Add nullable attributes for downlevel platforms.

Rx.NET/Source/Directory.build.targets

@@ -7,19 +7,19 @@
   </PropertyGroup>
     
   <PropertyGroup Condition="'$(TargetFramework)' == 'net472'">
-    <DefineConstants>$(DefineConstants);HAS_WINFORMS;HAS_WPF;HAS_WINRT;HAS_DISPATCHER;HAS_REMOTING;DESKTOPCLR</DefineConstants>
+    <DefineConstants>$(DefineConstants);HAS_WINFORMS;HAS_WPF;HAS_WINRT;HAS_DISPATCHER;HAS_REMOTING;DESKTOPCLR;NO_NULLABLE_ATTRIBUTES</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'uap10.0'">
     <TargetPlatformVersion>10.0.19041.0</TargetPlatformVersion>
     <TargetPlatformMinVersion>10.0.15063.0</TargetPlatformMinVersion>
-    <DefineConstants>$(DefineConstants);HAS_WINRT;NO_CODE_COVERAGE_ATTRIBUTE;NO_SERIALIZABLE;CRIPPLED_REFLECTION;NO_THREAD;NO_TRACE;WINDOWS</DefineConstants>
+    <DefineConstants>$(DefineConstants);HAS_WINRT;NO_CODE_COVERAGE_ATTRIBUTE;NO_SERIALIZABLE;CRIPPLED_REFLECTION;NO_THREAD;NO_TRACE;WINDOWS;NO_NULLABLE_ATTRIBUTES</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'uap10.0.16299'">
-    <DefineConstants>$(DefineConstants);HAS_WINRT;WINDOWS;HAS_OS_XAML;LEGACY_WINRT</DefineConstants>
+    <DefineConstants>$(DefineConstants);HAS_WINRT;WINDOWS;HAS_OS_XAML;LEGACY_WINRT;NO_NULLABLE_ATTRIBUTES</DefineConstants>
     <TargetPlatformVersion>10.0.19041.0</TargetPlatformVersion>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'netcoreapp2.1'">
-    <DefineConstants>$(DefineConstants);HAS_WINRT</DefineConstants>
+    <DefineConstants>$(DefineConstants);HAS_WINRT;NO_NULLABLE_ATTRIBUTES</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1'">
     <DefineConstants>$(DefineConstants);HAS_WINRT;HAS_WINFORMS;HAS_WPF;HAS_DISPATCHER;DESKTOPCLR</DefineConstants>

Rx.NET/Source/src/System.Reactive/Diagnostics/CodeAnalysis/NullableAttributes.cs

@@ -0,0 +1,148 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if NO_NULLABLE_ATTRIBUTES
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <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)]
+    internal sealed class AllowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+    internal sealed class DisallowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class MaybeNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output will not be null even if the corresponding type allows it. Specifies that an input argument was not null when the call returns.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class NotNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class MaybeNullWhenAttribute : 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 may be null.
+        /// </param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <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; }
+    }
+
+    /// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+    internal sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the associated parameter name.</summary>
+        /// <param name="parameterName">
+        /// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+        /// </param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>Gets the associated parameter name.</summary>
+        public string ParameterName { get; }
+    }
+
+    /// <summary>Applied to a method that will never return under any circumstance.</summary>
+    [AttributeUsage(AttributeTargets.Method, Inherited = false)]
+    internal sealed class DoesNotReturnAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class DoesNotReturnIfAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified parameter value.</summary>
+        /// <param name="parameterValue">
+        /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+        /// the associated parameter matches this value.
+        /// </param>
+        public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+        /// <summary>Gets the condition parameter value.</summary>
+        public bool ParameterValue { get; }
+    }
+
+    /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values.</summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+    internal sealed class MemberNotNullAttribute : Attribute
+    {
+        /// <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>Gets field or property member names.</summary>
+        public string[] Members { get; }
+    }
+
+    /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values when returning with the specified return value condition.</summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+    internal sealed class MemberNotNullWhenAttribute : Attribute
+    {
+        /// <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 null.
+        /// </param>
+        /// <param name="member">
+        /// The field or property member that is promised to be not-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 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;
+        }
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+    }
+}
+
+#endif