diff --git a/src/GraphQLParser/Annotations/DoesNotReturnAttribute.cs b/src/GraphQLParser/Annotations/DoesNotReturnAttribute.cs
deleted file mode 100644
index abebe684..00000000
--- a/src/GraphQLParser/Annotations/DoesNotReturnAttribute.cs
+++ /dev/null
@@ -1,12 +0,0 @@
-#if NETSTANDARD2_0
-
-// https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs#L107
-namespace System.Diagnostics.CodeAnalysis;
-
-/// <summary>Applied to a method that will never return under any circumstance.</summary>
-[AttributeUsage(AttributeTargets.Method, Inherited = false)]
-internal sealed class DoesNotReturnAttribute : Attribute
-{
-}
-
-#endif
diff --git a/src/GraphQLParser/Annotations/IsExternalInit.cs b/src/GraphQLParser/Annotations/IsExternalInit.cs
deleted file mode 100644
index 332aab13..00000000
--- a/src/GraphQLParser/Annotations/IsExternalInit.cs
+++ /dev/null
@@ -1,16 +0,0 @@
-#if !NET5_0_OR_GREATER
-
-using System.ComponentModel;
-
-namespace System.Runtime.CompilerServices;
-
-/// <summary>
-/// Reserved to be used by the compiler for tracking metadata.
-/// This class should not be used by developers in source code.
-/// </summary>
-[EditorBrowsable(EditorBrowsableState.Never)]
-internal static class IsExternalInit
-{
-}
-
-#endif
diff --git a/src/GraphQLParser/Annotations/NotNullWhenAttribute.cs b/src/GraphQLParser/Annotations/NotNullWhenAttribute.cs
deleted file mode 100644
index 5c2b3568..00000000
--- a/src/GraphQLParser/Annotations/NotNullWhenAttribute.cs
+++ /dev/null
@@ -1,23 +0,0 @@
-#if NETSTANDARD2_0
-
-// https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs#L69
-namespace System.Diagnostics.CodeAnalysis;
-
-/// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
-[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
-internal sealed class NotNullWhenAttribute : Attribute
-{
-    /// <summary>Initializes the attribute with the specified return value condition.</summary>
-    /// <param name="returnValue">
-    /// The return value condition. If the method returns this value, the associated parameter will not be null.
-    /// </param>
-    public NotNullWhenAttribute(bool returnValue)
-    {
-        ReturnValue = returnValue;
-    }
-
-    /// <summary>Gets the return value condition.</summary>
-    public bool ReturnValue { get; }
-}
-
-#endif
diff --git a/src/GraphQLParser/GraphQLParser.csproj b/src/GraphQLParser/GraphQLParser.csproj
index 0782e98c..a4d48ff2 100644
--- a/src/GraphQLParser/GraphQLParser.csproj
+++ b/src/GraphQLParser/GraphQLParser.csproj
@@ -13,6 +13,10 @@
   <ItemGroup>
     <PackageReference Include="System.Memory" Version="4.5.5" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
+    <PackageReference Include="PolySharp" Version="1.13.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup Condition="'$(IsTestProject)' != 'true'">