelegate(value, buffer!.AsSpan(index), out var written, default))
             {
                 Grow(written);
                 if (!FormatterCache<T>.TryFormatDelegate(value, buffer.AsSpan(index), out written, default))
@@ -326,7 +326,7 @@ public void Insert(int index, ReadOnlySpan<char> value, int count)
             var newSize = index + value.Length * count;
             var newBuffer = ArrayPool<char>.Shared.Rent(Math.Max(DefaultBufferSize, newSize));
 
-            buffer.AsSpan(0, index).CopyTo(newBuffer);
+            buffer!.AsSpan(0, index).CopyTo(newBuffer);
             int newBufferIndex = index;
 
             for (int i = 0; i < count; i++)
@@ -338,7 +338,7 @@ public void Insert(int index, ReadOnlySpan<char> value, int count)
             int remainLnegth = this.index - index;
             buffer.AsSpan(index, remainLnegth).CopyTo(newBuffer.AsSpan(newBufferIndex));
 
-            if (buffer!.Length != ThreadStaticBufferSize)
+            if (buffer.Length != ThreadStaticBufferSize)
             {
                 if (buffer != null)
                 {
@@ -462,7 +462,7 @@ public void Replace(ReadOnlySpan<char> oldValue, ReadOnlySpan<char> newValue, in
 
             var newBuffer = ArrayPool<char>.Shared.Rent(Math.Max(DefaultBufferSize, Length + (newValue.Length - oldValue.Length) * matchCount));
 
-            buffer.AsSpan(0, startIndex).CopyTo(newBuffer);
+            buffer!.AsSpan(0, startIndex).CopyTo(newBuffer);
             int newBufferIndex = startIndex;
 
             for (int i = startIndex; i < endIndex; i += oldValue.Length)
@@ -482,7 +482,7 @@ public void Replace(ReadOnlySpan<char> oldValue, ReadOnlySpan<char> newValue, in
                 i += pos;
             }
 
-            if (buffer!.Length != ThreadStaticBufferSize)
+            if (buffer.Length != ThreadStaticBufferSize)
             {
                 ArrayPool<char>.Shared.Return(buffer);
             }
@@ -541,7 +541,7 @@ public void Remove(int startIndex, int length)
             }
 
             int remain = startIndex + length;
-            buffer.AsSpan(remain, Length - remain).CopyTo(buffer.AsSpan(startIndex));
+            buffer!.AsSpan(remain, Length - remain).CopyTo(buffer.AsSpan(startIndex));
             index -= length;
         }
 
@@ -558,7 +558,7 @@ public bool TryCopyTo(Span<char> destination, out int charsWritten)
             }
 
             charsWritten = index;
-            buffer.AsSpan(0, index).CopyTo(destination);
+            buffer!.AsSpan(0, index).CopyTo(destination);
             return true;
         }
 
@@ -568,7 +568,7 @@ public override string ToString()
             if (index == 0)
                 return string.Empty;
 
-            return new string(buffer, 0, index);
+            return new string(buffer!, 0, index);
         }
 
         // IBufferWriter
@@ -621,7 +621,7 @@ void AppendFormatInternal<T>(T arg, int width, ReadOnlySpan<char> format, string
             {
                 width *= -1;
 
-                if (!FormatterCache<T>.TryFormatDelegate(arg, buffer.AsSpan(index), out var charsWritten, format))
+                if (!FormatterCache<T>.TryFormatDelegate(arg, buffer!.AsSpan(index), out var charsWritten, format))
                 {
                     Grow(charsWritten);
                     if (!FormatterCache<T>.TryFormatDelegate(arg, buffer.AsSpan(index), out charsWritten, format))
@@ -642,7 +642,7 @@ void AppendFormatInternal<T>(T arg, int width, ReadOnlySpan<char> format, string
             {
                 if (typeof(T) == typeof(string))
                 {
-                    var s = Unsafe.As<string>(arg);
+                    var s = Unsafe.As<string>(arg)!;
                     int padding = width - s.Length;
                     if (padding > 0)
                     {
@@ -731,7 +731,7 @@ static FormatterCache()
                     }
                 }
 
-                TryFormatDelegate = formatter;
+                TryFormatDelegate = formatter!;
             }
 
             static bool TryFormatString(T value, Span<char> dest, out int written, ReadOnlySpan<char> format)
@@ -762,7 +762,7 @@ static bool TryFormatDefault(T value, Span<char> dest, out int written, ReadOnly
                     value.ToString();
 
                 // also use this length when result is false.
-                written = s.Length;
+                written = s!.Length;
                 return s.AsSpan().TryCopyTo(dest);
             }
         }
diff --git a/src/ZString/Utf8ValueStringBuilder.cs b/src/ZString/Utf8ValueStringBuilder.cs
index 19df488..87cec87 100644
--- a/src/ZString/Utf8ValueStringBuilder.cs
+++ b/src/ZString/Utf8ValueStringBuilder.cs
@@ -52,13 +52,13 @@ static Utf8ValueStringBuilder()
         public int Length => index;
 
         /// <summary>Get the written buffer data.</summary>
-        public ReadOnlySpan<byte> AsSpan() => buffer.AsSpan(0, index);
+        public ReadOnlySpan<byte> AsSpan() => buffer!.AsSpan(0, index);
 
         /// <summary>Get the written buffer data.</summary>
-        public ReadOnlyMemory<byte> AsMemory() => buffer.AsMemory(0, index);
+        public ReadOnlyMemory<byte> AsMemory() => buffer!.AsMemory(0, index);
 
         /// <summary>Get the written buffer data.</summary>
-        public ArraySegment<byte> AsArraySegment() => new ArraySegment<byte>(buffer, 0, index);
+        public ArraySegment<byte> AsArraySegment() => new ArraySegment<byte>(buffer!, 0, index);
 
         /// <summary>
         /// Initializes a new instance
@@ -292,7 +292,7 @@ public void AppendLiteral(ReadOnlySpan<byte> value)
         /// <summary>Appends the string representation of a specified value to this instance.</summary>
         public void Append<T>(T value)
         {
-            if (!FormatterCache<T>.TryFormatDelegate(value, buffer.AsSpan(index), out var written, default))
+            if (!FormatterCache<T>.TryFormatDelegate(value, buffer!.AsSpan(index), out var written, default))
             {
                 Grow(written);
                 if (!FormatterCache<T>.TryFormatDelegate(value, buffer.AsSpan(index), out written, default))
@@ -330,20 +330,20 @@ public bool TryCopyTo(Span<byte> destination, out int bytesWritten)
             }
 
             bytesWritten = index;
-            buffer.AsSpan(0, index).CopyTo(destination);
+            buffer!.AsSpan(0, index).CopyTo(destination);
             return true;
         }
 
         /// <summary>Write inner buffer to stream.</summary>
         public Task WriteToAsync(Stream stream)
         {
-            return stream.WriteAsync(buffer, 0, index);
+            return stream.WriteAsync(buffer!, 0, index);
         }
 
         /// <summary>Write inner buffer to stream.</summary>
         public Task WriteToAsync(Stream stream, CancellationToken cancellationToken)
         {
-            return stream.WriteAsync(buffer, 0, index, cancellationToken);
+            return stream.WriteAsync(buffer!, 0, index, cancellationToken);
         }
 
         /// <summary>Encode the innner utf8 buffer to a System.String.</summary>
@@ -352,7 +352,7 @@ public override string ToString()
             if (index == 0)
                 return string.Empty;
 
-            return UTF8NoBom.GetString(buffer, 0, index);
+            return UTF8NoBom.GetString(buffer!, 0, index);
         }
 
         // IBufferWriter
@@ -411,7 +411,7 @@ private void AppendFormatInternal<T>(T arg, int width, StandardFormat format, st
             {
                 width *= -1;
 
-                if (!FormatterCache<T>.TryFormatDelegate(arg, buffer.AsSpan(index), out var charsWritten, format))
+                if (!FormatterCache<T>.TryFormatDelegate(arg, buffer!.AsSpan(index), out var charsWritten, format))
                 {
                     Grow(charsWritten);
                     if (!FormatterCache<T>.TryFormatDelegate(arg, buffer.AsSpan(index), out charsWritten, format))
@@ -432,7 +432,7 @@ private void AppendFormatInternal<T>(T arg, int width, StandardFormat format, st
             {
                 if (typeof(T) == typeof(string))
                 {
-                    var s = Unsafe.As<string>(arg);
+                    var s = Unsafe.As<string>(arg)!;
                     int padding = width - s.Length;
                     if (padding > 0)
                     {
@@ -460,7 +460,7 @@ private void AppendFormatInternal<T>(T arg, int width, StandardFormat format, st
                         Append(' ', padding);  // TODO Fill Method is too slow.
                     }
 
-                    s.CopyTo(GetSpan(charsWritten));
+                    s.Slice(0, charsWritten).CopyTo(GetSpan(charsWritten));
                     Advance(charsWritten);
                 }
             }
@@ -513,7 +513,7 @@ static FormatterCache()
                     }
                 }
 
-                TryFormatDelegate = formatter;
+                TryFormatDelegate = formatter!;
             }
 
             static bool TryFormatDefault(T value, Span<byte> dest, out int written, StandardFormat format)
@@ -524,12 +524,12 @@ static bool TryFormatDefault(T value, Span<byte> dest, out int written, Standard
                     return true;
                 }
 
-                var s = typeof(T) == typeof(string) ? Unsafe.As<string>(value) :
+                var s = typeof(T) == typeof(string) ? Unsafe.As<string>(value)! :
                     (value is IFormattable formattable && format != default) ? formattable.ToString(format.ToString(), null) :
                     value.ToString();
 
                 // also use this length when result is false.
-                written = UTF8NoBom.GetMaxByteCount(s.Length);
+                written = UTF8NoBom.GetMaxByteCount(s!.Length);
                 if (dest.Length < written)
                 {
                     return false;
diff --git a/src/ZString/ZString.cs b/src/ZString/ZString.cs
index c0db0a7..6506a64 100644
--- a/src/ZString/ZString.cs
+++ b/src/ZString/ZString.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
@@ -270,7 +270,7 @@ static string JoinInternal<T>(ReadOnlySpan<char> separator, ReadOnlySpan<T> valu
             }
             else if (typeof(T) == typeof(string) && values.Length == 1)
             {
-                return Unsafe.As<string>(values[0]);
+                return Unsafe.As<string>(values[0])!;
             }
 
             var sb = new Utf16ValueStringBuilder(true);
diff --git a/src/ZString/ZString.csproj b/src/ZString/ZString.csproj
index f30c041..a5775e4 100644
--- a/src/ZString/ZString.csproj
+++ b/src/ZString/ZString.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFrameworks>netstandard2.1;netstandard2.0;net6.0;net7.0</TargetFrameworks>
+		<TargetFrameworks>netstandard2.1;netstandard2.0;net6.0;net9.0</TargetFrameworks>
 		<RootNamespace>Cysharp.Text</RootNamespace>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
 		<GenerateDocumentationFile>true</GenerateDocumentationFile>
@@ -30,11 +30,11 @@
 	</ItemGroup>
 
 	<ItemGroup Condition="$(TargetFramework) == 'netstandard2.1'">
-		<PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.0.0" />
+		<PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.1.2" />
 	</ItemGroup>
 
 	<ItemGroup Condition="$(TargetFramework) == 'netstandard2.0'">
-		<PackageReference Include="System.Memory" Version="4.5.3" />
+		<PackageReference Include="System.Memory" Version="4.6.3" />
 	</ItemGroup>
 
 	<ItemGroup>
diff --git a/src/ZString/ZStringWriter.cs b/src/ZString/ZStringWriter.cs
index c89b89e..700dfdd 100644
--- a/src/ZString/ZStringWriter.cs
+++ b/src/ZString/ZStringWriter.cs
@@ -82,7 +82,7 @@ public override void Write(char[] buffer, int index, int count)
             sb.Append(buffer, index, count);
         }
 
-        public override void Write(string value)
+        public override void Write(string? value)
         {
             AssertNotDisposed();
 
@@ -98,7 +98,7 @@ public override Task WriteAsync(char value)
             return Task.CompletedTask;
         }
 
-        public override Task WriteAsync(string value)
+        public override Task WriteAsync(string? value)
         {
             Write(value);
             return Task.CompletedTask;
@@ -116,7 +116,7 @@ public override Task WriteLineAsync(char value)
             return Task.CompletedTask;
         }
 
-        public override Task WriteLineAsync(string value)
+        public override Task WriteLineAsync(string? value)
         {
             WriteLine(value);
             return Task.CompletedTask;
diff --git a/tests/ZString.Tests/NestedStringBuilder.cs b/tests/ZString.Tests/NestedStringBuilder.cs
index 8a9074d..543ab7c 100644
--- a/tests/ZString.Tests/NestedStringBuilder.cs
+++ b/tests/ZString.Tests/NestedStringBuilder.cs
@@ -61,7 +61,6 @@ public void NotDisposedUtf16()
                 }
                 );
             }
-            Utf16ValueStringBuilder.scratchBufferUsed.IsSameOrEqualTo(true);
             Utf16ValueStringBuilder.scratchBufferUsed = false;
         }
 
@@ -115,7 +114,6 @@ public void NotDisposedUtf8()
                 }
                 );
             }
-            Utf8ValueStringBuilder.scratchBufferUsed.IsSameOrEqualTo(true);
             Utf8ValueStringBuilder.scratchBufferUsed = false;
         }
 
diff --git a/tests/ZString.Tests/ZString.Tests.csproj b/tests/ZString.Tests/ZString.Tests.csproj
index c53bd2f..597dd37 100644
--- a/tests/ZString.Tests/ZString.Tests.csproj
+++ b/tests/ZString.Tests/ZString.Tests.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>net7.0</TargetFramework>
+        <TargetFramework>net9.0</TargetFramework>
         <IsPackable>false</IsPackable>
         <RootNamespace>ZStringTests</RootNamespace>
         <SignAssembly>true</SignAssembly>
@@ -9,14 +9,14 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="FluentAssertions" Version="5.10.2" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
-        <PackageReference Include="xunit" Version="2.4.1" />
-        <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1">
+        <PackageReference Include="FluentAssertions" Version="8.3.0" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.0" />
+        <PackageReference Include="xunit" Version="2.9.3" />
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.1.0">
             <PrivateAssets>all</PrivateAssets>
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
         </PackageReference>
-        <PackageReference Include="coverlet.collector" Version="1.2.0">
+        <PackageReference Include="coverlet.collector" Version="6.0.4">
             <PrivateAssets>all</PrivateAssets>
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
         </PackageReference>