Allow changing an open connection if it's not owned

Fixes dotnet#30704

src/EFCore.Relational/Storage/RelationalConnection.cs

@@ -181,7 +181,7 @@ public virtual void SetDbConnection(DbConnection? value, bool contextOwnsConnect
     {
         if (!ReferenceEquals(_connection, value))
         {
-            if (_openedCount > 0)
+            if (_connectionOwned && _openedCount > 0)
             {
                 throw new InvalidOperationException(RelationalStrings.CannotChangeWhenOpen);
             }

test/EFCore.Relational.Tests/RelationalConnectionTest.cs

@@ -1012,6 +1012,48 @@ public void Throws_when_rollback_is_called_without_active_transaction()
                 () => connection.RollbackTransaction()).Message);
     }
 
+    [ConditionalFact]
+    public void Throws_when_changing_DbConnection_if_current_is_open_and_owned()
+    {
+        using var connection = new FakeRelationalConnection(
+            CreateOptions(new FakeRelationalOptionsExtension().WithConnectionString("Database=FrodoLives")));
+        Assert.Equal(0, connection.DbConnections.Count);
+
+        connection.Open();
+
+        Assert.Throws<InvalidOperationException>(() => connection.DbConnection = new FakeDbConnection("Fake"));
+    }
+
+    [ConditionalFact]
+    public void Disposes_when_changing_DbConnection_if_current_is_owned_and_not_open()
+    {
+        using var connection = new FakeRelationalConnection(
+            CreateOptions(new FakeRelationalOptionsExtension().WithConnectionString("Database=FrodoLives")));
+        Assert.Equal(0, connection.DbConnections.Count);
+
+        var dbConnection = connection.DbConnection;
+
+        Assert.Raises<EventArgs>(
+            h => dbConnection.Disposed += h.Invoke,
+            h => dbConnection.Disposed -= h.Invoke,
+            () => connection.DbConnection = new FakeDbConnection("Fake"));
+    }
+
+    [ConditionalFact]
+    public void Does_not_dispose_when_changing_DbConnection_if_current_is_open_and_not_owned()
+    {
+        using var connection = new FakeRelationalConnection();
+        Assert.Equal(0, connection.DbConnections.Count);
+
+        var dbConnection = new FakeDbConnection("Database=FrodoLives");
+        connection.DbConnection = dbConnection;
+        connection.Open();
+
+        connection.DbConnection = new FakeDbConnection("Database=FrodoLives");
+
+        Assert.Equal(ConnectionState.Open, dbConnection.State);
+    }
+
     private static IDbContextOptions CreateOptions(params RelationalOptionsExtension[] optionsExtensions)
     {
         var optionsBuilder = new DbContextOptionsBuilder();