diff --git a/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs b/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs
index c4c20f1fba3b40..2c397dcd075211 100644
--- a/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs
+++ b/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs
@@ -3,6 +3,8 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
@@ -30,7 +32,7 @@ namespace Microsoft.Extensions.Internal
     static class ActivatorUtilities
     {
         private static readonly MethodInfo GetServiceInfo =
-            GetMethodInfo<Func<IServiceProvider, Type, Type, bool, object>>((sp, t, r, c) => GetService(sp, t, r, c));
+            GetMethodInfo<Func<IServiceProvider, Type, Type, bool, object?>>((sp, t, r, c) => GetService(sp, t, r, c));
 
         /// <summary>
         /// Instantiate a type with constructor arguments provided directly and/or from an <see cref="IServiceProvider"/>.
@@ -105,7 +107,7 @@ public static object CreateInstance(IServiceProvider provider, Type instanceType
         /// </returns>
         public static ObjectFactory CreateFactory(Type instanceType, Type[] argumentTypes)
         {
-            FindApplicableConstructor(instanceType, argumentTypes, out ConstructorInfo constructor, out int?[] parameterMap);
+            FindApplicableConstructor(instanceType, argumentTypes, out ConstructorInfo? constructor, out int?[]? parameterMap);
 
             var provider = Expression.Parameter(typeof(IServiceProvider), "provider");
             var argumentArray = Expression.Parameter(typeof(object[]), "argumentArray");
@@ -159,7 +161,7 @@ private static MethodInfo GetMethodInfo<T>(Expression<T> expr)
             return mc.Method;
         }
 
-        private static object GetService(IServiceProvider sp, Type type, Type requiredBy, bool isDefaultParameterRequired)
+        private static object? GetService(IServiceProvider sp, Type type, Type requiredBy, bool isDefaultParameterRequired)
         {
             var service = sp.GetService(type);
             if (service == null && !isDefaultParameterRequired)
@@ -216,25 +218,28 @@ private static void FindApplicableConstructor(
             Type instanceType,
             Type[] argumentTypes,
             out ConstructorInfo matchingConstructor,
-            out int?[] parameterMap)
+            out int?[] matchingParameterMap)
         {
-            matchingConstructor = null;
-            parameterMap = null;
+            ConstructorInfo? constructorInfo = null;
+            int?[]? parameterMap = null;
 
-            if (!TryFindPreferredConstructor(instanceType, argumentTypes, ref matchingConstructor, ref parameterMap) &&
-                !TryFindMatchingConstructor(instanceType, argumentTypes, ref matchingConstructor, ref parameterMap))
+            if (!TryFindPreferredConstructor(instanceType, argumentTypes, ref constructorInfo, ref parameterMap) &&
+                !TryFindMatchingConstructor(instanceType, argumentTypes, ref constructorInfo, ref parameterMap))
             {
                 var message = $"A suitable constructor for type '{instanceType}' could not be located. Ensure the type is concrete and services are registered for all parameters of a public constructor.";
                 throw new InvalidOperationException(message);
             }
+
+            matchingConstructor = constructorInfo;
+            matchingParameterMap = parameterMap;
         }
 
         // Tries to find constructor based on provided argument types
         private static bool TryFindMatchingConstructor(
             Type instanceType,
             Type[] argumentTypes,
-            ref ConstructorInfo matchingConstructor,
-            ref int?[] parameterMap)
+            [NotNullWhen(true)] ref ConstructorInfo? matchingConstructor,
+            [NotNullWhen(true)] ref int?[]? parameterMap)
         {
             foreach (var constructor in instanceType.GetTypeInfo().DeclaredConstructors)
             {
@@ -255,15 +260,21 @@ private static bool TryFindMatchingConstructor(
                 }
             }
 
-            return matchingConstructor != null;
+            if (matchingConstructor != null)
+            {
+                Debug.Assert(parameterMap != null);
+                return true;
+            }
+
+            return false;
         }
 
         // Tries to find constructor marked with ActivatorUtilitiesConstructorAttribute
         private static bool TryFindPreferredConstructor(
             Type instanceType,
             Type[] argumentTypes,
-            ref ConstructorInfo matchingConstructor,
-            ref int?[] parameterMap)
+            [NotNullWhen(true)] ref ConstructorInfo? matchingConstructor,
+            [NotNullWhen(true)] ref int?[]? parameterMap)
         {
             var seenPreferred = false;
             foreach (var constructor in instanceType.GetTypeInfo().DeclaredConstructors)
@@ -291,7 +302,13 @@ private static bool TryFindPreferredConstructor(
                 }
             }
 
-            return matchingConstructor != null;
+            if (matchingConstructor != null)
+            {
+                Debug.Assert(parameterMap != null);
+                return true;
+            }
+
+            return false;
         }
 
         // Creates an injective parameterMap from givenParameterTypes to assignable constructorParameters.
@@ -334,13 +351,13 @@ private struct ConstructorMatcher
         {
             private readonly ConstructorInfo _constructor;
             private readonly ParameterInfo[] _parameters;
-            private readonly object[] _parameterValues;
+            private readonly object?[] _parameterValues;
 
             public ConstructorMatcher(ConstructorInfo constructor)
             {
                 _constructor = constructor;
                 _parameters = _constructor.GetParameters();
-                _parameterValues = new object[_parameters.Length];
+                _parameterValues = new object?[_parameters.Length];
             }
 
             public int Match(object[] givenParameters)
diff --git a/src/libraries/Common/src/Extensions/ParameterDefaultValue/ParameterDefaultValue.cs b/src/libraries/Common/src/Extensions/ParameterDefaultValue/ParameterDefaultValue.cs
index c55b7a3cb7fca9..abcbf454dcfa5a 100644
--- a/src/libraries/Common/src/Extensions/ParameterDefaultValue/ParameterDefaultValue.cs
+++ b/src/libraries/Common/src/Extensions/ParameterDefaultValue/ParameterDefaultValue.cs
@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Reflection;
 
@@ -11,7 +13,7 @@ internal class ParameterDefaultValue
     {
         private static readonly Type _nullable = typeof(Nullable<>);
 
-        public static bool TryGetDefaultValue(ParameterInfo parameter, out object defaultValue)
+        public static bool TryGetDefaultValue(ParameterInfo parameter, out object? defaultValue)
         {
             bool hasDefaultValue;
             var tryToGetDefaultValue = true;
diff --git a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.cs b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.cs
index 733838e7e5f223..aac4f5d2c027d5 100644
--- a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.cs
+++ b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.cs
@@ -22,7 +22,7 @@ public ActivatorUtilitiesConstructorAttribute() { }
     public partial interface IServiceCollection : System.Collections.Generic.ICollection<Microsoft.Extensions.DependencyInjection.ServiceDescriptor>, System.Collections.Generic.IEnumerable<Microsoft.Extensions.DependencyInjection.ServiceDescriptor>, System.Collections.Generic.IList<Microsoft.Extensions.DependencyInjection.ServiceDescriptor>, System.Collections.IEnumerable
     {
     }
-    public partial interface IServiceProviderFactory<TContainerBuilder>
+    public partial interface IServiceProviderFactory<TContainerBuilder> where TContainerBuilder : notnull
     {
         TContainerBuilder CreateBuilder(Microsoft.Extensions.DependencyInjection.IServiceCollection services);
         System.IServiceProvider CreateServiceProvider(TContainerBuilder containerBuilder);
@@ -71,9 +71,9 @@ public partial class ServiceDescriptor
         public ServiceDescriptor(System.Type serviceType, System.Func<System.IServiceProvider, object> factory, Microsoft.Extensions.DependencyInjection.ServiceLifetime lifetime) { }
         public ServiceDescriptor(System.Type serviceType, object instance) { }
         public ServiceDescriptor(System.Type serviceType, System.Type implementationType, Microsoft.Extensions.DependencyInjection.ServiceLifetime lifetime) { }
-        public System.Func<System.IServiceProvider, object> ImplementationFactory { get { throw null; } }
-        public object ImplementationInstance { get { throw null; } }
-        public System.Type ImplementationType { get { throw null; } }
+        public System.Func<System.IServiceProvider, object>? ImplementationFactory { get { throw null; } }
+        public object? ImplementationInstance { get { throw null; } }
+        public System.Type? ImplementationType { get { throw null; } }
         public Microsoft.Extensions.DependencyInjection.ServiceLifetime Lifetime { get { throw null; } }
         public System.Type ServiceType { get { throw null; } }
         public static Microsoft.Extensions.DependencyInjection.ServiceDescriptor Describe(System.Type serviceType, System.Func<System.IServiceProvider, object> implementationFactory, Microsoft.Extensions.DependencyInjection.ServiceLifetime lifetime) { throw null; }
@@ -107,9 +107,10 @@ public static partial class ServiceProviderServiceExtensions
     {
         public static Microsoft.Extensions.DependencyInjection.IServiceScope CreateScope(this System.IServiceProvider provider) { throw null; }
         public static object GetRequiredService(this System.IServiceProvider provider, System.Type serviceType) { throw null; }
-        public static T GetRequiredService<T>(this System.IServiceProvider provider) { throw null; }
-        public static System.Collections.Generic.IEnumerable<object> GetServices(this System.IServiceProvider provider, System.Type serviceType) { throw null; }
+        public static T GetRequiredService<T>(this System.IServiceProvider provider) where T : notnull { throw null; }
+        public static System.Collections.Generic.IEnumerable<object?> GetServices(this System.IServiceProvider provider, System.Type serviceType) { throw null; }
         public static System.Collections.Generic.IEnumerable<T> GetServices<T>(this System.IServiceProvider provider) { throw null; }
+        [return: System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
         public static T GetService<T>(this System.IServiceProvider provider) { throw null; }
     }
 }
diff --git a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.csproj b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.csproj
index 0adff566b0bfd4..942dbe0ee15295 100644
--- a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.csproj
+++ b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/ref/Microsoft.Extensions.DependencyInjection.Abstractions.csproj
@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="Microsoft.Extensions.DependencyInjection.Abstractions.cs" />
diff --git a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/IServiceProviderFactory.cs b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/IServiceProviderFactory.cs
index 0d0a66bab676ab..a8b71bce3db761 100644
--- a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/IServiceProviderFactory.cs
+++ b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/IServiceProviderFactory.cs
@@ -9,7 +9,7 @@ namespace Microsoft.Extensions.DependencyInjection
     /// <summary>
     /// Provides an extension point for creating a container specific builder and an <see cref="IServiceProvider"/>.
     /// </summary>
-    public interface IServiceProviderFactory<TContainerBuilder>
+    public interface IServiceProviderFactory<TContainerBuilder> where TContainerBuilder : notnull
     {
         /// <summary>
         /// Creates a container builder from an <see cref="IServiceCollection"/>.
diff --git a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/Microsoft.Extensions.DependencyInjection.Abstractions.csproj b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/Microsoft.Extensions.DependencyInjection.Abstractions.csproj
index 167a6ebd19e972..17654fa1889d07 100644
--- a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/Microsoft.Extensions.DependencyInjection.Abstractions.csproj
+++ b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/Microsoft.Extensions.DependencyInjection.Abstractions.csproj
@@ -4,6 +4,7 @@
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <DefineConstants>$(DefineConstants);ActivatorUtilities_In_DependencyInjection</DefineConstants>
     <EnableDefaultItems>true</EnableDefaultItems>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceDescriptor.cs b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceDescriptor.cs
index bb850efe12facd..409aa6da34af5e 100644
--- a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceDescriptor.cs
+++ b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceDescriptor.cs
@@ -100,13 +100,13 @@ private ServiceDescriptor(Type serviceType, ServiceLifetime lifetime)
         public Type ServiceType { get; }
 
         /// <inheritdoc />
-        public Type ImplementationType { get; }
+        public Type? ImplementationType { get; }
 
         /// <inheritdoc />
-        public object ImplementationInstance { get; }
+        public object? ImplementationInstance { get; }
 
         /// <inheritdoc />
-        public Func<IServiceProvider, object> ImplementationFactory { get; }
+        public Func<IServiceProvider, object>? ImplementationFactory { get; }
 
         /// <inheritdoc />
         public override string ToString()
diff --git a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceProviderServiceExtensions.cs b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceProviderServiceExtensions.cs
index 1a7ffad12ad7d8..830009056e57ac 100644
--- a/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceProviderServiceExtensions.cs
+++ b/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceProviderServiceExtensions.cs
@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Microsoft.Extensions.DependencyInjection
 {
@@ -18,6 +19,7 @@ public static class ServiceProviderServiceExtensions
         /// <typeparam name="T">The type of service object to get.</typeparam>
         /// <param name="provider">The <see cref="IServiceProvider"/> to retrieve the service object from.</param>
         /// <returns>A service object of type <typeparamref name="T"/> or null if there is no such service.</returns>
+        [return: MaybeNull]
         public static T GetService<T>(this IServiceProvider provider)
         {
             if (provider == null)
@@ -69,7 +71,7 @@ public static object GetRequiredService(this IServiceProvider provider, Type ser
         /// <param name="provider">The <see cref="IServiceProvider"/> to retrieve the service object from.</param>
         /// <returns>A service object of type <typeparamref name="T"/>.</returns>
         /// <exception cref="System.InvalidOperationException">There is no service of type <typeparamref name="T"/>.</exception>
-        public static T GetRequiredService<T>(this IServiceProvider provider)
+        public static T GetRequiredService<T>(this IServiceProvider provider) where T : notnull
         {
             if (provider == null)
             {
@@ -101,7 +103,7 @@ public static IEnumerable<T> GetServices<T>(this IServiceProvider provider)
         /// <param name="provider">The <see cref="IServiceProvider"/> to retrieve the services from.</param>
         /// <param name="serviceType">An object that specifies the type of service object to get.</param>
         /// <returns>An enumeration of services of type <paramref name="serviceType"/>.</returns>
-        public static IEnumerable<object> GetServices(this IServiceProvider provider, Type serviceType)
+        public static IEnumerable<object?> GetServices(this IServiceProvider provider, Type serviceType)
         {
             if (provider == null)
             {