Add async disposal to transactions in samples (#4960)

Fixes #4959

Author: roji

samples/core/Saving/Transactions/ControllingTransaction.cs

@@ -17,7 +17,7 @@ public static async Task Run()
 
         #region Transaction
         using var context = new BloggingContext();
-        using var transaction = await context.Database.BeginTransactionAsync();
+        await using var transaction = await context.Database.BeginTransactionAsync();
 
         try
         {

samples/core/Saving/Transactions/ExternalDbTransaction.cs

@@ -25,7 +25,7 @@ public static async Task Run()
         using var connection = new SqlConnection(connectionString);
         await connection.OpenAsync();
 
-        using var transaction = (SqlTransaction)await connection.BeginTransactionAsync();
+        await using var transaction = (SqlTransaction)await connection.BeginTransactionAsync();
         try
         {
             // Run raw ADO.NET command in the transaction

samples/core/Saving/Transactions/ManagingSavepoints.cs

@@ -16,7 +16,7 @@ public static async Task Run()
 
         #region Savepoints
         using var context = new BloggingContext();
-        using var transaction = await context.Database.BeginTransactionAsync();
+        await using var transaction = await context.Database.BeginTransactionAsync();
 
         try
         {

samples/core/Saving/Transactions/SharingTransaction.cs

@@ -30,7 +30,7 @@ public static async Task Run()
             .Options;
 
         using var context1 = new BloggingContext(options);
-        using var transaction = await context1.Database.BeginTransactionAsync();
+        await using var transaction = await context1.Database.BeginTransactionAsync();
         try
         {
             context1.Blogs.Add(new Blog { Url = "http://blogs.msdn.com/dotnet" });

samples/core/Testing/BloggingWebApi/Controllers/BloggingController.cs

@@ -46,7 +46,7 @@ public async Task<ActionResult> AddBlog(string name, string url)
     public async Task<ActionResult> UpdateBlogUrl(string name, string url)
     {
         // Note: it isn't usually necessary to start a transaction for updating. This is done here for illustration purposes only.
-        using var transaction = await _context.Database.BeginTransactionAsync(IsolationLevel.Serializable);
+        await using var transaction = await _context.Database.BeginTransactionAsync(IsolationLevel.Serializable);
 
         var blog = await _context.Blogs.FirstOrDefaultAsync(b => b.Name == name);
         if (blog is null)