Show warning in ChangeSig dialog for duplicated param names

No warning in AddParam dialog yet

src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialog.xaml

@@ -211,13 +211,20 @@
                             </DataTemplate>
                         </DataGridTemplateColumn.CellTemplate>
                     </DataGridTemplateColumn>
-                    <DataGridTextColumn x:Name="parameterHeader" Binding="{Binding ParameterName, Mode=OneWay}" Width="*" IsReadOnly="True">
-                        <DataGridTextColumn.ElementStyle>
-                            <Style TargetType="TextBlock">
-                                <Setter Property="Padding" Value="{StaticResource ResourceKey=cellPadding}" />
-                            </Style>
-                        </DataGridTextColumn.ElementStyle>
-                    </DataGridTextColumn>
+                    <DataGridTemplateColumn x:Name="parameterHeader" Width="*" IsReadOnly="True">
+                        <DataGridTemplateColumn.CellTemplate>
+                            <DataTemplate>
+                                <StackPanel Orientation="Horizontal">
+                                    <imaging:CrispImage 
+                                        Moniker="{x:Static imagecatalog:KnownMonikers.StatusWarning}" 
+                                        Visibility="{Binding HasParameterNameConflict, Mode=TwoWay}"
+                                        vs2:ImageThemingUtilities.ImageBackgroundColor="White"
+                                        Margin="0,0,5,0"/>
+                                    <TextBlock Text="{Binding ParameterName, Mode=OneWay}"/>
+                                </StackPanel>
+                            </DataTemplate>
+                        </DataGridTemplateColumn.CellTemplate>
+                    </DataGridTemplateColumn>
                     <DataGridTextColumn x:Name="defaultHeader" Binding="{Binding DefaultValue, Mode=OneWay}" Width="*" IsReadOnly="True">
                         <DataGridTextColumn.ElementStyle>
                             <Style TargetType="TextBlock">

src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialogViewModel.ParameterViewModels.cs

@@ -29,14 +29,16 @@ public abstract class ParameterViewModel
             public abstract bool IsRequired { get; }
             public abstract string DefaultValue { get; }
 
+            public abstract string InitialIndex { get; }
+            public abstract string Modifier { get; }
+            public abstract string Default { get; }
+
             public ParameterViewModel(ChangeSignatureDialogViewModel changeSignatureDialogViewModel)
             {
                 ChangeSignatureDialogViewModel = changeSignatureDialogViewModel;
             }
 
-            public abstract string InitialIndex { get; }
-            public abstract string Modifier { get; }
-            public abstract string Default { get; }
+            public Visibility HasParameterNameConflict { get; set; }
 
             public virtual Visibility TypeWarningVisibility => Visibility.Collapsed;
 

src/VisualStudio/Core/Def/Implementation/ChangeSignature/ChangeSignatureDialogViewModel.cs

@@ -9,6 +9,7 @@
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
+using System.Windows;
 using System.Windows.Controls;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.ChangeSignature;
@@ -41,6 +42,8 @@ internal partial class ChangeSignatureDialogViewModel : AbstractNotifyPropertyCh
         private ImmutableArray<SymbolDisplayPart> _declarationParts;
         private bool _previewChanges;
 
+        private readonly Dictionary<string, List<ParameterViewModel>> _parameterNameOverlapMap = new Dictionary<string, List<ParameterViewModel>>();
+
         /// <summary>
         /// The document where the symbol we are changing signature is defined.
         /// </summary>
@@ -82,6 +85,8 @@ internal partial class ChangeSignatureDialogViewModel : AbstractNotifyPropertyCh
                 _paramsParameter = new ExistingParameterViewModel(this, parameters.ParamsParameter, initialDisplayIndex++);
             }
 
+            UpdateNameConflictMarkers();
+
             var selectedIndex = parameters.SelectedIndex;
             // Currently, we do not support editing the ThisParameter. 
             // Therefore, if there is such parameter, we should move the selectedIndex.
@@ -104,6 +109,42 @@ internal partial class ChangeSignatureDialogViewModel : AbstractNotifyPropertyCh
             }
         }
 
+        private void UpdateNameConflictMarkers()
+        {
+            var parameterNameOverlapMap = new Dictionary<string, List<ParameterViewModel>>();
+            foreach (var parameter in AllParameters)
+            {
+                if (!parameter.IsRemoved)
+                {
+                    parameterNameOverlapMap
+                        .GetOrAdd(parameter.ParameterName, _ => new List<ParameterViewModel>())
+                        .Add(parameter);
+                }
+                else
+                {
+                    parameter.HasParameterNameConflict = Visibility.Collapsed;
+                }
+            }
+
+            foreach (var parameterName in parameterNameOverlapMap.Keys)
+            {
+                var matchingParameters = parameterNameOverlapMap[parameterName];
+                if (matchingParameters.Count > 1)
+                {
+                    foreach (var matchingParameter in matchingParameters)
+                    {
+                        matchingParameter.HasParameterNameConflict = Visibility.Visible;
+                    }
+                }
+                else
+                {
+                    matchingParameters.Single().HasParameterNameConflict = Visibility.Collapsed;
+                }
+            }
+
+            NotifyPropertyChanged(nameof(AllParameters));
+        }
+
         public AddParameterDialogViewModel CreateAddParameterDialogViewModel()
             => new AddParameterDialogViewModel(_document, _insertPosition);
 
@@ -209,12 +250,14 @@ internal void Remove()
                 AllParameters[_selectedIndex!.Value].IsRemoved = true;
             }
 
+            UpdateNameConflictMarkers();
             RemoveRestoreNotifyPropertyChanged();
         }
 
         internal void Restore()
         {
             AllParameters[_selectedIndex!.Value].IsRemoved = false;
+            UpdateNameConflictMarkers();
             RemoveRestoreNotifyPropertyChanged();
         }
 
@@ -229,6 +272,7 @@ internal void AddParameter(AddedParameter addedParameter)
                 _parametersWithDefaultValues.Add(new AddedParameterViewModel(this, addedParameter));
             }
 
+            UpdateNameConflictMarkers();
             RemoveRestoreNotifyPropertyChanged();
         }