如何在 C# 中 List<string> 类型的公共属性中添加值？
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How to add value in a public property of List<string> type in c#?
提问by sunk
private List<string> _Baseline = new List<string>();
public List<string> Baseline
{
get { return _Baseline; }
set { _Baseline = value; }
}
How can I set this property? It does not let me add using the add method; it throws an "object reference null" error.
如何设置此属性？它不允许我使用 add 方法添加；它会引发“对象引用为空”错误。
回答by Mehrdad Afshari
Do you initialize the class (using new) that holds this property before use?
您是否new在使用前初始化（使用）保存此属性的类？
There are two possible cases (assuming your code is in MyClassclass):
有两种可能的情况（假设您的代码在MyClass课堂上）：
//External code:
MyClass x = new MyClass();
x.Baseline = null; // Somewhere it'll be set to null.
x.Baseline.Add("Something"); // NullReferenceException
Or:
或者：
//External code:
MyClass x = null; // Somewhere the class itself is set to null.
x.Baseline.Add("Something"); // NullReferenceException
回答by Diego Jancic
It should work if you did what you wrote here. I guess you are using generics, and I can't see them in your post.
如果你做了你在这里写的东西，它应该可以工作。我猜您正在使用泛型，而我在您的帖子中看不到它们。
If you have a complex expression, split it. For example, change ObjectA.Prop.Other.Xyz.Add(..) to:
如果您有一个复杂的表达式，请将其拆分。例如，将 ObjectA.Prop.Other.Xyz.Add(..) 更改为：
SomeClass a = ObjectA.Prop;
SomeClass2 b = a.Other;
SomeClass3 c = b.Xyz;
c.Add(...)
this way you will find quickly where the null reference is.
这样你会很快找到空引用的位置。
回答by Nick
You might need to use IList:
您可能需要使用 IList：
private IList<string> _Baseline = new List<string>();
public IList<string> Baseline
{
get { return _Baseline; }
set { _Baseline = value; }
}
回答by Muad'Dib
There are 3 possibilities:
有3种可能：
- your
list<string>is null - the object containing your
list<string>is null - the item you are inserting is null
- 你
list<string>是空的 - 包含您的对象
list<string>为空 - 您插入的项目为空
1 is addressed when you assign a new List to it
1 在您为其分配新列表时得到解决
2 and 3 we can not ascertain from the code you post here.
2 和 3 我们无法从您在此处发布的代码中确定。
if you do not intend to allow assignment of a new list object outside of your class, then you do not, as noted elsewhere, need a setter. You can either remove it or declare it private or protected, like this....
如果你不打算允许在你的类之外分配一个新的列表对象，那么你不需要像其他地方提到的那样需要一个 setter。您可以将其删除或将其声明为私有或受保护，就像这样....
public List<string> Baseline
{
get { return _Baseline; }
protected set { _Baseline = value; }
}
回答by Amay Kulkarni
i got the same error since the declaration was not proper:
由于声明不正确，我遇到了同样的错误：
private List<string> _Baseline; - incorrect
private List<string> _Baseline = new List<string>(); - correct
