Use C# 7.3 compatible null check and simplify unary negation

CHANGELOG.md

@@ -15,6 +15,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 * Remove redundant empty string coalesce in string comparison - [#500](https://github.com/icsharpcode/CodeConverter/issues/500)
 * Convert VB comparison operators - [#396](https://github.com/icsharpcode/CodeConverter/issues/396)
 * Convert Redim Preserve of 1D array to Array.Resize - [#501](https://github.com/icsharpcode/CodeConverter/issues/501)
+* Use C#7.3 compatible null check
 
 ### C# -> VB
 

CodeConverter/CSharp/CommonConversions.cs

@@ -661,10 +661,9 @@ public static CSSyntax.BinaryExpressionSyntax NotNothingComparison(ExpressionSyn
 
         public static ExpressionSyntax NothingComparison(ExpressionSyntax otherArgument, bool isReferenceType)
         {
-            if (isReferenceType) {
-                return SyntaxFactory.IsPatternExpression(otherArgument, SyntaxFactory.ConstantPattern(SyntaxFactory.LiteralExpression(CSSyntaxKind.NullLiteralExpression)));
-            }
-            return SyntaxFactory.BinaryExpression(CSSyntaxKind.EqualsExpression, otherArgument, SyntaxFactory.LiteralExpression(CSSyntaxKind.DefaultLiteralExpression));
+            // Old project style doesn't support is pattern expressions (or indeed anything beyond c#7.3), so can't use "x is null"
+            var literalKind = isReferenceType ? CSSyntaxKind.NullLiteralExpression : CSSyntaxKind.DefaultLiteralExpression;
+            return SyntaxFactory.BinaryExpression(CSSyntaxKind.EqualsExpression, otherArgument, SyntaxFactory.LiteralExpression(literalKind));
         }
     }
 }

CodeConverter/CSharp/ExpressionNodeVisitor.cs

@@ -675,7 +675,7 @@ private ExpressionSyntax AsBool(VBSyntax.UnaryExpressionSyntax node, ExpressionS
 
         private async Task<ExpressionSyntax> NegateAndSimplifyOrNullAsync(VBSyntax.UnaryExpressionSyntax node, ExpressionSyntax expr)
         {
-            if (await _operatorConverter.ConvertNothingComparisonOrNullAsync(node.Operand, true) is ExpressionSyntax nothingComparison) {
+            if (await _operatorConverter.ConvertNothingComparisonOrNullAsync(node.Operand.SkipParens(), true) is ExpressionSyntax nothingComparison) {
                 return nothingComparison;
             } else if (expr is BinaryExpressionSyntax bes && bes.OperatorToken.IsKind(SyntaxKind.EqualsToken)) {
                 return bes.WithOperatorToken(SyntaxFactory.Token(SyntaxKind.ExclamationEqualsToken));

Tests/CSharp/ExpressionTests/ExpressionTests.cs

@@ -356,6 +356,24 @@ internal partial class TestClass
 }");
         }
 
+        [Fact]
+        public async Task GenericComparisonAsync()
+        {
+            await TestConversionVisualBasicToCSharpAsync(@"Public Class GenericComparison
+    Public Sub m(Of T)(p As T)
+        If p Is Nothing Then Return
+    End Sub
+End Class", @"
+public partial class GenericComparison
+{
+    public void m<T>(T p)
+    {
+        if (p == null)
+            return;
+    }
+}");
+        }
+
         [Fact]
         public async Task AccessSharedThroughInstanceAsync()
         {
@@ -1308,7 +1326,7 @@ public partial class Foo
 
     protected void OnBar(EventArgs e)
     {
-        if (Bar is null)
+        if (Bar == null)
         {
             Debug.WriteLine(""No subscriber"");
         }

Tests/CSharp/StatementTests/StatementTests.cs

@@ -1390,7 +1390,7 @@ internal partial class TestClass
 {
     private void TestMethod(object nullObject)
     {
-        if (nullObject is null)
+        if (nullObject == null)
             throw new ArgumentNullException(nameof(nullObject));
         lock (nullObject)
             Console.WriteLine(nullObject);
@@ -1411,7 +1411,7 @@ internal partial class TestClass
 {
     private void TestMethod(object nullObject)
     {
-        if (nullObject is null)
+        if (nullObject == null)
             throw new ArgumentNullException(nameof(nullObject));
     }
 }");

Tests/CSharp/TypeCastTests.cs

@@ -382,7 +382,7 @@ public partial class MultipleCasts
 {
     public static T ToGenericParameter<T>(object Value)
     {
-        if (Value is null)
+        if (Value == null)
         {
             return default;
         }