diff --git a/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.cs b/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.cs
index feef64fc2dde15..c9719bcdbbc920 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.cs
@@ -15,23 +15,24 @@ public partial class ChainedConfigurationProvider : Microsoft.Extensions.Configu
     {
         public ChainedConfigurationProvider(Microsoft.Extensions.Configuration.ChainedConfigurationSource source) { }
         public void Dispose() { }
-        public System.Collections.Generic.IEnumerable<string> GetChildKeys(System.Collections.Generic.IEnumerable<string> earlierKeys, string parentPath) { throw null; }
+        public System.Collections.Generic.IEnumerable<string> GetChildKeys(System.Collections.Generic.IEnumerable<string> earlierKeys, string? parentPath) { throw null; }
         public Microsoft.Extensions.Primitives.IChangeToken GetReloadToken() { throw null; }
         public void Load() { }
-        public void Set(string key, string value) { }
-        public bool TryGet(string key, out string value) { throw null; }
+        public void Set(string key, string? value) { }
+        public bool TryGet(string key, out string? value) { throw null; }
     }
     public partial class ChainedConfigurationSource : Microsoft.Extensions.Configuration.IConfigurationSource
     {
         public ChainedConfigurationSource() { }
-        public Microsoft.Extensions.Configuration.IConfiguration Configuration { get { throw null; } set { } }
+        [System.Diagnostics.CodeAnalysis.DisallowNull]
+        public Microsoft.Extensions.Configuration.IConfiguration? Configuration { get { throw null; } set { } }
         public bool ShouldDisposeConfiguration { get { throw null; } set { } }
         public Microsoft.Extensions.Configuration.IConfigurationProvider Build(Microsoft.Extensions.Configuration.IConfigurationBuilder builder) { throw null; }
     }
     public sealed partial class ConfigurationManager : Microsoft.Extensions.Configuration.IConfigurationBuilder, Microsoft.Extensions.Configuration.IConfigurationRoot, System.IDisposable
     {
         public ConfigurationManager() { }
-        public string this[string key] { get { throw null; } set { throw null; } }
+        public string? this[string key] { get { throw null; } set { throw null; } }
         public IConfigurationSection GetSection(string key) { throw null; }
         public System.Collections.Generic.IEnumerable<IConfigurationSection> GetChildren() { throw null; }
         public void Dispose() { throw null; }
@@ -55,19 +56,19 @@ public partial class ConfigurationKeyComparer : System.Collections.Generic.IComp
     {
         public ConfigurationKeyComparer() { }
         public static Microsoft.Extensions.Configuration.ConfigurationKeyComparer Instance { get { throw null; } }
-        public int Compare(string x, string y) { throw null; }
+        public int Compare(string? x, string? y) { throw null; }
     }
     public abstract partial class ConfigurationProvider : Microsoft.Extensions.Configuration.IConfigurationProvider
     {
         protected ConfigurationProvider() { }
-        protected System.Collections.Generic.IDictionary<string, string> Data { get { throw null; } set { } }
-        public virtual System.Collections.Generic.IEnumerable<string> GetChildKeys(System.Collections.Generic.IEnumerable<string> earlierKeys, string parentPath) { throw null; }
+        protected System.Collections.Generic.IDictionary<string, string?> Data { get { throw null; } set { } }
+        public virtual System.Collections.Generic.IEnumerable<string> GetChildKeys(System.Collections.Generic.IEnumerable<string> earlierKeys, string? parentPath) { throw null; }
         public Microsoft.Extensions.Primitives.IChangeToken GetReloadToken() { throw null; }
         public virtual void Load() { }
         protected void OnReload() { }
-        public virtual void Set(string key, string value) { }
+        public virtual void Set(string key, string? value) { }
         public override string ToString() { throw null; }
-        public virtual bool TryGet(string key, out string value) { throw null; }
+        public virtual bool TryGet(string key, out string? value) { throw null; }
     }
     public partial class ConfigurationReloadToken : Microsoft.Extensions.Primitives.IChangeToken
     {
@@ -75,12 +76,12 @@ public ConfigurationReloadToken() { }
         public bool ActiveChangeCallbacks { get { throw null; } }
         public bool HasChanged { get { throw null; } }
         public void OnReload() { }
-        public System.IDisposable RegisterChangeCallback(System.Action<object> callback, object state) { throw null; }
+        public System.IDisposable RegisterChangeCallback(System.Action<object?> callback, object? state) { throw null; }
     }
     public partial class ConfigurationRoot : Microsoft.Extensions.Configuration.IConfiguration, Microsoft.Extensions.Configuration.IConfigurationRoot, System.IDisposable
     {
         public ConfigurationRoot(System.Collections.Generic.IList<Microsoft.Extensions.Configuration.IConfigurationProvider> providers) { }
-        public string this[string key] { get { throw null; } set { } }
+        public string? this[string key] { get { throw null; } set { } }
         public System.Collections.Generic.IEnumerable<Microsoft.Extensions.Configuration.IConfigurationProvider> Providers { get { throw null; } }
         public void Dispose() { }
         public System.Collections.Generic.IEnumerable<Microsoft.Extensions.Configuration.IConfigurationSection> GetChildren() { throw null; }
@@ -91,10 +92,10 @@ public void Reload() { }
     public partial class ConfigurationSection : Microsoft.Extensions.Configuration.IConfiguration, Microsoft.Extensions.Configuration.IConfigurationSection
     {
         public ConfigurationSection(Microsoft.Extensions.Configuration.IConfigurationRoot root, string path) { }
-        public string this[string key] { get { throw null; } set { } }
+        public string? this[string key] { get { throw null; } set { } }
         public string Key { get { throw null; } }
         public string Path { get { throw null; } }
-        public string Value { get { throw null; } set { } }
+        public string? Value { get { throw null; } set { } }
         public System.Collections.Generic.IEnumerable<Microsoft.Extensions.Configuration.IConfigurationSection> GetChildren() { throw null; }
         public Microsoft.Extensions.Primitives.IChangeToken GetReloadToken() { throw null; }
         public Microsoft.Extensions.Configuration.IConfigurationSection GetSection(string key) { throw null; }
@@ -102,7 +103,7 @@ public ConfigurationSection(Microsoft.Extensions.Configuration.IConfigurationRoo
     public static partial class MemoryConfigurationBuilderExtensions
     {
         public static Microsoft.Extensions.Configuration.IConfigurationBuilder AddInMemoryCollection(this Microsoft.Extensions.Configuration.IConfigurationBuilder configurationBuilder) { throw null; }
-        public static Microsoft.Extensions.Configuration.IConfigurationBuilder AddInMemoryCollection(this Microsoft.Extensions.Configuration.IConfigurationBuilder configurationBuilder, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string>> initialData) { throw null; }
+        public static Microsoft.Extensions.Configuration.IConfigurationBuilder AddInMemoryCollection(this Microsoft.Extensions.Configuration.IConfigurationBuilder configurationBuilder, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string?>>? initialData) { throw null; }
     }
     public abstract partial class StreamConfigurationProvider : Microsoft.Extensions.Configuration.ConfigurationProvider
     {
@@ -114,23 +115,24 @@ public override void Load() { }
     public abstract partial class StreamConfigurationSource : Microsoft.Extensions.Configuration.IConfigurationSource
     {
         protected StreamConfigurationSource() { }
-        public System.IO.Stream Stream { get { throw null; } set { } }
+        [System.Diagnostics.CodeAnalysis.DisallowNull]
+        public System.IO.Stream? Stream { get { throw null; } set { } }
         public abstract Microsoft.Extensions.Configuration.IConfigurationProvider Build(Microsoft.Extensions.Configuration.IConfigurationBuilder builder);
     }
 }
 namespace Microsoft.Extensions.Configuration.Memory
 {
-    public partial class MemoryConfigurationProvider : Microsoft.Extensions.Configuration.ConfigurationProvider, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string>>, System.Collections.IEnumerable
+    public partial class MemoryConfigurationProvider : Microsoft.Extensions.Configuration.ConfigurationProvider, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string?>>, System.Collections.IEnumerable
     {
         public MemoryConfigurationProvider(Microsoft.Extensions.Configuration.Memory.MemoryConfigurationSource source) { }
-        public void Add(string key, string value) { }
-        public System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<string, string>> GetEnumerator() { throw null; }
+        public void Add(string key, string? value) { }
+        public System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<string, string?>> GetEnumerator() { throw null; }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
     }
     public partial class MemoryConfigurationSource : Microsoft.Extensions.Configuration.IConfigurationSource
     {
         public MemoryConfigurationSource() { }
-        public System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string>> InitialData { get { throw null; } set { } }
+        public System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string?>>? InitialData { get { throw null; } set { } }
         public Microsoft.Extensions.Configuration.IConfigurationProvider Build(Microsoft.Extensions.Configuration.IConfigurationBuilder builder) { throw null; }
     }
 }
diff --git a/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.csproj b/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.csproj
index 311697da639012..0ab0a2842d7181 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.csproj
+++ b/src/libraries/Microsoft.Extensions.Configuration/ref/Microsoft.Extensions.Configuration.csproj
@@ -1,10 +1,15 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreAppCurrent);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="Microsoft.Extensions.Configuration.cs" />
     <ProjectReference Include="$(LibrariesProjectRoot)Microsoft.Extensions.Configuration.Abstractions\ref\Microsoft.Extensions.Configuration.Abstractions.csproj" />
     <ProjectReference Include="$(LibrariesProjectRoot)Microsoft.Extensions.Primitives\ref\Microsoft.Extensions.Primitives.csproj" />
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">
+    <ProjectReference Include="$(LibrariesProjectRoot)System.Collections\ref\System.Collections.csproj" />
+    <ProjectReference Include="$(LibrariesProjectRoot)System.Runtime\ref\System.Runtime.csproj" />
+  </ItemGroup>
 </Project>
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationProvider.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationProvider.cs
index a240f3da8e7d11..eb9bed00211458 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationProvider.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationProvider.cs
@@ -40,7 +40,7 @@ public ChainedConfigurationProvider(ChainedConfigurationSource source)
         /// <param name="key">The key.</param>
         /// <param name="value">The value.</param>
         /// <returns><c>True</c> if a value for the specified key was found, otherwise <c>false</c>.</returns>
-        public bool TryGet(string key, out string value)
+        public bool TryGet(string key, out string? value)
         {
             value = _config[key];
             return !string.IsNullOrEmpty(value);
@@ -51,7 +51,7 @@ public bool TryGet(string key, out string value)
         /// </summary>
         /// <param name="key">The key.</param>
         /// <param name="value">The value.</param>
-        public void Set(string key, string value) => _config[key] = value;
+        public void Set(string key, string? value) => _config[key] = value;
 
         /// <summary>
         /// Returns a change token if this provider supports change tracking, null otherwise.
@@ -74,7 +74,7 @@ public void Load() { }
         /// <returns>The child keys.</returns>
         public IEnumerable<string> GetChildKeys(
             IEnumerable<string> earlierKeys,
-            string parentPath)
+            string? parentPath)
         {
             IConfiguration section = parentPath == null ? _config : _config.GetSection(parentPath);
             var keys = new List<string>();
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationSource.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationSource.cs
index 27391aead76fda..b5a9c68c522db4 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationSource.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationSource.cs
@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace Microsoft.Extensions.Configuration
 {
     /// <summary>
@@ -11,7 +13,8 @@ public class ChainedConfigurationSource : IConfigurationSource
         /// <summary>
         /// The chained configuration.
         /// </summary>
-        public IConfiguration Configuration { get; set; }
+        [DisallowNull]
+        public IConfiguration? Configuration { get; set; }
 
         /// <summary>
         /// Whether the chained configuration should be disposed when the
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationKeyComparer.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationKeyComparer.cs
index b5dc31fdffe733..00cca781d47f46 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationKeyComparer.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationKeyComparer.cs
@@ -27,7 +27,7 @@ public class ConfigurationKeyComparer : IComparer<string>
         /// <param name="x">First string.</param>
         /// <param name="y">Second string.</param>
         /// <returns>Less than 0 if x is less than y, 0 if x is equal to y and greater than 0 if x is greater than y.</returns>
-        public int Compare(string x, string y)
+        public int Compare(string? x, string? y)
         {
             string[] xParts = x?.Split(_keyDelimiterArray, StringSplitOptions.RemoveEmptyEntries) ?? Array.Empty<string>();
             string[] yParts = y?.Split(_keyDelimiterArray, StringSplitOptions.RemoveEmptyEntries) ?? Array.Empty<string>();
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationManager.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationManager.cs
index 8ce08d822426c6..6fe4c1a0046634 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationManager.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationManager.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using Microsoft.Extensions.Configuration.Memory;
@@ -38,7 +39,7 @@ public ConfigurationManager()
         }
 
         /// <inheritdoc/>
-        public string this[string key]
+        public string? this[string key]
         {
             get
             {
@@ -338,7 +339,7 @@ public bool Remove(KeyValuePair<string, object> item)
                 return wasRemoved;
             }
 
-            public bool TryGetValue(string key, out object value)
+            public bool TryGetValue(string key, [NotNullWhen(true)] out object? value)
             {
                 return _properties.TryGetValue(key, out value);
             }
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationProvider.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationProvider.cs
index e337bd31483314..9fbeded4fc3de6 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationProvider.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationProvider.cs
@@ -21,13 +21,13 @@ public abstract class ConfigurationProvider : IConfigurationProvider
         /// </summary>
         protected ConfigurationProvider()
         {
-            Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+            Data = new Dictionary<string, string?>(StringComparer.OrdinalIgnoreCase);
         }
 
         /// <summary>
         /// The configuration key value pairs for this provider.
         /// </summary>
-        protected IDictionary<string, string> Data { get; set; }
+        protected IDictionary<string, string?> Data { get; set; }
 
         /// <summary>
         /// Attempts to find a value with the given key, returns true if one is found, false otherwise.
@@ -35,7 +35,7 @@ protected ConfigurationProvider()
         /// <param name="key">The key to lookup.</param>
         /// <param name="value">The value found at key if one is found.</param>
         /// <returns>True if key has a value, false otherwise.</returns>
-        public virtual bool TryGet(string key, out string value)
+        public virtual bool TryGet(string key, out string? value)
             => Data.TryGetValue(key, out value);
 
         /// <summary>
@@ -43,7 +43,7 @@ public virtual bool TryGet(string key, out string value)
         /// </summary>
         /// <param name="key">The configuration key to set.</param>
         /// <param name="value">The value to set.</param>
-        public virtual void Set(string key, string value)
+        public virtual void Set(string key, string? value)
             => Data[key] = value;
 
         /// <summary>
@@ -60,13 +60,13 @@ public virtual void Load()
         /// <returns>The list of keys for this provider.</returns>
         public virtual IEnumerable<string> GetChildKeys(
             IEnumerable<string> earlierKeys,
-            string parentPath)
+            string? parentPath)
         {
             var results = new List<string>();
 
             if (parentPath is null)
             {
-                foreach (KeyValuePair<string, string> kv in Data)
+                foreach (KeyValuePair<string, string?> kv in Data)
                 {
                     results.Add(Segment(kv.Key, 0));
                 }
@@ -75,7 +75,7 @@ public virtual IEnumerable<string> GetChildKeys(
             {
                 Debug.Assert(ConfigurationPath.KeyDelimiter == ":");
 
-                foreach (KeyValuePair<string, string> kv in Data)
+                foreach (KeyValuePair<string, string?> kv in Data)
                 {
                     if (kv.Key.Length > parentPath.Length &&
                         kv.Key.StartsWith(parentPath, StringComparison.OrdinalIgnoreCase) &&
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationReloadToken.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationReloadToken.cs
index fad07ebfdacb97..7aceba29fe55b0 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationReloadToken.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationReloadToken.cs
@@ -33,7 +33,7 @@ public class ConfigurationReloadToken : IChangeToken
         /// <param name="callback">The callback to invoke.</param>
         /// <param name="state">State to be passed into the callback.</param>
         /// <returns>The <see cref="CancellationToken"/> registration.</returns>
-        public IDisposable RegisterChangeCallback(Action<object> callback, object state) => _cts.Token.Register(callback, state);
+        public IDisposable RegisterChangeCallback(Action<object?> callback, object? state) => _cts.Token.Register(callback, state);
 
         /// <summary>
         /// Used to trigger the change token when a reload occurs.
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationRoot.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationRoot.cs
index cfe56a20978e57..397ef5160b1ec7 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationRoot.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationRoot.cs
@@ -47,7 +47,7 @@ public ConfigurationRoot(IList<IConfigurationProvider> providers)
         /// </summary>
         /// <param name="key">The configuration key.</param>
         /// <returns>The configuration value.</returns>
-        public string this[string key]
+        public string? this[string key]
         {
             get => GetConfiguration(_providers, key);
             set => SetConfiguration(_providers, key, value);
@@ -111,13 +111,13 @@ public void Dispose()
             }
         }
 
-        internal static string GetConfiguration(IList<IConfigurationProvider> providers, string key)
+        internal static string? GetConfiguration(IList<IConfigurationProvider> providers, string key)
         {
             for (int i = providers.Count - 1; i >= 0; i--)
             {
                 IConfigurationProvider provider = providers[i];
 
-                if (provider.TryGet(key, out string value))
+                if (provider.TryGet(key, out string? value))
                 {
                     return value;
                 }
@@ -126,7 +126,7 @@ internal static string GetConfiguration(IList<IConfigurationProvider> providers,
             return null;
         }
 
-        internal static void SetConfiguration(IList<IConfigurationProvider> providers, string key, string value)
+        internal static void SetConfiguration(IList<IConfigurationProvider> providers, string key, string? value)
         {
             if (providers.Count == 0)
             {
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationSection.cs b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationSection.cs
index da3fbb733a3a11..0896e587d4082d 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationSection.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationSection.cs
@@ -14,7 +14,7 @@ public class ConfigurationSection : IConfigurationSection
     {
         private readonly IConfigurationRoot _root;
         private readonly string _path;
-        private string _key;
+        private string? _key;
 
         /// <summary>
         /// Initializes a new instance.
@@ -61,7 +61,7 @@ public string Key
         /// <summary>
         /// Gets or sets the section value.
         /// </summary>
-        public string Value
+        public string? Value
         {
             get
             {
@@ -78,13 +78,12 @@ public string Value
         /// </summary>
         /// <param name="key">The configuration key.</param>
         /// <returns>The configuration value.</returns>
-        public string this[string key]
+        public string? this[string key]
         {
             get
             {
                 return _root[ConfigurationPath.Combine(Path, key)];
             }
-
             set
             {
                 _root[ConfigurationPath.Combine(Path, key)] = value;
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/InternalConfigurationRootExtensions.cs b/src/libraries/Microsoft.Extensions.Configuration/src/InternalConfigurationRootExtensions.cs
index bff5c527104008..7fd17a98acf475 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/InternalConfigurationRootExtensions.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/InternalConfigurationRootExtensions.cs
@@ -18,7 +18,7 @@ internal static class InternalConfigurationRootExtensions
         /// <param name="root">Configuration from which to retrieve sub-sections.</param>
         /// <param name="path">Key of a section of which children to retrieve.</param>
         /// <returns>Immediate children sub-sections of section specified by key.</returns>
-        internal static IEnumerable<IConfigurationSection> GetChildrenImplementation(this IConfigurationRoot root, string path)
+        internal static IEnumerable<IConfigurationSection> GetChildrenImplementation(this IConfigurationRoot root, string? path)
         {
             return root.Providers
                 .Aggregate(Enumerable.Empty<string>(),
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationBuilderExtensions.cs b/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationBuilderExtensions.cs
index 996f8a876b9c37..15ed46a26a5b98 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationBuilderExtensions.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationBuilderExtensions.cs
@@ -36,7 +36,7 @@ public static IConfigurationBuilder AddInMemoryCollection(this IConfigurationBui
         /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>
         public static IConfigurationBuilder AddInMemoryCollection(
             this IConfigurationBuilder configurationBuilder,
-            IEnumerable<KeyValuePair<string, string>> initialData)
+            IEnumerable<KeyValuePair<string, string?>>? initialData)
         {
             if (configurationBuilder == null)
             {
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationProvider.cs b/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationProvider.cs
index 84a0b142e0fbc6..0e7f0be02fec78 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationProvider.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationProvider.cs
@@ -10,7 +10,7 @@ namespace Microsoft.Extensions.Configuration.Memory
     /// <summary>
     /// In-memory implementation of <see cref="IConfigurationProvider"/>
     /// </summary>
-    public class MemoryConfigurationProvider : ConfigurationProvider, IEnumerable<KeyValuePair<string, string>>
+    public class MemoryConfigurationProvider : ConfigurationProvider, IEnumerable<KeyValuePair<string, string?>>
     {
         private readonly MemoryConfigurationSource _source;
 
@@ -29,7 +29,7 @@ public MemoryConfigurationProvider(MemoryConfigurationSource source)
 
             if (_source.InitialData != null)
             {
-                foreach (KeyValuePair<string, string> pair in _source.InitialData)
+                foreach (KeyValuePair<string, string?> pair in _source.InitialData)
                 {
                     Data.Add(pair.Key, pair.Value);
                 }
@@ -41,7 +41,7 @@ public MemoryConfigurationProvider(MemoryConfigurationSource source)
         /// </summary>
         /// <param name="key">The configuration key.</param>
         /// <param name="value">The configuration value.</param>
-        public void Add(string key, string value)
+        public void Add(string key, string? value)
         {
             Data.Add(key, value);
         }
@@ -50,7 +50,7 @@ public void Add(string key, string value)
         /// Returns an enumerator that iterates through the collection.
         /// </summary>
         /// <returns>An enumerator that can be used to iterate through the collection.</returns>
-        public IEnumerator<KeyValuePair<string, string>> GetEnumerator()
+        public IEnumerator<KeyValuePair<string, string?>> GetEnumerator()
         {
             return Data.GetEnumerator();
         }
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationSource.cs b/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationSource.cs
index e11891595e4142..412df6534c1109 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationSource.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/MemoryConfigurationSource.cs
@@ -13,7 +13,7 @@ public class MemoryConfigurationSource : IConfigurationSource
         /// <summary>
         /// The initial key value configuration pairs.
         /// </summary>
-        public IEnumerable<KeyValuePair<string, string>> InitialData { get; set; }
+        public IEnumerable<KeyValuePair<string, string?>>? InitialData { get; set; }
 
         /// <summary>
         /// Builds the <see cref="MemoryConfigurationProvider"/> for this source.
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/Microsoft.Extensions.Configuration.csproj b/src/libraries/Microsoft.Extensions.Configuration/src/Microsoft.Extensions.Configuration.csproj
index bc0bf6e7cc443a..10fd1c6acdcb60 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/Microsoft.Extensions.Configuration.csproj
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/Microsoft.Extensions.Configuration.csproj
@@ -1,8 +1,11 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreAppCurrent);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <Nullable>enable</Nullable>
     <EnableDefaultItems>true</EnableDefaultItems>
+    <!-- Use targeting pack references instead of granular ones in the project file. -->
+    <DisableImplicitAssemblyReferences>false</DisableImplicitAssemblyReferences>
     <PackageDescription>Implementation of key-value pair based configuration for Microsoft.Extensions.Configuration. Includes the memory configuration provider.</PackageDescription>
   </PropertyGroup>
 
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/Resources/Strings.resx b/src/libraries/Microsoft.Extensions.Configuration/src/Resources/Strings.resx
index 941bf0cc348733..8f6038211e50dc 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/Resources/Strings.resx
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/Resources/Strings.resx
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <root>
   <!-- 
     Microsoft ResX Schema 
@@ -126,4 +126,7 @@
   <data name="StreamConfigurationProvidersAlreadyLoaded" xml:space="preserve">
     <value>StreamConfigurationProviders cannot be loaded more than once.</value>
   </data>
+  <data name="StreamConfigurationSourceStreamCannotBeNull" xml:space="preserve">
+    <value>Source.Stream cannot be null.</value>
+  </data>
 </root>
\ No newline at end of file
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationProvider.cs b/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationProvider.cs
index fb279be1b4c5ff..6d8917a188fd65 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationProvider.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationProvider.cs
@@ -42,6 +42,12 @@ public override void Load()
             {
                 throw new InvalidOperationException(SR.StreamConfigurationProvidersAlreadyLoaded);
             }
+
+            if (Source.Stream == null)
+            {
+                throw new InvalidOperationException(SR.StreamConfigurationSourceStreamCannotBeNull);
+            }
+
             Load(Source.Stream);
             _loaded = true;
         }
diff --git a/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationSource.cs b/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationSource.cs
index 3fdf7414435095..f857b9b313d00f 100644
--- a/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationSource.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration/src/StreamConfigurationSource.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 
 namespace Microsoft.Extensions.Configuration
@@ -13,7 +14,8 @@ public abstract class StreamConfigurationSource : IConfigurationSource
         /// <summary>
         /// The stream containing the configuration data.
         /// </summary>
-        public Stream Stream { get; set; }
+        [DisallowNull]
+        public Stream? Stream { get; set; }
 
         /// <summary>
         /// Builds the <see cref="StreamConfigurationProvider"/> for this source.
diff --git a/src/libraries/Microsoft.Extensions.Options.ConfigurationExtensions/tests/OptionsBuidlerConfigurationExtensionsTests.cs b/src/libraries/Microsoft.Extensions.Options.ConfigurationExtensions/tests/OptionsBuidlerConfigurationExtensionsTests.cs
index 9d4764c19c0b40..387beac8cc4349 100644
--- a/src/libraries/Microsoft.Extensions.Options.ConfigurationExtensions/tests/OptionsBuidlerConfigurationExtensionsTests.cs
+++ b/src/libraries/Microsoft.Extensions.Options.ConfigurationExtensions/tests/OptionsBuidlerConfigurationExtensionsTests.cs
@@ -66,7 +66,7 @@ public static void BindConfiguration_UsesConfigurationSectionPath()
         {
             const string configSectionName = "Test";
             const string messageValue = "This is a test";
-            var configEntries = new Dictionary<string, string>
+            var configEntries = new Dictionary<string, string?>
             {
                 [ConfigurationPath.Combine(configSectionName, nameof(FakeOptions.Message))] = messageValue
             };
@@ -88,7 +88,7 @@ public static void BindConfiguration_UsesConfigurationSectionPath()
         public static void BindConfiguration_UsesConfigurationRootIfSectionNameIsEmptyString()
         {
             const string messageValue = "This is a test";
-            var configEntries = new Dictionary<string, string>
+            var configEntries = new Dictionary<string, string?>
             {
                 [nameof(FakeOptions.Message)] = messageValue
             };
@@ -114,7 +114,7 @@ public static void BindConfiguration_UpdatesOptionOnConfigurationUpdateWithEmpty
 
             FakeConfigurationSource configSource = new()
             {
-                InitialData = new Dictionary<string, string>
+                InitialData = new Dictionary<string, string?>
                 {
                     [nameof(FakeOptions.Message)] = messageValue1,
                 }
@@ -151,7 +151,7 @@ public static void BindConfiguration_UpdatesOptionOnConfigurationUpdate()
 
             FakeConfigurationSource configSource = new()
             {
-                InitialData = new Dictionary<string, string>
+                InitialData = new Dictionary<string, string?>
                 {
                     [messageConfigKey] = messageValue1
                 }