refactor: apply Open-Closed Principle to ExecuteTimestampQuery for extensibility

src/Delta.EF/Delta.EF.csproj

@@ -13,5 +13,17 @@
     <Compile Include="..\Delta\DeltaExtensions_Shared.cs">
       <Link>DeltaExtensions_Shared.cs</Link>
     </Compile>
+    <Compile Include="..\Delta/TimestampQueries/NpgsqlTimestampQueryExecutor.cs">
+      <Link>TimestampQueries/NpgsqlTimestampQueryExecutor.cs</Link>
+    </Compile>
+    <Compile Include="..\Delta/TimestampQueries/SqlServerTimestampQueryExecutor.cs">
+      <Link>TimestampQueries/SqlServerTimestampQueryExecutor.cs</Link>
+    </Compile>
+    <Compile Include="..\Delta/TimestampQueries/TimestampQueryExecutorFactory.cs">
+      <Link>TimestampQueries/TimestampQueryExecutorFactory.cs</Link>
+    </Compile>
+    <Compile Include="..\Delta/TimestampQueries/ITimestampQueryExecutor.cs">
+      <Link>TimestampQueries/ITimestampQueryExecutor.cs</Link>
+    </Compile>
   </ItemGroup>
 </Project>

src/Delta/DeltaExtensions_Sql.cs

@@ -1,5 +1,3 @@
-// ReSharper disable UseRawString
-
 namespace Delta;
 
 public static partial class DeltaExtensions
@@ -39,41 +37,7 @@ public static async Task<string> GetLastTimeStamp(this DbConnection connection,
 
     static async Task<string> ExecuteTimestampQuery(DbCommand command, Cancel cancel = default)
     {
-        var name = command.GetType().Name;
-        if (name == "SqlCommand")
-        {
-            command.CommandText = @"
--- begin-snippet: SqlServerTimestamp
-declare @changeTracking bigint = change_tracking_current_version();
-declare @timeStamp bigint = convert(bigint, @@dbts);
-
-if (@changeTracking is null)
-  select cast(@timeStamp as varchar)
-else
-  select cast(@timeStamp as varchar) + '-' + cast(@changeTracking as varchar)
--- end-snippet
-";
-            return (string) (await command.ExecuteScalarAsync(cancel))!;
-        }
-
-        if (name == "NpgsqlCommand")
-        {
-            command.CommandText = @"
--- begin-snippet: PostgresTimestamp
-select pg_last_committed_xact();
--- end-snippet
-";
-            var result = (object[]?) await command.ExecuteScalarAsync(cancel);
-            // null on first run after SET track_commit_timestamp to 'on'
-            if (result is null)
-            {
-                return string.Empty;
-            }
-
-            var xid = (uint) result[0];
-            return xid.ToString();
-        }
-
-        throw new("Unsupported type " + name);
+        var timestampQueryExecutor = TimestampQueryExecutorFactory.Create(command);
+        return await timestampQueryExecutor.Execute(command, cancel);
     }
 }

src/Delta/TimestampQueries/ITimestampQueryExecutor.cs

@@ -0,0 +1,6 @@
+namespace Delta;
+
+interface ITimestampQueryExecutor
+{
+    Task<string> Execute(DbCommand command, Cancel cancel);
+}

src/Delta/TimestampQueries/NpgsqlTimestampQueryExecutor.cs

@@ -0,0 +1,23 @@
+// ReSharper disable UseRawString
+
+namespace Delta;
+class NpgsqlTimestampQueryExecutor : ITimestampQueryExecutor
+{
+    public async Task<string> Execute(DbCommand command, Cancel cancel)
+    {
+        command.CommandText = @"
+        -- begin-snippet: PostgresTimestamp
+        select pg_last_committed_xact();
+        -- end-snippet
+        ";
+        var result = (object[]?)await command.ExecuteScalarAsync(cancel);
+        // null on first run after SET track_commit_timestamp to 'on'
+        if (result is null)
+        {
+            return string.Empty;
+        }
+
+        var xid = (uint)result[0];
+        return xid.ToString();
+    }
+}

src/Delta/TimestampQueries/SqlServerTimestampQueryExecutor.cs

@@ -0,0 +1,21 @@
+// ReSharper disable UseRawString
+
+namespace Delta;
+class SqlServerTimestampQueryExecutor : ITimestampQueryExecutor
+{
+    public async Task<string> Execute(DbCommand command, Cancel cancel)
+    {
+        command.CommandText = @"
+        -- begin-snippet: SqlServerTimestamp
+        declare @changeTracking bigint = change_tracking_current_version();
+        declare @timeStamp bigint = convert(bigint, @@dbts);
+
+        if (@changeTracking is null)
+          select cast(@timeStamp as varchar)
+        else
+          select cast(@timeStamp as varchar) + '-' + cast(@changeTracking as varchar)
+        -- end-snippet
+        ";
+        return (string)(await command.ExecuteScalarAsync(cancel))!;
+    }
+}

src/Delta/TimestampQueries/TimestampQueryExecutorFactory.cs

@@ -0,0 +1,12 @@
+namespace Delta;
+
+static class TimestampQueryExecutorFactory
+{
+    public static ITimestampQueryExecutor Create(DbCommand command) =>
+        command.GetType().Name switch
+        {
+            "SqlCommand" => new SqlServerTimestampQueryExecutor(),
+            "NpgsqlCommand" => new NpgsqlTimestampQueryExecutor(),
+            _ => throw new NotSupportedException("Unsupported type " + command.GetType().Name)
+        };
+}