C# 对象引用未设置为对象的实例

Question

我有以下代码:

public void SetUser(User user)
{
   string streetNumber = "";

   if (user.Address.StreetNo != null)
      streetNumber = user.Address.StreetNo.ToString();
   else
     streetNumber = "";
}

我得到了永远流行

Object reference not set to an instance of an object.

问题。

Answer 1

public void SetUser(User user)
{
   string streetNumber = "";

   if (user != null && user.Address != null && user.Address.StreetNo != null) {
      streetNumber = user.Address.StreetNo.ToString();
   }
}

考虑到@CKoenig 的建议，如果 user 或 user.Address 为 null，则以下抛出异常:

public void SetUser(User user)
{

   if (user == null) {
       throw new System.ArgumentNullException("user", "user cannot be null");
   }

   if (user.Address == null) {
       throw new System.ArgumentNullException("Address", "Address cannot be null");
   }
   
   string streetNumber = "";

   if (user.Address.StreetNo != null) {
      streetNumber = user.Address.StreetNo.ToString();
   }
}