diff --git a/Source/ReactiveProperty.Core/IReactiveProperty.cs b/Source/ReactiveProperty.Core/IReactiveProperty.cs
index a69ec1a4..56c42779 100644
--- a/Source/ReactiveProperty.Core/IReactiveProperty.cs
+++ b/Source/ReactiveProperty.Core/IReactiveProperty.cs
@@ -12,7 +12,7 @@ public interface IReactiveProperty : IReadOnlyReactiveProperty, IHasErrors, INot
     /// Gets or sets the value.
     /// </summary>
     /// <value>The value.</value>
-    new object Value { get; set; }
+    new object? Value { get; set; }
 
     /// <summary>
     /// Forces the notify.
diff --git a/Source/ReactiveProperty.Core/IReadOnlyReactiveProperty.cs b/Source/ReactiveProperty.Core/IReadOnlyReactiveProperty.cs
index 13bbfc42..982bf8b3 100644
--- a/Source/ReactiveProperty.Core/IReadOnlyReactiveProperty.cs
+++ b/Source/ReactiveProperty.Core/IReadOnlyReactiveProperty.cs
@@ -11,7 +11,7 @@ public interface IReadOnlyReactiveProperty : INotifyPropertyChanged
     /// Gets the value.
     /// </summary>
     /// <value>The value.</value>
-    object Value { get; }
+    object? Value { get; }
 }
 
 /// <summary>
diff --git a/Source/ReactiveProperty.Core/Internals/AccessorCache.cs b/Source/ReactiveProperty.Core/Internals/AccessorCache.cs
index 6505474e..9c4b4889 100644
--- a/Source/ReactiveProperty.Core/Internals/AccessorCache.cs
+++ b/Source/ReactiveProperty.Core/Internals/AccessorCache.cs
@@ -24,7 +24,7 @@ internal static class AccessorCache<TType>
     public static Func<TType, TProperty> LookupGet<TProperty>(Expression<Func<TType, TProperty>> propertySelector, out string propertyName)
     {
         propertyName = ExpressionTreeUtils.GetPropertyName(propertySelector);
-        Delegate accessor;
+        Delegate? accessor;
 
         lock (s_getCache)
         {
@@ -48,7 +48,7 @@ public static Func<TType, TProperty> LookupGet<TProperty>(Expression<Func<TType,
     public static Func<TType, TProperty> LookupNestedGet<TProperty>(Expression<Func<TType, TProperty>> propertySelector, out string propertyName)
     {
         propertyName = ExpressionTreeUtils.GetPropertyPath(propertySelector);
-        Delegate accessor;
+        Delegate? accessor;
 
         lock (s_getCache)
         {
@@ -72,7 +72,7 @@ public static Func<TType, TProperty> LookupNestedGet<TProperty>(Expression<Func<
     public static Action<TType, TProperty> LookupSet<TProperty>(Expression<Func<TType, TProperty>> propertySelector, out string propertyName)
     {
         propertyName = ExpressionTreeUtils.GetPropertyName(propertySelector);
-        Delegate accessor;
+        Delegate? accessor;
 
         lock (s_setCache)
         {
@@ -113,7 +113,7 @@ private static Dictionary<string, Delegate> GetGetCacheByType(Type type)
             }
 
             var accessorType = GetAccessorCacheTypeByType(type);
-            cache = (Dictionary<string, Delegate>)accessorType.GetField("s_getCache", BindingFlags.Static | BindingFlags.NonPublic).GetValue(null);
+            cache = (Dictionary<string, Delegate>)accessorType.GetField("s_getCache", BindingFlags.Static | BindingFlags.NonPublic)!.GetValue(null)!;
             _getCache.Add(type, cache);
             return cache;
         }
@@ -129,7 +129,7 @@ private static Dictionary<string, Delegate> GetSetCacheByType(Type type)
             }
 
             var accessorType = GetAccessorCacheTypeByType(type);
-            cache = (Dictionary<string, Delegate>)accessorType.GetField("s_setCache", BindingFlags.Static | BindingFlags.NonPublic).GetValue(null);
+            cache = (Dictionary<string, Delegate>)accessorType.GetField("s_setCache", BindingFlags.Static | BindingFlags.NonPublic)!.GetValue(null)!;
             _setCache.Add(type, cache);
             return cache;
         }
@@ -182,7 +182,7 @@ public static Delegate LookupSet(Type type, string propertyName)
 
     private static Delegate CreateAndCacheGetAccessor(Type type, string propertyName, Dictionary<string, Delegate> cache)
     {
-        var propertyInfo = type.GetProperty(propertyName);
+        var propertyInfo = type.GetProperty(propertyName) ?? throw new ArgumentException($"{propertyName} is not found on {type.FullName}");
         var accessor = CreateGetAccessor(type, propertyInfo);
         cache.Add(propertyName, accessor);
         return accessor;
@@ -190,7 +190,7 @@ private static Delegate CreateAndCacheGetAccessor(Type type, string propertyName
 
     private static Delegate CreateAndCacheSetAccessor(Type type, string propertyName, Dictionary<string, Delegate> cache)
     {
-        var propertyInfo = type.GetProperty(propertyName);
+        var propertyInfo = type.GetProperty(propertyName) ?? throw new ArgumentException($"{propertyName} is not found on {type.FullName}");
         var accessor = CreateSetAccessor(type, propertyInfo);
         cache.Add(propertyName, accessor);
         return accessor;
diff --git a/Source/ReactiveProperty.Core/Internals/ExpressionTreeUtils.cs b/Source/ReactiveProperty.Core/Internals/ExpressionTreeUtils.cs
index c5e4be89..73d5e0e5 100644
--- a/Source/ReactiveProperty.Core/Internals/ExpressionTreeUtils.cs
+++ b/Source/ReactiveProperty.Core/Internals/ExpressionTreeUtils.cs
@@ -9,9 +9,10 @@ internal static class ExpressionTreeUtils
 {
     public static string GetPropertyPath<TType, TProperty>(Expression<Func<TType, TProperty>> propertySelector)
     {
-        if (!(propertySelector.Body is MemberExpression memberExpression))
+        var memberExpression = propertySelector.Body as MemberExpression;
+        if (memberExpression == null)
         {
-            if (!(propertySelector.Body is UnaryExpression unaryExpression)) { throw new ArgumentException(nameof(propertySelector)); }
+            if (propertySelector.Body is not UnaryExpression unaryExpression) { throw new ArgumentException(nameof(propertySelector)); }
             memberExpression = unaryExpression.Operand as MemberExpression;
             if (memberExpression == null) { throw new ArgumentException(nameof(propertySelector)); }
         }
@@ -29,7 +30,8 @@ public static string GetPropertyPath<TType, TProperty>(Expression<Func<TType, TP
 
     public static string GetPropertyName<TType, TProperty>(Expression<Func<TType, TProperty>> propertySelector)
     {
-        if (!(propertySelector.Body is MemberExpression memberExpression))
+        var memberExpression = propertySelector.Body as MemberExpression;
+        if (memberExpression == null)
         {
             if (!(propertySelector.Body is UnaryExpression unaryExpression)) { throw new ArgumentException(nameof(propertySelector)); }
             memberExpression = unaryExpression.Operand as MemberExpression;
diff --git a/Source/ReactiveProperty.Core/Internals/ObserverNode.cs b/Source/ReactiveProperty.Core/Internals/ObserverNode.cs
index e1035f67..8fbb5ec5 100644
--- a/Source/ReactiveProperty.Core/Internals/ObserverNode.cs
+++ b/Source/ReactiveProperty.Core/Internals/ObserverNode.cs
@@ -8,11 +8,11 @@ namespace Reactive.Bindings.Internals;
 internal sealed class ObserverNode<T> : IObserver<T>, IDisposable
 {
     private readonly IObserver<T> _observer;
-    private IObserverLinkedList<T> _list;
+    private IObserverLinkedList<T>? _list;
 
-    public ObserverNode<T> Previous { get; set; }
+    public ObserverNode<T>? Previous { get; set; }
 
-    public ObserverNode<T> Next { get; set; }
+    public ObserverNode<T>? Next { get; set; }
 
     public ObserverNode(IObserverLinkedList<T> list, IObserver<T> observer)
     {
diff --git a/Source/ReactiveProperty.Core/ReactivePropertySlim.cs b/Source/ReactiveProperty.Core/ReactivePropertySlim.cs
index c5e86abf..07df0eca 100644
--- a/Source/ReactiveProperty.Core/ReactivePropertySlim.cs
+++ b/Source/ReactiveProperty.Core/ReactivePropertySlim.cs
@@ -22,14 +22,14 @@ public class ReactivePropertySlim<T> : IReactiveProperty<T>, IObserverLinkedList
 
     private ReactivePropertyMode _mode; // None = 0, DistinctUntilChanged = 1, RaiseLatestValueOnSubscribe = 2, Disposed = (1 << 9)
     private readonly IEqualityComparer<T> _equalityComparer;
-    private ObserverNode<T> _root;
-    private ObserverNode<T> _last;
+    private ObserverNode<T>? _root;
+    private ObserverNode<T>? _last;
 
     /// <summary>
     /// Occurs when a property value changes.
     /// </summary>
     /// <returns></returns>
-    public event PropertyChangedEventHandler PropertyChanged;
+    public event PropertyChangedEventHandler? PropertyChanged;
 
     /// <summary>
     /// Gets or sets the value.
@@ -64,7 +64,7 @@ public T Value
     /// <value><c>true</c> if this instance is disposed; otherwise, <c>false</c>.</value>
     public bool IsDisposed => (int)_mode == IsDisposedFlagNumber;
 
-    object IReactiveProperty.Value
+    object? IReactiveProperty.Value
     {
         get
         {
@@ -73,11 +73,11 @@ object IReactiveProperty.Value
 
         set
         {
-            Value = (T)value;
+            Value = (T)value!;
         }
     }
 
-    object IReadOnlyReactiveProperty.Value
+    object? IReadOnlyReactiveProperty.Value
     {
         get
         {
@@ -105,7 +105,7 @@ object IReadOnlyReactiveProperty.Value
     /// <param name="initialValue">The initial value.</param>
     /// <param name="mode">The mode.</param>
     /// <param name="equalityComparer">The equality comparer.</param>
-    public ReactivePropertySlim(T initialValue = default, ReactivePropertyMode mode = ReactivePropertyMode.Default, IEqualityComparer<T> equalityComparer = null)
+    public ReactivePropertySlim(T initialValue = default!, ReactivePropertyMode mode = ReactivePropertyMode.Default, IEqualityComparer<T>? equalityComparer = null)
     {
         _latestValue = initialValue;
         _mode = mode;
@@ -162,7 +162,7 @@ public IDisposable Subscribe(IObserver<T> observer)
         }
         else
         {
-            _last.Next = next;
+            _last!.Next = next;
             next.Previous = _last;
             _last = next;
         }
@@ -216,7 +216,7 @@ public void Dispose()
     /// Returns a <see cref="string"/> that represents this instance.
     /// </summary>
     /// <returns>A <see cref="string"/> that represents this instance.</returns>
-    public override string ToString()
+    public override string? ToString()
     {
         return (_latestValue == null)
             ? "null"
@@ -231,7 +231,7 @@ public override string ToString()
 
     IObservable<bool> IHasErrors.ObserveHasErrors => throw new NotSupportedException();
 
-    event EventHandler<DataErrorsChangedEventArgs> INotifyDataErrorInfo.ErrorsChanged
+    event EventHandler<DataErrorsChangedEventArgs>? INotifyDataErrorInfo.ErrorsChanged
     {
         add
         {
@@ -241,7 +241,7 @@ event EventHandler<DataErrorsChangedEventArgs> INotifyDataErrorInfo.ErrorsChange
         }
     }
 
-    IEnumerable INotifyDataErrorInfo.GetErrors(string propertyName)
+    IEnumerable INotifyDataErrorInfo.GetErrors(string? propertyName)
     {
         return System.Linq.Enumerable.Empty<object>();
     }
@@ -260,17 +260,17 @@ public class ReadOnlyReactivePropertySlim<T> : IReadOnlyReactiveProperty<T>, IOb
     // minimize field count
     private T _latestValue;
 
-    private IDisposable _sourceSubscription;
+    private IDisposable? _sourceSubscription;
     private ReactivePropertyMode _mode; // None = 0, DistinctUntilChanged = 1, RaiseLatestValueOnSubscribe = 2, Disposed = (1 << 9)
     private readonly IEqualityComparer<T> _equalityComparer;
-    private ObserverNode<T> _root;
-    private ObserverNode<T> _last;
+    private ObserverNode<T>? _root;
+    private ObserverNode<T>? _last;
 
     /// <summary>
     /// Occurs when a property value changes.
     /// </summary>
     /// <returns></returns>
-    public event PropertyChangedEventHandler PropertyChanged;
+    public event PropertyChangedEventHandler? PropertyChanged;
 
     /// <summary>
     /// Gets the value.
@@ -290,7 +290,7 @@ public T Value
     /// <value><c>true</c> if this instance is disposed; otherwise, <c>false</c>.</value>
     public bool IsDisposed => (int)_mode == IsDisposedFlagNumber;
 
-    object IReadOnlyReactiveProperty.Value => Value;
+    object? IReadOnlyReactiveProperty.Value => Value;
 
     private bool IsDistinctUntilChanged => (_mode & ReactivePropertyMode.DistinctUntilChanged) == ReactivePropertyMode.DistinctUntilChanged;
 
@@ -305,7 +305,7 @@ public T Value
     /// <param name="initialValue">The initial value.</param>
     /// <param name="mode">The mode.</param>
     /// <param name="equalityComparer">The equality comparer.</param>
-    public ReadOnlyReactivePropertySlim(IObservable<T> source, T initialValue = default, ReactivePropertyMode mode = ReactivePropertyMode.DistinctUntilChanged | ReactivePropertyMode.RaiseLatestValueOnSubscribe, IEqualityComparer<T> equalityComparer = null)
+    public ReadOnlyReactivePropertySlim(IObservable<T> source, T initialValue = default!, ReactivePropertyMode mode = ReactivePropertyMode.DistinctUntilChanged | ReactivePropertyMode.RaiseLatestValueOnSubscribe, IEqualityComparer<T>? equalityComparer = null)
     {
         _latestValue = initialValue;
         _mode = mode;
@@ -347,7 +347,7 @@ public IDisposable Subscribe(IObserver<T> observer)
         }
         else
         {
-            _last.Next = next;
+            _last!.Next = next;
             next.Previous = _last;
             _last = next;
         }
@@ -444,7 +444,7 @@ void IObserver<T>.OnCompleted()
     /// Returns a <see cref="string"/> that represents this instance.
     /// </summary>
     /// <returns>A <see cref="string"/> that represents this instance.</returns>
-    public override string ToString()
+    public override string? ToString()
     {
         return (_latestValue == null)
             ? "null"
@@ -468,7 +468,7 @@ public static class ReadOnlyReactivePropertySlim
     /// <param name="mode">The mode.</param>
     /// <param name="equalityComparer">The equality comparer.</param>
     /// <returns></returns>
-    public static ReadOnlyReactivePropertySlim<T> ToReadOnlyReactivePropertySlim<T>(this IObservable<T> source, T initialValue = default, ReactivePropertyMode mode = ReactivePropertyMode.DistinctUntilChanged | ReactivePropertyMode.RaiseLatestValueOnSubscribe, IEqualityComparer<T> equalityComparer = null)
+    public static ReadOnlyReactivePropertySlim<T> ToReadOnlyReactivePropertySlim<T>(this IObservable<T> source, T initialValue = default!, ReactivePropertyMode mode = ReactivePropertyMode.DistinctUntilChanged | ReactivePropertyMode.RaiseLatestValueOnSubscribe, IEqualityComparer<T>? equalityComparer = null)
     {
         return new ReadOnlyReactivePropertySlim<T>(source, initialValue, mode, equalityComparer);
     }
diff --git a/Source/ReactiveProperty.NETStandard/ReactiveProperty.cs b/Source/ReactiveProperty.NETStandard/ReactiveProperty.cs
index 2ee1c1b6..6400b5a4 100644
--- a/Source/ReactiveProperty.NETStandard/ReactiveProperty.cs
+++ b/Source/ReactiveProperty.NETStandard/ReactiveProperty.cs
@@ -92,6 +92,7 @@ public ReactiveProperty()
     {
     }
 
+#nullable disable
     /// <summary>
     /// PropertyChanged raise on ReactivePropertyScheduler
     /// </summary>
@@ -101,6 +102,7 @@ public ReactiveProperty(
         IEqualityComparer<T> equalityComparer = null)
         : this(ReactivePropertyScheduler.Default, initialValue, mode, equalityComparer)
     { }
+#nullable enable
 
     /// <summary>
     /// PropertyChanged raise on selected scheduler
diff --git a/Source/SharedProperties.csproj b/Source/SharedProperties.csproj
index 51491b80..772d7d64 100644
--- a/Source/SharedProperties.csproj
+++ b/Source/SharedProperties.csproj
@@ -12,6 +12,7 @@
     <RepositoryUrl>https://github.com/runceel/ReactiveProperty</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <LangVersion>10.0</LangVersion>
+    <Nullable>enable</Nullable>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <IncludeSymbols>true</IncludeSymbols>
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>