Use null propagation (#595)

Author: gpetrou

Src/IronPython/Compiler/Ast/AndExpression.cs

@@ -72,12 +72,8 @@ public override Type Type {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_left != null) {
-                    _left.Walk(walker);
-                }
-                if (_right != null) {
-                    _right.Walk(walker);
-                }
+                _left?.Walk(walker);
+                _right?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/Arg.cs

@@ -46,9 +46,7 @@ internal Argument GetArgumentInfo() {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_expression != null) {
-                    _expression.Walk(walker);
-                }
+                _expression?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/AugmentedAssignStatement.cs

@@ -72,12 +72,8 @@ private static PythonOperationKind PythonOperatorToAction(PythonOperator op) {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_left != null) {
-                    _left.Walk(walker);
-                }
-                if (_right != null) {
-                    _right.Walk(walker);
-                }
+                _left?.Walk(walker);
+                _right?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/BackQuoteExpression.cs

@@ -31,9 +31,7 @@ public override MSAst.Expression Reduce() {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_expression != null) {
-                    _expression.Walk(walker);
-                }
+                _expression?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/ClassDefinition.cs

@@ -288,9 +288,8 @@ public override void Walk(PythonWalker walker) {
                         b.Walk(walker);
                     }
                 }
-                if (_body != null) {
-                    _body.Walk(walker);
-                }
+
+                _body?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/Comprehension.cs

@@ -106,9 +106,7 @@ public override string NodeName {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_item != null) {
-                    _item.Walk(walker);
-                }
+                _item?.Walk(walker);
                 if (_iterators != null) {
                     foreach (ComprehensionIterator ci in _iterators) {
                         ci.Walk(walker);
@@ -169,9 +167,7 @@ public override string NodeName {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_item != null) {
-                    _item.Walk(walker);
-                }
+                _item?.Walk(walker);
                 if (_iterators != null) {
                     foreach (ComprehensionIterator ci in _iterators) {
                         ci.Walk(walker);
@@ -244,12 +240,8 @@ public override string NodeName {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_key != null) {
-                    _key.Walk(walker);
-                }
-                if (_value != null) {
-                    _value.Walk(walker);
-                }
+                _key?.Walk(walker);
+                _value?.Walk(walker);
                 if (_iterators != null) {
                     foreach (ComprehensionIterator ci in _iterators) {
                         ci.Walk(walker);

Src/IronPython/Compiler/Ast/ComprehensionFor.cs

@@ -39,12 +39,8 @@ internal override MSAst.Expression Transform(MSAst.Expression body) {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_lhs != null) {
-                    _lhs.Walk(walker);
-                }
-                if (_list != null) {
-                    _list.Walk(walker);
-                }
+                _lhs?.Walk(walker);
+                _list?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/ComprehensionIf.cs

@@ -34,9 +34,7 @@ internal override MSAst.Expression Transform(MSAst.Expression body) {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_test != null) {
-                    _test.Walk(walker);
-                }
+                _test?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/ConditionalExpression.cs

@@ -50,15 +50,9 @@ public override MSAst.Expression Reduce() {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_testExpr != null) {
-                    _testExpr.Walk(walker);
-                }
-                if (_trueExpr != null) {
-                    _trueExpr.Walk(walker);
-                }
-                if (_falseExpr != null) {
-                    _falseExpr.Walk(walker);
-                }
+                _testExpr?.Walk(walker);
+                _trueExpr?.Walk(walker);
+                _falseExpr?.Walk(walker);
             }
             walker.PostWalk(this);
         }

Src/IronPython/Compiler/Ast/ExecStatement.cs

@@ -65,15 +65,9 @@ public override MSAst.Expression Reduce() {
 
         public override void Walk(PythonWalker walker) {
             if (walker.Walk(this)) {
-                if (_code != null) {
-                    _code.Walk(walker);
-                }
-                if (_locals != null) {
-                    _locals.Walk(walker);
-                }
-                if (_globals != null) {
-                    _globals.Walk(walker);
-                }
+                _code?.Walk(walker);
+                _locals?.Walk(walker);
+                _globals?.Walk(walker);
             }
             walker.PostWalk(this);
         }