Working, with tests.

src/EditorFeatures/CSharpTest/InitializeParameter/InitializeMemberFromParameterTests.cs

@@ -1618,5 +1618,126 @@ public C(int i, string s)
     }
 }");
         }
+
+        [WorkItem(35665, "https://github.com/dotnet/roslyn/issues/35665")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInitializeParameter)]
+        public async Task TestGenerateRemainingFields1()
+        {
+            await TestInRegularAndScript1Async(
+@"
+class C
+{
+    public C([||]int i, int j, int k)
+    {
+    }
+}",
+@"
+class C
+{
+    private readonly int i;
+    private readonly int j;
+    private readonly int k;
+
+    public C(int i, int j, int k)
+    {
+        this.i = i;
+        this.j = j;
+        this.k = k;
+    }
+}", index: 3);
+        }
+
+        [WorkItem(35665, "https://github.com/dotnet/roslyn/issues/35665")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInitializeParameter)]
+        public async Task TestGenerateRemainingFields2()
+        {
+            await TestInRegularAndScript1Async(
+@"
+class C
+{
+    private readonly int i;
+
+    public C(int i, [||]int j, int k)
+    {
+        this.i = i;
+    }
+}",
+@"
+class C
+{
+    private readonly int i;
+    private readonly int j;
+    private readonly int k;
+
+    public C(int i, int j, int k)
+    {
+        this.i = i;
+        this.j = j;
+        this.k = k;
+    }
+}", index: 2);
+        }
+
+        [WorkItem(35665, "https://github.com/dotnet/roslyn/issues/35665")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInitializeParameter)]
+        public async Task TestGenerateRemainingFields3()
+        {
+            await TestInRegularAndScript1Async(
+@"
+class C
+{
+    private readonly int j;
+
+    public C([||]int i, int j, int k)
+    {
+        this.j = j;
+    }
+}",
+@"
+class C
+{
+    private readonly int i;
+    private readonly int j;
+    private readonly int k;
+
+    public C(int i, int j, int k)
+    {
+        this.i = i;
+        this.j = j;
+        this.k = k;
+    }
+}", index: 2);
+        }
+
+        [WorkItem(35665, "https://github.com/dotnet/roslyn/issues/35665")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInitializeParameter)]
+        public async Task TestGenerateRemainingFields4()
+        {
+            await TestInRegularAndScript1Async(
+@"
+class C
+{
+    private readonly int k;
+
+    public C([||]int i, int j, int k)
+    {
+        this.k = k;
+    }
+}",
+@"
+class C
+{
+    private readonly int i;
+    private readonly int j;
+    private readonly int k;
+
+    public C(int i, int j, int k)
+    {
+        this.i = i;
+        this.j = j;
+        this.k = k;
+    }
+}", index: 2);
+        }
     }
 }

src/Features/CSharp/Portable/InitializeParameter/CSharpAddParameterCheckCodeRefactoringProvider.cs

@@ -35,8 +35,8 @@ protected override SyntaxNode GetTypeBlock(SyntaxNode node)
         protected override SyntaxNode GetBody(SyntaxNode functionDeclaration)
             => InitializeParameterHelpers.GetBody(functionDeclaration);
 
-        protected override void InsertStatement(SyntaxEditor editor, SyntaxNode functionDeclaration, IMethodSymbol method, SyntaxNode statementToAddAfterOpt, StatementSyntax statement)
-            => InitializeParameterHelpers.InsertStatement(editor, functionDeclaration, method, statementToAddAfterOpt, statement);
+        protected override void InsertStatement(SyntaxEditor editor, SyntaxNode functionDeclaration, bool returnsVoid, SyntaxNode statementToAddAfterOpt, StatementSyntax statement)
+            => InitializeParameterHelpers.InsertStatement(editor, functionDeclaration, returnsVoid, statementToAddAfterOpt, statement);
 
         protected override bool IsImplicitConversion(Compilation compilation, ITypeSymbol source, ITypeSymbol destination)
             => InitializeParameterHelpers.IsImplicitConversion(compilation, source, destination);

src/Features/CSharp/Portable/InitializeParameter/CSharpInitializeMemberFromParameterCodeRefactoringProvider.cs

@@ -16,6 +16,7 @@ namespace Microsoft.CodeAnalysis.CSharp.InitializeParameter
     [ExtensionOrder(Before = PredefinedCodeRefactoringProviderNames.Wrapping)]
     internal class CSharpInitializeMemberFromParameterCodeRefactoringProvider :
         AbstractInitializeMemberFromParameterCodeRefactoringProvider<
+            BaseTypeDeclarationSyntax,
             ParameterSyntax,
             StatementSyntax,
             ExpressionSyntax>
@@ -34,8 +35,8 @@ protected override SyntaxNode GetTypeBlock(SyntaxNode node)
         protected override SyntaxNode TryGetLastStatement(IBlockOperation blockStatementOpt)
             => InitializeParameterHelpers.TryGetLastStatement(blockStatementOpt);
 
-        protected override void InsertStatement(SyntaxEditor editor, SyntaxNode functionDeclaration, IMethodSymbol method, SyntaxNode statementToAddAfterOpt, StatementSyntax statement)
-            => InitializeParameterHelpers.InsertStatement(editor, functionDeclaration, method, statementToAddAfterOpt, statement);
+        protected override void InsertStatement(SyntaxEditor editor, SyntaxNode functionDeclaration, bool returnsVoid, SyntaxNode statementToAddAfterOpt, StatementSyntax statement)
+            => InitializeParameterHelpers.InsertStatement(editor, functionDeclaration, returnsVoid, statementToAddAfterOpt, statement);
 
         protected override bool IsImplicitConversion(Compilation compilation, ITypeSymbol source, ITypeSymbol destination)
             => InitializeParameterHelpers.IsImplicitConversion(compilation, source, destination);

src/Features/CSharp/Portable/InitializeParameter/InitializeParameterHelpers.cs

@@ -62,7 +62,7 @@ public static SyntaxNode TryGetLastStatement(IBlockOperation blockStatementOpt)
         public static void InsertStatement(
             SyntaxEditor editor,
             SyntaxNode functionDeclaration,
-            IMethodSymbol method,
+            bool returnsVoid,
             SyntaxNode statementToAddAfterOpt,
             StatementSyntax statement)
         {
@@ -72,7 +72,7 @@ public static SyntaxNode TryGetLastStatement(IBlockOperation blockStatementOpt)
             {
                 var semicolonToken = TryGetSemicolonToken(functionDeclaration) ?? SyntaxFactory.Token(SyntaxKind.SemicolonToken);
 
-                if (!TryConvertExpressionBodyToStatement(body, semicolonToken, !method.ReturnsVoid, out var convertedStatement))
+                if (!TryConvertExpressionBodyToStatement(body, semicolonToken, !returnsVoid, out var convertedStatement))
                 {
                     return;
                 }

src/Features/Core/Portable/InitializeParameter/AbstractAddParameterCheckCodeRefactoringProvider.cs

@@ -345,7 +345,7 @@ private bool IsNullCheck(IOperation operand1, IOperation operand2, IParameterSym
             // signature.
             var statementToAddAfter = GetStatementToAddNullCheckAfter(
                 semanticModel, parameter, blockStatementOpt, cancellationToken);
-            InsertStatement(editor, functionDeclaration, method, statementToAddAfter, nullCheckStatement);
+            InsertStatement(editor, functionDeclaration, method.ReturnsVoid, statementToAddAfter, nullCheckStatement);
 
             var newRoot = editor.GetChangedRoot();
             return document.WithSyntaxRoot(newRoot);

src/Features/Core/Portable/InitializeParameter/AbstractInitializeParameterCodeRefactoringProvider.cs

@@ -53,7 +53,7 @@ internal abstract partial class AbstractInitializeParameterCodeRefactoringProvid
             CancellationToken cancellationToken);
 
         protected abstract void InsertStatement(
-            SyntaxEditor editor, SyntaxNode functionDeclaration, IMethodSymbol method,
+            SyntaxEditor editor, SyntaxNode functionDeclaration, bool returnsVoid,
             SyntaxNode? statementToAddAfterOpt, TStatementSyntax statement);
 
         public override async Task ComputeRefactoringsAsync(CodeRefactoringContext context)

src/Features/VisualBasic/Portable/InitializeParameter/VisualBasicAddParameterCheckCodeRefactoringProvider.vb

@@ -35,7 +35,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.InitializeParameter
             Return InitializeParameterHelpers.GetBody(functionDeclaration)
         End Function
 
-        Protected Overrides Sub InsertStatement(editor As SyntaxEditor, functionDeclaration As SyntaxNode, method As IMethodSymbol, statementToAddAfterOpt As SyntaxNode, statement As StatementSyntax)
+        Protected Overrides Sub InsertStatement(editor As SyntaxEditor, functionDeclaration As SyntaxNode, returnsVoid As Boolean, statementToAddAfterOpt As SyntaxNode, statement As StatementSyntax)
             InitializeParameterHelpers.InsertStatement(editor, functionDeclaration, statementToAddAfterOpt, statement)
         End Sub
 

src/Features/VisualBasic/Portable/InitializeParameter/VisualBasicInitializeMemberFromParameterCodeRefactoringProvider.vb

@@ -15,6 +15,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.InitializeParameter
     <ExtensionOrder(Before:=PredefinedCodeRefactoringProviderNames.Wrapping)>
     Friend Class VisualBasicInitializeMemberFromParameterCodeRefactoringProvider
         Inherits AbstractInitializeMemberFromParameterCodeRefactoringProvider(Of
+            TypeBlockSyntax,
             ParameterSyntax,
             StatementSyntax,
             ExpressionSyntax)
@@ -39,7 +40,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.InitializeParameter
             Return InitializeParameterHelpers.IsImplicitConversion(compilation, source, destination)
         End Function
 
-        Protected Overrides Sub InsertStatement(editor As SyntaxEditor, functionDeclaration As SyntaxNode, method As IMethodSymbol, statementToAddAfterOpt As SyntaxNode, statement As StatementSyntax)
+        Protected Overrides Sub InsertStatement(editor As SyntaxEditor, functionDeclaration As SyntaxNode, returnsVoid As Boolean, statementToAddAfterOpt As SyntaxNode, statement As StatementSyntax)
             InitializeParameterHelpers.InsertStatement(editor, functionDeclaration, statementToAddAfterOpt, statement)
         End Sub