不是100％不确定这是否是问题-但是 PrincipalSearcher 确实返回了 Principal 对象。

Not 100% sure if that's the problem - but PrincipalSearcher really returns a list of Principal objects.

因此，如果-由于某种原因-搜索者返回的内容不是 UserPrincipal ，那么您的对象结果将为空-不是 .DisplayName 属性。

So if - for whatever reason - your searcher would return something that's not a UserPrincipal, then your object result would be null - not it's .DisplayName property.

因此，您应该将支票更改为：

So you should change your checking to:

foreach (UserPrincipal result in search.FindAll())
{
    if (result != null && result.DisplayName != null)
    {
        comboBox2.Items.Add(result.DisplayName);
    }
}