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GerstITS.Examples.Transactions/TransactionRunner.cs

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+using GerstITS.Data;
+using GerstITS.Examples.Transactions.Entities;
+
+namespace GerstITS.Examples.Transactions;
+
+/// <summary>
+/// Demonstrates the <see cref="IRepository" /> transaction API:
+/// <list type="bullet">
+///   <item>legacy auto-commit behaviour when no transaction is started,</item>
+///   <item>a successful transaction that spans a Save and a Delete,</item>
+///   <item>an automatic rollback on exception that discards partial work,</item>
+///   <item>the guard that prevents starting a second, nested transaction.</item>
+/// </list>
+/// </summary>
+internal sealed class TransactionRunner
+{
+    #region Fields
+
+    private readonly IRepository _repository;
+    private readonly ProductDbContext _dbContext;
+
+    #endregion
+
+    #region Constructors
+
+    public TransactionRunner(IRepository repository,
+                             ProductDbContext dbContext)
+    {
+        _repository = repository;
+        _dbContext = dbContext;
+    }
+
+    #endregion
+
+    #region Methods
+
+    public void Run()
+    {
+        // The SQLite in-memory database starts out empty, so create the schema once up front.
+        _dbContext.Database.EnsureCreated();
+
+        RunAutoCommitBehavior();
+        RunSuccessfulTransaction();
+        RunRollbackOnException();
+        RunNestedTransactionGuard();
+    }
+
+    private void RunAutoCommitBehavior()
+    {
+        Print("=== Legacy auto-commit (no BeginTransaction) ===");
+
+        // Without BeginTransaction() every Save/Delete auto-commits immediately (unchanged legacy behaviour).
+        _repository.Save(new ProductEntity { Name = "Obsolete Widget", Price = 9.99m });
+
+        Print($"Products after auto-committed insert: {NamesInStore()}");
+    }
+
+    private void RunSuccessfulTransaction()
+    {
+        Print(string.Empty);
+        Print("=== Successful transaction spanning a Save and a Delete ===");
+
+        var obsolete = _repository.Query<ProductEntity>()
+                                  .Single(product => product.Name == "Obsolete Widget");
+
+        using (var transaction = _repository.BeginTransaction())
+        {
+            _repository.Save(new ProductEntity { Name = "New Gadget", Price = 19.99m });
+            _repository.Delete(obsolete);
+
+            transaction.Commit();
+        }
+
+        Print($"Products after committed transaction: {NamesInStore()}");
+    }
+
+    private void RunRollbackOnException()
+    {
+        Print(string.Empty);
+        Print("=== Rollback on exception (partial work is discarded) ===");
+
+        try
+        {
+            using var transaction = _repository.BeginTransaction();
+
+            _repository.Save(new ProductEntity { Name = "Half-Baked Product", Price = 1.00m });
+
+            // Something fails before Commit() is reached. Because the transaction is disposed without a
+            // prior Commit(), the framework rolls it back automatically - so the insert above is undone.
+            throw new InvalidOperationException("Simulated failure after partial work.");
+        }
+        catch (InvalidOperationException exception)
+        {
+            Print($"Caught expected exception: {exception.Message}");
+        }
+
+        Print($"Products after rolled-back transaction: {NamesInStore()} (the half-baked product was discarded)");
+    }
+
+    private void RunNestedTransactionGuard()
+    {
+        Print(string.Empty);
+        Print("=== Starting a second transaction while one is active throws ===");
+
+        using var transaction = _repository.BeginTransaction();
+
+        try
+        {
+            _repository.BeginTransaction();
+        }
+        catch (InvalidOperationException exception)
+        {
+            Print($"Caught expected exception: {exception.Message}");
+        }
+
+        transaction.Rollback();
+    }
+
+    private string NamesInStore()
+    {
+        var names = _repository.Query<ProductEntity>()
+                               .OrderBy(product => product.Name)
+                               .Select(product => product.Name)
+                               .ToList();
+
+        return names.Count == 0
+                   ? "<none>"
+                   : string.Join(", ", names);
+    }
+
+    private static void Print(string message)
+    {
+        Console.WriteLine(message);
+    }
+
+    #endregion
+}