From 6c3bfeda802f55040624f0de33f644b46a7d9b6f Mon Sep 17 00:00:00 2001
From: Julien Couvreur <jcouv@users.noreply.github.com>
Date: Tue, 2 Jan 2018 19:02:51 -0800
Subject: [PATCH] AddParameter should handle and bail out on omitted argument
 (#23933)

---
 .../AddParameter/AddParameterTests.vb         | 19 +++++++++++++++++++
 .../AbstractAddParameterCodeFixProvider.cs    | 12 ++++++++----
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/src/EditorFeatures/VisualBasicTest/AddParameter/AddParameterTests.vb b/src/EditorFeatures/VisualBasicTest/AddParameter/AddParameterTests.vb
index 1d1985ef571..6253ab14048 100644
--- a/src/EditorFeatures/VisualBasicTest/AddParameter/AddParameterTests.vb
+++ b/src/EditorFeatures/VisualBasicTest/AddParameter/AddParameterTests.vb
@@ -13,6 +13,25 @@ Namespace Microsoft.CodeAnalysis.Editor.VisualBasic.UnitTests.AddParameter
             Return (Nothing, New VisualBasicAddParameterCodeFixProvider())
         End Function
 
+        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddParameter)>
+        <WorkItem(23927, "https://github.com/dotnet/roslyn/issues/23927")>
+        Public Async Function TestMissingOnOmittedArgument() As Task
+            Await TestMissingAsync(
+"Public Module Module1
+    Private Class C
+        Public Sub New(Arg1 As Integer)
+        End Sub
+
+        Public Sub New(Arg1 As Integer, Arg2 As Integer)
+        End Sub
+    End Class
+
+    Public Sub Main()
+        Dim x = New [|C|](, 0)
+    End Sub
+End Module")
+        End Function
+
         <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddParameter)>
         Public Async Function TestMissingWithImplicitConstructor() As Task
             Await TestMissingAsync(
diff --git a/src/Features/Core/Portable/AddParameter/AbstractAddParameterCodeFixProvider.cs b/src/Features/Core/Portable/AddParameter/AbstractAddParameterCodeFixProvider.cs
index fa25ad5d739..ac408bc0357 100644
--- a/src/Features/Core/Portable/AddParameter/AbstractAddParameterCodeFixProvider.cs
+++ b/src/Features/Core/Portable/AddParameter/AbstractAddParameterCodeFixProvider.cs
@@ -442,10 +442,14 @@ private int NonParamsParameterCount(IMethodSymbol method)
 
                     // Now check the type of the argument versus the type of the parameter.  If they
                     // don't match, then this is the argument we should make the parameter for.
-                    var expressionOfArgumment = syntaxFacts.GetExpressionOfArgument(argument);
-                    var argumentTypeInfo = semanticModel.GetTypeInfo(expressionOfArgumment);
-                    var isNullLiteral = syntaxFacts.IsNullLiteralExpression(expressionOfArgumment);
-                    var isDefaultLiteral = syntaxFacts.IsDefaultLiteralExpression(expressionOfArgumment);
+                    var expressionOfArgument = syntaxFacts.GetExpressionOfArgument(argument);
+                    if (expressionOfArgument is null)
+                    {
+                        return null;
+                    }
+                    var argumentTypeInfo = semanticModel.GetTypeInfo(expressionOfArgument);
+                    var isNullLiteral = syntaxFacts.IsNullLiteralExpression(expressionOfArgument);
+                    var isDefaultLiteral = syntaxFacts.IsDefaultLiteralExpression(expressionOfArgument);
 
                     if (argumentTypeInfo.Type == null && argumentTypeInfo.ConvertedType == null)
                     {
-- 
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