请求VB.NET中ReadObjects泛型方法的调用示例代码及说明

先定义示例实体类 #

假设你有一个对应数据库表的实体类，比如Customer：

Public Class Customer
    Public Property Id As Integer
    Public Property Name As String
    Public Property Email As String
End Class

调用方式1：使用Lambda表达式（匿名函数） #

直接在调用时编写转换逻辑，适合简单场景：

' 编写查询SQL
Dim sqlQuery As String = "SELECT Id, Name, Email FROM Customers"

' 调用ReadObjects，指定泛型类型为Customer，传入Lambda转换逻辑
Dim customers As IEnumerable(Of Customer) = ReadObjects(Of Customer)(sqlQuery, Function(reader)
    Return New Customer With {
        ' 通过GetOrdinal获取列索引，再读取对应类型的值
        .Id = reader.GetInt32(reader.GetOrdinal("Id")),
        .Name = reader.GetString(reader.GetOrdinal("Name")),
        ' 处理可能的DBNull值，避免空引用异常
        .Email = If(reader.IsDBNull(reader.GetOrdinal("Email")), Nothing, reader.GetString(reader.GetOrdinal("Email")))
    }
End Function)

' 遍历输出结果
For Each customer In customers
    Console.WriteLine($"ID: {customer.Id}, 姓名: {customer.Name}, 邮箱: {customer.Email}")
Next

调用方式2：使用单独的转换方法 #

如果转换逻辑复杂或需要复用，可以定义单独的转换方法，通过AddressOf传入：

' 定义通用的转换方法，将IDataReader映射为Customer对象
Private Shared Function MapToCustomer(reader As IDataReader) As Customer
    Return New Customer With {
        .Id = reader.GetInt32(reader.GetOrdinal("Id")),
        .Name = reader.GetString(reader.GetOrdinal("Name")),
        .Email = If(reader.IsDBNull(reader.GetOrdinal("Email")), Nothing, reader.GetString(reader.GetOrdinal("Email")))
    }
End Function

' 调用ReadObjects，传入转换方法的引用
Dim sqlQuery As String = "SELECT Id, Name, Email FROM Customers"
Dim customers As IEnumerable(Of Customer) = ReadObjects(Of Customer)(sqlQuery, AddressOf MapToCustomer)

关键注意点 #

• 必须明确指定泛型参数Of T（比如Of Customer），让方法知道要返回的集合类型
• 读取列值时，GetOrdinal("列名")比直接写索引数字更健壮，能避免数据库列顺序变化导致的错误
• 务必处理DBNull情况（比如允许为空的列），否则会抛出类型转换异常

内容的提问来源于stack exchange，提问作者Bern Bilazon