面试题：Visual Basic优化JSON解析性能及自定义扩展

Question

在一个包含大量JSON数据（假设每个JSON对象有多个层级嵌套且数据量达到数千条）的场景下，用Visual Basic实现高效的JSON解析。同时，请说明如何通过自定义扩展来提高解析特定格式JSON数据的灵活性和性能，例如对某些特定键值对的快速查找优化等。

Answer 1

使用Visual Basic实现高效JSON解析

1. 引入JSON库：在Visual Basic中，通常可以使用Newtonsoft.Json库（也称为Json.NET）来进行JSON解析。首先需要在项目中添加对该库的引用。如果使用的是NuGet包管理器，可以通过以下步骤添加：
   • 在Visual Studio中，右键点击项目，选择“管理NuGet程序包”。
   • 在NuGet包管理器中，搜索“Newtonsoft.Json”，然后点击“安装”。
2. 基本的JSON解析示例：假设JSON数据如下：

{
    "name": "John",
    "age": 30,
    "address": {
        "city": "New York",
        "country": "USA"
    }
}

以下是使用Newtonsoft.Json库解析上述JSON的代码：

Imports Newtonsoft.Json

Module Module1
    Sub Main()
        Dim json = "{ ""name"": ""John"", ""age"": 30, ""address"": { ""city"": ""New York"", ""country"": ""USA"" }}"
        Dim result = JsonConvert.DeserializeObject(Of Dictionary(Of String, Object))(json)
        Console.WriteLine("Name: " & result("name"))
        Console.WriteLine("Age: " & result("age"))
        Dim address = result("address")
        Console.WriteLine("City: " & (address("city")))
        Console.WriteLine("Country: " & (address("country")))
    End Sub
End Module

3. 处理多层级嵌套和大量数据：对于多层级嵌套和数千条数据，可以定义相应的类结构来进行更方便的解析。例如，假设JSON数据是一个包含多个人员信息的数组：

[
    {
        "name": "John",
        "age": 30,
        "address": {
            "city": "New York",
            "country": "USA"
        }
    },
    {
        "name": "Jane",
        "age": 25,
        "address": {
            "city": "Los Angeles",
            "country": "USA"
        }
    }
]

定义类：

Public Class Address
    Public Property city As String
    Public Property country As String
End Class

Public Class Person
    Public Property name As String
    Public Property age As Integer
    Public Property address As Address
End Class

解析代码：

Imports Newtonsoft.Json

Module Module1
    Sub Main()
        Dim json = "[{ ""name"": ""John"", ""age"": 30, ""address"": { ""city"": ""New York"", ""country"": ""USA"" }}, { ""name"": ""Jane"", ""age"": 25, ""address"": { ""city"": ""Los Angeles"", ""country"": ""USA"" }}]"
        Dim people = JsonConvert.DeserializeObject(Of List(Of Person))(json)
        For Each person In people
            Console.WriteLine("Name: " & person.name)
            Console.WriteLine("Age: " & person.age)
            Console.WriteLine("City: " & person.address.city)
            Console.WriteLine("Country: " & person.address.country)
        Next
    End Sub
End Module

通过自定义扩展提高解析特定格式JSON数据的灵活性和性能

1. 自定义扩展方法：可以通过定义扩展方法来优化特定键值对的查找。例如，假设我们经常需要根据“name”字段查找人员信息。

Imports System.Runtime.CompilerServices

Module PersonExtensions
    <Extension>
    Public Function FindPersonByName(people As List(Of Person), nameToFind As String) As Person
        For Each person In people
            If person.name = nameToFind Then
                Return person
            End If
        Next
        Return Nothing
    End Function
End Module

2. 使用扩展方法：

Imports Newtonsoft.Json

Module Module1
    Sub Main()
        Dim json = "[{ ""name"": ""John"", ""age"": 30, ""address"": { ""city"": ""New York"", ""country"": ""USA"" }}, { ""name"": ""Jane"", ""age"": 25, ""address"": { ""city"": ""Los Angeles"", ""country"": ""USA"" }}]"
        Dim people = JsonConvert.DeserializeObject(Of List(Of Person))(json)
        Dim john = people.FindPersonByName("John")
        If john IsNot Nothing Then
            Console.WriteLine("John's age: " & john.age)
        End If
    End Sub
End Module

3. 性能优化：对于大规模数据，为了进一步提高查找性能，可以考虑使用更高效的数据结构，如Dictionary(Of String, Person)。在解析JSON数据后，可以将数据填充到这个字典中，这样通过键查找就可以达到接近O(1)的时间复杂度。

Imports Newtonsoft.Json

Module Module1
    Sub Main()
        Dim json = "[{ ""name"": ""John"", ""age"": 30, ""address"": { ""city"": ""New York"", ""country"": ""USA"" }}, { ""name"": ""Jane"", ""age"": 25, ""address"": { ""city"": ""Los Angeles"", ""country"": ""USA"" }}]"
        Dim people = JsonConvert.DeserializeObject(Of List(Of Person))(json)
        Dim nameToPersonDict = New Dictionary(Of String, Person)()
        For Each person In people
            nameToPersonDict.Add(person.name, person)
        Next
        If nameToPersonDict.ContainsKey("John") Then
            Dim john = nameToPersonDict("John")
            Console.WriteLine("John's age: " & john.age)
        End If
    End Sub
End Module