Implemented preview TODO, modified exception handling

- Better handles unexpected exceptions
- Expected exceptions should be OperationException
- Leaves files unchanged even if the exception occurs at *any* point of the operation
- TODO: Probably have to optimize this.
- Operations now operate on IList<T> instead of List<T> to maintain compatibility with AvaloniaList<T>
- Slightly better comments

BFR/MainWindow.xaml.cs

@@ -43,7 +43,9 @@ namespace BFR
             foreach (var file in Files)
                 file.Path = file.OldPath;
 
-            // TODO: Apply operations
+            // Apply operations
+            foreach (var operation in Operations)
+                operation.ApplyTo(Files);
 
             // Refresh the file list to guarantee that changes are displayed. TODO: Find a better way to do this.
             Files.ReplaceAll(new List<FileModel>(Files));

BFR/MainWindowUIProperties.cs

@@ -3,6 +3,7 @@ using Avalonia.Collections;
 using Avalonia.Controls;
 
 using BFR.DataModels;
+using BFR.Operations;
 
 namespace BFR
 {
@@ -14,6 +15,7 @@ namespace BFR
 
         public AvaloniaList<FileModel> AllFiles { get; } = new AvaloniaList<FileModel>();
         public AvaloniaList<FileModel> Files { get; } = new AvaloniaList<FileModel>();
+        public AvaloniaList<Operation> Operations { get; } = new AvaloniaList<Operation>();
 
         // Filters
         public string FilterExtension { get; set; } = "";

BFR/Operations/Operation.cs

@@ -1,15 +1,16 @@
 using System;
 using System.Collections.Generic;
-
+using System.Linq;
 using Avalonia;
 
 using BFR.DataModels;
+using BFR.Helpers;
 
 namespace BFR.Operations
 {
     public abstract class Operation : AvaloniaObject
     {
-        // Needs to be an avalonia property to update the UI with with any potential error.
+        // Needs to be avalonia property to update UI with any potential error.
         public AvaloniaProperty<string> errorProperty = AvaloniaProperty.Register<Operation, string>(nameof(Error), defaultValue: "");
         public string Error { get => GetValue(errorProperty); set => SetValue(errorProperty, value); }
         public bool IsEnabled { get; set; } = true;
@@ -17,13 +18,25 @@ namespace BFR.Operations
         public abstract string Description { get; }
         public abstract OperationType OperationType { get; }
 
-        protected abstract void ApplyToInternal(List<FileModel> files);
+        protected abstract void ApplyToInternal(IList<FileModel> files);
         
-        // Updates the UI with the appropriate error if the operation fails
-        public void ApplyTo(List<FileModel> files)
+        public void ApplyTo(IList<FileModel> files)
         {
-            try { ApplyToInternal(files); Error = ""; }
-            catch (Exception e) { Error = e.Message; }
+            // Backup of all changes thus far, in case something goes wrong
+            var backup = files.Select(file => file.Path).ToList();
+            try
+            {
+                ApplyToInternal(files); // Try to apply operation
+                Error = "";             // If successul, remove any previous error
+            }  
+            catch (OperationException e) { Error = e.Message; } // If expected, show user a concise and descriptive message
+            catch (Exception e) { Error = $"Unexpected {e}"; }  // If not, hint with the exception thrown that this is unexpected behaviour
+            finally
+            {
+                if (Error != "") // If something went wrong, restore backup
+                    for (int i = 0; i < files.Count; i++)
+                        files[i].Path = backup[i];
+            }
         }
     }
 }

BFR/Operations/OperationException.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace BFR.Operations
+{
+    public class OperationException : Exception
+    {
+        public OperationException(string message) :
+            base(message)
+        { }
+    }
+}

BFR/Operations/Overwrite.cs

@@ -15,7 +15,7 @@ namespace BFR.Operations
         public string Replacement { get; set; } = "";
         public bool FullName { get; set; } = false;
 
-        protected override void ApplyToInternal(List<FileModel> files)
+        protected override void ApplyToInternal(IList<FileModel> files)
         {
             // Fail conditions: No fail conditions
             // Apply Operation