✨ Improve nullable annotations for net7.0 multi-targeting (#1175)

Inspired by article: https://www.meziantou.net/how-to-use-nullable-reference-types-in-dotnet-standard-2-0-and-dotnet-.htm

Related to #1164 

### Changes
- Add `NullableAttributes.cs` via `Directory.Build.props` to shim annotation attributes for netstandard2.0 and older .NET versions
- Fix nullable warnings, observed when multitargeting net7.0 + netstandard2.0
- Add `[NotNullWhen(true)]` to all `Try` methods
- Remove redundant `!` nullable suppressions

Author: angularsen

CodeGen/Generators/UnitsNetGen/QuantityGenerator.cs

@@ -37,6 +37,7 @@ public string Generate()
             Writer.WL(GeneratedFileHeader);
             Writer.WL(@"
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Linq;
 using System.Runtime.Serialization;
@@ -761,7 +762,7 @@ private void GenerateEqualityAndComparison()
         /// <summary>Indicates strict equality of two <see cref=""{_quantity.Name}""/> quantities, where both <see cref=""Value"" /> and <see cref=""Unit"" /> are exactly equal.</summary>
         /// <remarks>Consider using <see cref=""Equals({_quantity.Name}, {_valueType}, ComparisonType)""/> to check equality across different units and to specify a floating-point number error tolerance.</remarks>
         [Obsolete(""Consider using Equals(Angle, {_valueType}, ComparisonType) to check equality across different units and to specify a floating-point number error tolerance."")]
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {{
             if (obj is null || !(obj is {_quantity.Name} otherQuantity))
                 return false;
@@ -793,7 +794,7 @@ public bool Equals({_quantity.Name} other)
         ///         <item><term> Greater than zero</term><description> This instance follows <paramref name=""obj"" /> in the sort order.</description></item>
         ///     </list>
         /// </returns>
-        public int CompareTo(object obj)
+        public int CompareTo(object? obj)
         {{
             if (obj is null) throw new ArgumentNullException(nameof(obj));
             if (!(obj is {_quantity.Name} otherQuantity)) throw new ArgumentException(""Expected type {_quantity.Name}."", nameof(obj));
@@ -995,15 +996,15 @@ double IQuantity.As(Enum unit)
         /// <param name=""unit"">The unit to convert to.</param>
         /// <param name=""converted"">The converted <see cref=""{_quantity.Name}""/> in <paramref name=""unit""/>, if successful.</param>
         /// <returns>True if successful, otherwise false.</returns>
-        private bool TryToUnit({_quantity.Name}Unit unit, out {_quantity.Name}? converted)
+        private bool TryToUnit({_quantity.Name}Unit unit, [NotNullWhen(true)] out {_quantity.Name}? converted)
         {{
             if (Unit == unit)
             {{
                 converted = this;
                 return true;
             }}
 
-            converted = (Unit, unit) switch
+            {_quantity.Name}? convertedOrNull = (Unit, unit) switch
             {{
                 // {_quantity.Name}Unit -> BaseUnit");
 
@@ -1031,10 +1032,17 @@ private bool TryToUnit({_quantity.Name}Unit unit, out {_quantity.Name}? converte
 
             Writer.WL();
             Writer.WL($@"
-                _ => null!
+                _ => null
             }};
 
-            return converted is not null;
+            if (convertedOrNull is null)
+            {{
+                converted = default;
+                return false;
+            }}
+
+            converted = convertedOrNull.Value;
+            return true;
         }}
 
         /// <inheritdoc />
@@ -1104,7 +1112,7 @@ public string ToString(IFormatProvider? provider)
         /// </summary>
         /// <param name=""format"">The format string.</param>
         /// <returns>The string representation.</returns>
-        public string ToString(string format)
+        public string ToString(string? format)
         {{
             return ToString(format, CultureInfo.CurrentCulture);
         }}
@@ -1116,7 +1124,7 @@ public string ToString(string format)
         /// <param name=""format"">The format string.</param>
         /// <param name=""provider"">Format to use for localization and number formatting. Defaults to <see cref=""CultureInfo.CurrentCulture"" /> if null.</param>
         /// <returns>The string representation.</returns>
-        public string ToString(string format, IFormatProvider? provider)
+        public string ToString(string? format, IFormatProvider? provider)
         {{
             return QuantityFormatter.Format<{_unitEnumName}>(this, format, provider);
         }}
@@ -1127,75 +1135,75 @@ public string ToString(string format, IFormatProvider? provider)
 
         private void GenerateIConvertibleMethods()
         {
-           Writer.WL($@"
+            Writer.WL($@"
         #region IConvertible Methods
 
         TypeCode IConvertible.GetTypeCode()
         {{
             return TypeCode.Object;
         }}
 
-        bool IConvertible.ToBoolean(IFormatProvider provider)
+        bool IConvertible.ToBoolean(IFormatProvider? provider)
         {{
             throw new InvalidCastException($""Converting {{typeof({_quantity.Name})}} to bool is not supported."");
         }}
 
-        byte IConvertible.ToByte(IFormatProvider provider)
+        byte IConvertible.ToByte(IFormatProvider? provider)
         {{
             return Convert.ToByte(_value);
         }}
 
-        char IConvertible.ToChar(IFormatProvider provider)
+        char IConvertible.ToChar(IFormatProvider? provider)
         {{
             throw new InvalidCastException($""Converting {{typeof({_quantity.Name})}} to char is not supported."");
         }}
 
-        DateTime IConvertible.ToDateTime(IFormatProvider provider)
+        DateTime IConvertible.ToDateTime(IFormatProvider? provider)
         {{
             throw new InvalidCastException($""Converting {{typeof({_quantity.Name})}} to DateTime is not supported."");
         }}
 
-        decimal IConvertible.ToDecimal(IFormatProvider provider)
+        decimal IConvertible.ToDecimal(IFormatProvider? provider)
         {{
             return Convert.ToDecimal(_value);
         }}
 
-        double IConvertible.ToDouble(IFormatProvider provider)
+        double IConvertible.ToDouble(IFormatProvider? provider)
         {{
             return Convert.ToDouble(_value);
         }}
 
-        short IConvertible.ToInt16(IFormatProvider provider)
+        short IConvertible.ToInt16(IFormatProvider? provider)
         {{
             return Convert.ToInt16(_value);
         }}
 
-        int IConvertible.ToInt32(IFormatProvider provider)
+        int IConvertible.ToInt32(IFormatProvider? provider)
         {{
             return Convert.ToInt32(_value);
         }}
 
-        long IConvertible.ToInt64(IFormatProvider provider)
+        long IConvertible.ToInt64(IFormatProvider? provider)
         {{
             return Convert.ToInt64(_value);
         }}
 
-        sbyte IConvertible.ToSByte(IFormatProvider provider)
+        sbyte IConvertible.ToSByte(IFormatProvider? provider)
         {{
             return Convert.ToSByte(_value);
         }}
 
-        float IConvertible.ToSingle(IFormatProvider provider)
+        float IConvertible.ToSingle(IFormatProvider? provider)
         {{
             return Convert.ToSingle(_value);
         }}
 
-        string IConvertible.ToString(IFormatProvider provider)
+        string IConvertible.ToString(IFormatProvider? provider)
         {{
             return ToString(""g"", provider);
         }}
 
-        object IConvertible.ToType(Type conversionType, IFormatProvider provider)
+        object IConvertible.ToType(Type conversionType, IFormatProvider? provider)
         {{
             if (conversionType == typeof({_quantity.Name}))
                 return this;
@@ -1209,17 +1217,17 @@ object IConvertible.ToType(Type conversionType, IFormatProvider provider)
                 throw new InvalidCastException($""Converting {{typeof({_quantity.Name})}} to {{conversionType}} is not supported."");
         }}
 
-        ushort IConvertible.ToUInt16(IFormatProvider provider)
+        ushort IConvertible.ToUInt16(IFormatProvider? provider)
         {{
             return Convert.ToUInt16(_value);
         }}
 
-        uint IConvertible.ToUInt32(IFormatProvider provider)
+        uint IConvertible.ToUInt32(IFormatProvider? provider)
         {{
             return Convert.ToUInt32(_value);
         }}
 
-        ulong IConvertible.ToUInt64(IFormatProvider provider)
+        ulong IConvertible.ToUInt64(IFormatProvider? provider)
         {{
             return Convert.ToUInt64(_value);
         }}

CodeGen/Generators/UnitsNetGen/StaticQuantityGenerator.cs

@@ -20,6 +20,7 @@ public string Generate()
 using System.Globalization;
 using UnitsNet.Units;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 #nullable enable
 
@@ -70,7 +71,7 @@ public static IQuantity FromQuantityInfo(QuantityInfo quantityInfo, QuantityValu
         /// <param name=""unit"">Unit enum value.</param>
         /// <param name=""quantity"">The resulting quantity if successful, otherwise <c>default</c>.</param>
         /// <returns><c>True</c> if successful with <paramref name=""quantity""/> assigned the value, otherwise <c>false</c>.</returns>
-        public static bool TryFrom(QuantityValue value, Enum unit, out IQuantity? quantity)
+        public static bool TryFrom(QuantityValue value, Enum unit, [NotNullWhen(true)] out IQuantity? quantity)
         {
             switch (unit)
             {");
@@ -102,7 +103,7 @@ public static bool TryFrom(QuantityValue value, Enum unit, out IQuantity? quanti
         /// <param name=""quantityString"">Quantity string representation, such as ""1.5 kg"". Must be compatible with given quantity type.</param>
         /// <param name=""quantity"">The resulting quantity if successful, otherwise <c>default</c>.</param>
         /// <returns>The parsed quantity.</returns>
-        public static bool TryParse(IFormatProvider? formatProvider, Type quantityType, string quantityString, out IQuantity? quantity)
+        public static bool TryParse(IFormatProvider? formatProvider, Type quantityType, string quantityString, [NotNullWhen(true)] out IQuantity? quantity)
         {
             quantity = default(IQuantity);
 

Directory.Build.props

@@ -2,9 +2,9 @@
   <!-- This file will be implicitly linked by all projects in folder -folders -->
   <PropertyGroup>
     <!-- Default to output to Artifacts folder -->
-    <OutputPath>$(MSBuildThisFileDirectory)Artifacts/$(MSBuildProjectName)</OutputPath> 
+    <OutputPath>$(MSBuildThisFileDirectory)Artifacts/$(MSBuildProjectName)</OutputPath>
     <!-- Specific output folder for .NET nanoFramework projects -->
-    <OutputPath Condition=" '$(TargetFrameworkIdentifier)' == '.NETnanoFramework'">$(MSBuildThisFileDirectory)Artifacts/UnitsNet.NanoFramework/$(MSBuildProjectName)</OutputPath> 
+    <OutputPath Condition=" '$(TargetFrameworkIdentifier)' == '.NETnanoFramework'">$(MSBuildThisFileDirectory)Artifacts/UnitsNet.NanoFramework/$(MSBuildProjectName)</OutputPath>
 
   </PropertyGroup>
 
@@ -17,6 +17,12 @@
   <PropertyGroup>
     <!-- Warning instead of compile error on obsolete errors.-->
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <!-- 612: obsolete, 618: obsolete with message -->
     <WarningsNotAsErrors>612,618</WarningsNotAsErrors>
   </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="$(MSBuildThisFileDirectory)/NullableAttributes.cs" />
+  </ItemGroup>
+
 </Project>