Convert Not(x IsNot Nothing) to x is null - fixes #1113

Author: GrahamTheCoder

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ### VB -> C#
 
+* Convert Not(x IsNot Nothing) to x is null [#1113](https://github.com/icsharpcode/CodeConverter/issues/1113)
 * Escape parameter names [#1092](https://github.com/icsharpcode/CodeConverter/issues/1092)
 
 ### C# -> VB

CodeConverter/CSharp/BuiltInVisualBasicOperatorSubstitutions.cs

@@ -40,7 +40,7 @@ public async Task<ExpressionSyntax> ConvertReferenceOrNothingComparisonOrNullAsy
             }
 
             var notted =
-                node.IsKind(VBasic.SyntaxKind.IsNotExpression, VBasic.SyntaxKind.NotEqualsExpression) ||
+                node.IsKind(VBasic.SyntaxKind.IsNotExpression, VBasic.SyntaxKind.NotEqualsExpression) ^
                 negateExpression;
             var isReferenceComparison = node.IsKind(VBasic.SyntaxKind.IsExpression, VBasic.SyntaxKind.IsNotExpression);
 

Tests/CSharp/ExpressionTests/ExpressionTests.cs

@@ -19,6 +19,7 @@ Private Sub TestMethod(a as String)
         result = Nothing = a
         result = a Is Nothing
         result = a IsNot Nothing
+        result = Not(a IsNot Nothing)
         result = a = a
         result = a = (""test"")
         result = ""test"" = a
@@ -36,6 +37,7 @@ private void TestMethod(string a)
         result = string.IsNullOrEmpty(a);
         result = a is null;
         result = a is not null;
+        result = a is null;
         result = (a ?? """") == (a ?? """");
         result = a == ""test"";
         result = ""test"" == a;