public override void VisitNullReferenceExpression(NullReferenceExpression e)
{
var call = e.Parent as InvocationExpression;
if (call != null)
{
// 解决 null 参数引起的调用不明问题：???? 如何判断有必要
var ti = e.Annotation<TypeInformation>();
e.ReplaceWith(e.Clone().CastTo(AstBuilder.ConvertType(ti.ExpectedType)));
}
base.VisitNullReferenceExpression(e);
}
