A propper way to avoid duplication code, using "using" statement and oracle transactions?

I have a class (data access layer) that is used to make all database queries. I use Dapper as micro-ORM.

The "normal" way of doing such queries is :

string req = @"select ...";
using (var db = new OracleConnection(this.con_str))
{
    return db.Execute(req, { p1, p2});
}

Now, I have multiples methods in this class and i need to make mutliple insersions. If one insersion fail, I need to rollback everithing that has been done.

So, instead of using, i need to use an existing connection and transaction

IDbTransaction t;
IDbConnection c;
...
dynamic MyInsersion(int p1, int p2){
    string req = @"select ...";
    return c.Execute(req, new { p1, p2 }, transaction: t);
}

My probleme is that myInsersion need to use the existing connection ONLY if c is not null. If c is null, then just ignore it and create a new connection, without transaction. I need a short code because i have A LOT of methods like this. I tried something like this :

using (var db = connection ?? new OracleConnection(this.con_str))

If the connection is not null, I use it, if not, I create a new connection. The problem is that the "using" statement automatically call db.Dispose(), which call db.close()

So I have to create two almost identical code blocks

if(c != null && c.State == ConnectionState.Open)
{
    return c.Execute(req, new { p1, p2 }, transaction: t);
}
else
{
    return c.Execute(req, new { p1, p2 });
}

The only way to avoid this code duplication I found is to use a lambda :

Action<IDbConnection, IDbTransaction> A = (db, tr) =>
{
    db.Execute(
        req,
        new { p1, p2 ... },
        transaction: tr);
};
if (connection != null)
{
    A(connection, transaction);
    return;
}

using (var db = new OracleConnection(this.Con_str))
{
    A(db, null);
}

but it seems pretty heavy to me, I'd like to know if there was a more elegant way to do that.

Thanks you !

I hope I've been clear enough, tell me if not.