How to set up boundary condition for VectorL2FESpace

I tried to set up u=g on the boundary for Stokes problem. I defined the DG FEspace as follows.

When I run the code, there is a error at the following line

gfu.components[0].Set(g, BND, definedon=mesh.Boundaries("left|right|top|bottom"))

VectorL2FESpace does not have an evaluator for BND!


The code was attached.
Could you please show me how to set up the boundary condition for VectorL2FESpace or fix this error? Thank you so much.

l2 space doesnot have dofs on the bdry. you need to impose dirichlet bc weakly

I'm a newbie, I don't understand what you've said. Could you please tell me what 'weakly' meant here? Do you know any examples about imposing dirichlet bc weakly with Ngsolve? Thank you.