C# Dictionary.Add 与 Dictionary[key]=value 的区别

Difference of Dictionary.Add vs Dictionary[key]=value

提问by rsg

What is the difference between the Dictionary.Addmethod and the indexer Dictionary[key] = value?

Dictionary.Add方法和索引器有Dictionary[key] = value什么区别？

采纳答案by Habib

Add-> Adds an item to the dictionary if item already exists in the dictionary an exception will be thrown.

添加-> 如果项目已存在于字典中，则将项目添加到字典中，将引发异常。

Indexer or Dictionary[Key]=> Add Or Update. If the key doesn't exist in the dictionary, a new item will be added. If the key exists then the value will be updated with the new value.

索引器或Dictionary[Key]=> 添加或更新。如果字典中不存在该键，则会添加一个新项目。如果键存在，则该值将使用新值更新。

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dictionary.addwill add a new item to the dictionary, dictionary[key]=valuewill set a value to an existing entry in the dictionary against a key. If the key is not present then it (indexer)will add the item in the dictionary.

dictionary.add将向字典添加新项目，dictionary[key]=value将值设置为字典中现有条目的键值。如果键不存在，那么它（索引器）将在字典中添加该项目。

Dictionary<string, string> dict = new Dictionary<string, string>();
dict.Add("Test", "Value1");
dict["OtherKey"] = "Value2"; //Adds a new element in dictionary 
Console.Write(dict["OtherKey"]);
dict["OtherKey"] = "New Value"; // Modify the value of existing element to new value
Console.Write(dict["OtherKey"]);

In the above example, in first place dict["OtherKey"] = "Value2";will add a new value in the dictionary because it doesn't exist, and in second place it will modify the value to New Value.

在上面的例子中，首先dict["OtherKey"] = "Value2";将在字典中添加一个新值，因为它不存在，其次将值修改为新值。

回答by Jeeva

dictionary.addadd an item to the dictionary whereas dictionary[key]=valueassigns a value to an already existing key.

dictionary.add将一个项目添加到字典中，同时dictionary[key]=value为一个已经存在的键分配一个值。

回答by xanatos

Dictionary.Addthrows an exception if the key is already present. []when used for setting an item doesn't (it does if you try to access it for read).

Dictionary.Add如果键已经存在，则抛出异常。[]用于设置项目时不会（如果您尝试访问它以进行读取，则不会）。

x.Add(key, value); // will throw if key already exists or key is null
x[key] = value; // will throw only if key is null
var y = x[key]; // will throw if key doesn't exists or key is null

回答by cdhowie

The behavior is identical when the key does not exist in the dictionary: the item will be added in both cases.

当字典中不存在键时，行为是相同的：在这两种情况下都将添加该项目。

The behavior differs when the key already exists. dictionary[key] = valuewill update the value mapped to the key, while dictionary.Add(key, value)will instead throw an ArgumentException.

当密钥已经存在时，行为会有所不同。 dictionary[key] = value将更新映射到键的值，而dictionary.Add(key, value)将抛出 ArgumentException。

回答by Jon Skeet

The documentation for Addmakes this very clear, I feel:

的文档Add对此非常清楚，我觉得：

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

您还可以使用该Item属性通过设置 中不存在的键的值来添加新元素Dictionary(Of TKey, TValue)；例如，myCollection[myKey] = myValue（在 Visual Basic 中，myCollection(myKey) = myValue）。但是，如果指定的键已存在于 中Dictionary(Of TKey, TValue)，则设置 Item 属性会覆盖旧值。相反，Add如果具有指定键的值已存在，则该方法将引发异常。

(Note that the Itemproperty corresponds to the indexer.)

（请注意，该Item属性对应于索引器。）

It's always worth consulting the documentation before asking a question...

在提出问题之前总是值得参考文档......