c# 问号

 

int m=3;
public void xiha(int c,int a)
{
     m=c+a;
}

xiha(1,2);

 

 

想要这样

xiha(1,null);

就用到?

public void xiha(int c,int? a)
{
     m=c+a;
}

m=c+null;