what is the career prospects for Winform developers?

Probably not so good in the medium to long term.

The desktop is becoming increasingly marginalized by mobile and web applications and, whilst I think there will continue to be a need for bespoke desktop applications, I expect this need to diminish in future. 

Worse still, Windows Forms faces strong competition in its own backyard from WPF which looked at one time as though it would replace the former for programming .NET desktop applications. Whilst this hasn't happened - partly because of fears that WPF was going to be deprecated by Microsoft - it remains a more powerful and flexible rival even if the learning curve is much steeper.