Added Add-Operation + UI

BFR/MainWindow.xaml

@@ -260,5 +260,25 @@
         </Grid>
       </Expander>
     </DataTemplate>
+    <DataTemplate DataType="{x:Type ops:Add}">
+      <Expander Classes="OperationExpander">
+        <Grid ColumnDefinitions="auto,*" RowDefinitions="auto,auto,auto,auto">
+          <TextBlock Grid.Row="0" Text="Mode:"/>
+          <ComboBox Grid.Row="0" Items="{Binding Modes}" Name="ModeSelector" SelectedIndex="{Binding Mode}" PropertyChanged="PreviewChanged">
+            <ComboBox.ItemTemplate>
+              <DataTemplate>
+                <TextBlock Text="{Binding Name}"/>
+              </DataTemplate>
+            </ComboBox.ItemTemplate>
+          </ComboBox>
+          <TextBlock Grid.Row="1" Text="Position:" IsVisible="{Binding #ModeSelector.SelectedItem.IsInsert}"/>
+          <NumericUpDown Grid.Row="1" Value="{Binding InsertPosition}" IsVisible="{Binding #ModeSelector.SelectedItem.IsInsert}" PropertyChanged="PreviewChanged"/>
+          <TextBlock Grid.Row="2" Text="Text:"/>
+          <TextBox Grid.Row="2" Text="{Binding Text}" PropertyChanged="PreviewChanged"/>
+          <TextBlock Grid.Row="3" Text="Full Name:"/>
+          <CheckBox Grid.Row="3" IsChecked="{Binding FullName}" PropertyChanged="PreviewChanged"/>
+        </Grid>
+      </Expander>
+    </DataTemplate>
   </Window.DataTemplates>
 </Window>

BFR/MainWindowUIProperties.cs

@@ -27,6 +27,7 @@ namespace BFR
             OperationType.Make<Replace>(),
             OperationType.Make<Remove>(),
             OperationType.Make<Number>(),
+            OperationType.Make<Add>(),
         };
 
         public static readonly AvaloniaProperty<bool> IsCommitButtonEnabledProperty =

BFR/Operations/Add.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Xml.Serialization;
+using System.Linq;
+
+using Avalonia;
+
+using BFR.DataModels;
+
+namespace BFR.Operations
+{
+    public class Add : Operation
+    {
+        public override string Name => nameof(Add);
+        public override string Description => "Adds the specified text to the file name.";
+
+        [XmlIgnore]
+        public static readonly AvaloniaProperty<AddMode> ModeProperty =
+            AvaloniaProperty.Register<MainWindow, AddMode>(nameof(Mode), AddMode.Prepend);
+        public AddMode Mode { get => GetValue(ModeProperty); set => SetValue(ModeProperty, value); }
+
+        public AddOperationMode[] Modes => AddOperationMode.Modes;
+
+        public int InsertPosition { get; set; } = 0;
+        public string Text { get; set; } = "";
+        public bool FullName { get; set; } = false;
+
+        protected override void ApplyToInternal(IList<FileModel> files)
+        {
+            // Fail conditions: Out of bounds insert
+            var shortestLength = files.Min(x => FullName ? x.FullName.Length : x.Name.Length);
+            if (Mode == AddMode.Insert && InsertPosition > shortestLength) throw new OperationException($"Index can't be greater than the shortest file name length ({shortestLength}).");
+
+            // Apply operation
+            foreach (var file in files)
+            {
+                var newName = FullName ? file.FullName : file.Name;
+
+                newName = Mode switch
+                {
+                    AddMode.Prepend => $"{Text}{newName}",
+                    AddMode.Append => $"{newName}{Text}",
+                    AddMode.Insert => newName.Insert(InsertPosition, Text),
+                    _ => newName
+                };
+
+                if (FullName) file.FullName = newName;
+                else file.Name = newName;
+            }
+        }
+    }
+}

BFR/Operations/AddMode.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+using BFR.Helpers;
+
+namespace BFR.Operations
+{
+    public class AddOperationMode : OperationMode<AddMode>
+    {
+        public bool IsPrepend => Index == AddMode.Prepend;
+        public bool IsAppend => Index == AddMode.Append;
+        public bool IsInsert => Index == AddMode.Insert;
+
+        public readonly static AddOperationMode[] Modes = All();
+
+        public AddOperationMode(AddMode index, string name, string description) :
+            base(index, name, description) { }
+
+        protected static AddOperationMode[] All() => ((IEnumerable<AddMode>)Enum.GetValues(typeof(AddMode))).Select(x =>
+            new AddOperationMode(
+                x,
+                x.GetAttribute<OperationModeAttribute>().DisplayName,
+                x.GetAttribute<OperationModeAttribute>().Description
+            )).ToArray();
+    }
+
+    public enum AddMode
+    {
+        [OperationMode(nameof(Prepend), "Inserts before the file name.")]
+        Prepend,
+        [OperationMode(nameof(Append), "Inserts after the file name.")]
+        Append,
+        [OperationMode(nameof(Insert), "Inserts at the specified index.")]
+        Insert
+    }
+}