[Java.Interop.Tools.JavaCallableWrappers] NRT Support (#1012)

Context: 3226a4b
Context: 7068f4b

Enable [C# 8 Nullable Reference Types][0] for
`Java.Interop.Tools.JavaCallableWrappers.dll`.

[0]: https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references

Author: jonpryor

src/Java.Interop.NamingCustomAttributes/Android.Runtime/RegisterAttribute.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 
 using Mono.Cecil;
@@ -10,9 +12,9 @@ namespace Android.Runtime {
 #endif  // !JCW_ONLY_TYPE_NAMES
 	sealed class RegisterAttribute : Attribute, Java.Interop.IJniNameProviderAttribute {
 
-		string connector;
+		string? connector;
 		string name;
-		string signature;
+		string? signature;
 
 		public RegisterAttribute (string name)
 		{
@@ -26,22 +28,22 @@ public RegisterAttribute (string name, string signature, string connector)
 			this.signature = signature;
 		}
 #if HAVE_CECIL
-		public RegisterAttribute (string name, CustomAttribute originAttribute)
+		public RegisterAttribute (string name, CustomAttribute? originAttribute)
 			: this (name)
 		{
 			OriginAttribute = originAttribute;
 		}
 
-		public RegisterAttribute (string name, string signature, string connector, CustomAttribute originAttribute)
+		public RegisterAttribute (string name, string signature, string connector, CustomAttribute? originAttribute)
 			: this (name, signature, connector)
 		{
 			OriginAttribute = originAttribute;
 		}
 
-		public CustomAttribute OriginAttribute { get; }
+		public CustomAttribute? OriginAttribute { get; }
 #endif  // HAVE_CECIL
 
-		public string Connector {
+		public string? Connector {
 			get { return connector; }
 			set { connector = value; }
 		}
@@ -51,7 +53,7 @@ public string Name {
 			set { name = value; }
 		}
 
-		public string Signature {
+		public string? Signature {
 			get { return signature; }
 			set { signature = value; }
 		}

src/Java.Interop.NamingCustomAttributes/Java.Interop/ExportAttribute.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 
 namespace Java.Interop {
@@ -15,15 +17,15 @@ public ExportAttribute ()
 		{
 		}
 
-		public ExportAttribute (string name)
+		public ExportAttribute (string? name)
 		{
 			Name = name;
 		}
 
-		public string                 Name                    {get; private set;}
-		public string                 SuperArgumentsString    {get; set;}
-		public Type []                Throws                  {get; set;}
-		internal string []            ThrownNames             {get; set;} // msbuild internal use
+		public string?                Name                    {get; private set;}
+		public string?                SuperArgumentsString    {get; set;}
+		public Type []?               Throws                  {get; set;}
+		internal string []?           ThrownNames             {get; set;} // msbuild internal use
 	}
 }
 

src/Java.Interop.Tools.Diagnostics/Java.Interop.Tools.Diagnostics/Diagnostic.cs

@@ -148,7 +148,7 @@ namespace Java.Interop.Tools.Diagnostics {
 	//
 
 	public static class Diagnostic {
-		public static void Error (int code, SequencePoint location, string message, params object[] args)
+		public static void Error (int code, SequencePoint? location, string message, params object[] args)
 		{
 			throw new XamarinAndroidException (code, message, args) {
 				Location = location,

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.csproj

@@ -4,6 +4,7 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <LangVersion>8.0</LangVersion>
+    <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/IdentifierValidator.cs

@@ -31,9 +31,9 @@ public static class IdentifierValidator
 		// We use [^ ...] to detect any character that is NOT a match.
 		static Regex validIdentifier = new Regex ($"[^{Identifier}]", RegexOptions.Compiled);
 
-		public static string CreateValidIdentifier (string identifier, bool useEncodedReplacements = false)
+		public static string CreateValidIdentifier (string? identifier, bool useEncodedReplacements = false)
 		{
-			if (string.IsNullOrWhiteSpace (identifier)) return string.Empty;
+			if (identifier == null || string.IsNullOrWhiteSpace (identifier)) return string.Empty;
 
 			var normalizedIdentifier = identifier.Normalize ();