Question

c# dictionary 给某键值赋值之前是否一定要add此键值？

Answer 1

我使用，好像是要先Add的

来个判断

Dictionary<keyType, valueType> dict = new ....;

void SetKeyValue(key, value)
{
    if (!dict.Contain(key))
        dict.Add(key, value);
    else
        dict[key] = value;
}

追问

是么，但是我用的时候似乎不需要判断，.net 4.0。
到底有无官方的说法？

追答

MSDN没说明？

即使没官方的说法，即使.NET 4.0帮你实现，原理也是，.NET自己去看有没有包含那个KEY，没有就ADD，有就直接修改那个KEY对于的值

无论要不要你判断，实际程序肯定都要去判断，只是是你敲代码判断或者.NET已经有代码帮你判断了，无论如何，肯定要判断的

程序也不会因为此判断就快或者慢多少纳秒毫秒

官方有说明的啊，我看的是 Add 方法中的Remarks

Remarks

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Dictionary; for example,myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) = myValue).
However, if the specified key already exists in the Dictionary, setting the Item
property overwrites the old value. In contrast, the Add method throws an exception if a value with the specified key already exists.

追问

追答

Dictionary Add Method

Remarks
--------------------------------------------------------------------------------

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Dictionary; for example,myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) = myValue). However, if the specified key already exists in the Dictionary, setting the Item property overwrites the old value. In contrast, the Add method throws an exception if a value with the specified key already exists.

你没看？
虽然是 Add Method，但是说了 myCollection[myKey] = myValue 的情况当myKey不存在时

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Dictionary; for example,myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) = myValue).

追问

这么说判断键是否存在就是不必要的了吧。上面那段是说值重复会报错。

追答

说了会报错是后半部分
你的问题是需要看前半段

可以不用判断，官方都这么说了