C# create default session

Hi i use C# driver for mongodb .
How can i create default session to avoid writing an ‘if else’ condition ?

What now

if(!session)
   return _x.FindOneAndUpdate<x>(c => c.UserId == userId && c.f== f, u, new 
    FindOneAndUpdateOptions<x>
        {
            IsUpsert = true,
            ReturnDocument = ReturnDocument.After 
        });
else 

    return _x.FindOneAndUpdate<x>(session, c => c.UserId == userId && c.f== f, u, new FindOneAndUpdateOptions<x>
        {
            IsUpsert = true,
            ReturnDocument = ReturnDocument.After 
        });

for example so
_x.FindOneAndUpdate<x>(session ?? Session.Default(),