diff --git a/src/Avalonia.Base/Metadata/NullableAttributes.cs b/src/Avalonia.Base/Metadata/NullableAttributes.cs
index 91f5e81863..b6f0f3a47c 100644
--- a/src/Avalonia.Base/Metadata/NullableAttributes.cs
+++ b/src/Avalonia.Base/Metadata/NullableAttributes.cs
@@ -1,6 +1,5 @@
 #pragma warning disable MA0048 // File name must match type name
 #define INTERNAL_NULLABLE_ATTRIBUTES
-#if NETSTANDARD2_0 ||  NETCOREAPP2_0 ||  NETCOREAPP2_1 ||  NETCOREAPP2_2 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
 
 // https://github.com/dotnet/corefx/blob/48363ac826ccf66fbe31a5dcb1dc2aab9a7dd768/src/Common/src/CoreLib/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
 
@@ -10,6 +9,7 @@
 
 namespace System.Diagnostics.CodeAnalysis
 {
+#if NETSTANDARD2_0 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
     /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
     [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
 #if INTERNAL_NULLABLE_ATTRIBUTES
@@ -136,5 +136,82 @@ namespace System.Diagnostics.CodeAnalysis
         /// <summary>Gets the condition parameter value.</summary>
         public bool ParameterValue { get; }
     }
-}
+#endif // NETSTANDARD2_0 attributes
+
+#if NETSTANDARD2_1 || NETSTANDARD2_0 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NETCOREAPP3_1 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
+    /// <summary>
+    /// Specifies that the method or property will ensure that the listed field and property members have
+    /// not-<see langword="null"/> values.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
 #endif
+    sealed class MemberNotNullAttribute : Attribute
+    {
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+
+        /// <summary>Initializes the attribute with a field or property member.</summary>
+        /// <param name="member">The field or property member that is promised to be not-null.</param>
+        public MemberNotNullAttribute(string member)
+        {
+            Members = new[] { member };
+        }
+
+        /// <summary>Initializes the attribute with the list of field and property members.</summary>
+        /// <param name="members">The list of field and property members that are promised to be not-null.</param>
+        public MemberNotNullAttribute(params string[] members)
+        {
+            Members = members;
+        }
+    }
+
+    /// <summary>
+    /// Specifies that the method or property will ensure that the listed field and property members have
+    /// non-<see langword="null"/> values when returning with the specified return value condition.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+    sealed class MemberNotNullWhenAttribute : Attribute
+    {
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+
+        /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value,
+        /// the associated parameter will not be <see langword="null"/>.
+        /// </param>
+        /// <param name="member">The field or property member that is promised to be not-<see langword="null"/>.</param>
+        public MemberNotNullWhenAttribute(bool returnValue, string member)
+        {
+            ReturnValue = returnValue;
+            Members = new[] { member };
+        }
+
+        /// <summary>Initializes the attribute with the specified return value condition and list of field and property members.
+        /// </summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value,
+        /// the associated parameter will not be <see langword="null"/>.
+        /// </param>
+        /// <param name="members">The list of field and property members that are promised to be not-null.</param>
+        public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
+        {
+            ReturnValue = returnValue;
+            Members = members;
+        }
+    }
+#endif // NETSTANDARD2_1 attributes
+}
+