两个选项。

首先使用条件表达式执行相同的条件运算符行为：

First use a conditional expression doing the same conditional operator behavior:

return this.Children != null ? this.Children.Count(f=>f!=null) > 0 : false;

其次，保护您的Children属性不受null：

Second, protect your Children property against the null:

return this.Children.Count(f => f != null) > 0;

public ObservableCollection<DirectoryItemViewModel> Children {
 get
{
  return _Children ?? Enumerable.Empty<DirectoryItemViewModel>();
}
            
set
{
  _Children = value;                
}
}

Personaly，我更喜欢第二种，因为每次我需要Children属性时，我都不需要保护我的代码免受null 。

Personaly, I prefer the second because each time I need the Children property I don't need to protect my code against the null.

问候，