VB.NET中如何编写一个函数来返回特定类型的参数对象？

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创建一个返回特定类型对象的函数，用户可以在参数中指定类型。以下是一个使用VB.NET 2010编写的示例代码：

vbPrivate Function TryThis(ByVal t As Type) As Object Dim n As Object=New t() Return nEnd Function

这段代码定义了一个名为 `TryThis` 的私有函数，它接受一个 `Type` 类型的参数 `t`，并返回一个新创建的 `t` 类型的对象。函数内部使用 `New t()` 来实例化对象。

如何创建一个返回特定类型对象的函数,该函数的用户在参数中指定(使用vb.net 2010)？

Private Function TryThis(ByVal t As Type) As Object Dim n = New t Return n End Function

上面的代码不起作用,但也许它可以解释我想要实现的目标.

使用此功能,我想从数据表中保存数据传输对象.客户端只需调用此函数,指定客户端需要哪个DTO,此函数将只创建该DTO并使用反射GetType.GetProperties()填充属性.

这是一个使用泛型的非常基本的例子.方法GetPropFromDatabase只使用一个Select Case,而不是你明显地进行真正的数据库查找调用.

Option Explicit On Option Strict On Public Class Form1 Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim X = CreateObject(Of TestClass)() Trace.WriteLine(X.PropertyA) Trace.WriteLine(X.PropertyB) Me.Close() End Sub ''//Of T as New means that the object must have a constructor Private Function CreateObject(Of T As New)() As T ''//Create our object Dim O As New T ''//Get the type properties Dim Props = GetType(T).GetProperties() ''//Loop through each property For Each P In Props ''//Set the value of our return type by property name P.SetValue(O, GetPropFromDatabase(P.Name), Nothing) Next ''//Return our object Return O End Function ''//This function would obviously do a lot more Private Shared Function GetPropFromDatabase(ByVal name As String) As String Select Case name Case "PropertyA" Return "Value1" Case "PropertyB" Return "Value2" End Select Throw New ApplicationException(String.Format("Unknown database column : {0}", name)) End Function End Class Public Class TestClass Public Property PropertyA As String Public Property PropertyB As String End Class

编辑

您可能必须在GetProperties()上使用BindingFlags,具体取决于对象的设置方式.

编辑2

您可能还想查看使用自定义属性.例如,如果您的数据库中有一个名为[First Name]的列,由于空间的原因,它显然不能作为对象属性存在.使用自定义属性,您可以标记某些属性,以便以特殊方式忽略或解析.下面的代码显示了上述代码的扩展版本.

Option Explicit On Option Strict On Public Class Form1 Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim X = CreateObject(Of TestClass)() Trace.WriteLine(X.PropertyA) Trace.WriteLine(X.PropertyB) Trace.WriteLine(X.FirstName) Me.Close() End Sub ''//Of T as New means that the object must have a constructor Private Function CreateObject(Of T As New)() As T ''//Create our object Dim O As New T ''//Get the type properties Dim Props = GetType(T).GetProperties() ''//Will hold the name of the database column to get the value of Dim PropName As String ''//Will hold our collection of attributes on the property Dim CustomAttributes() As Object ''//Loop through each property For Each P In Props ''//Default the value to the property name PropName = P.Name ''//Try to get any custom attributes for the property CustomAttributes = P.GetCustomAttributes(True) ''//See if we have anything to work with If (CustomAttributes IsNot Nothing) AndAlso (CustomAttributes.Count > 0) Then ''//Loop through each attribute For Each A In CustomAttributes ''//If the attribute is our custom one defined below If TypeOf A Is ColumnNameInDatabase Then ''//Use the manually set column name instead PropName = DirectCast(A, ColumnNameInDatabase).ColumnNameInDatabase ''//No reason to loop through any more attributes so exit Exit For End If Next End If ''//Set the value of our return type by property name P.SetValue(O, GetPropFromDatabase(PropName), Nothing) Next ''//Return our object Return O End Function ''//This function would obviously do a lot more Private Shared Function GetPropFromDatabase(ByVal name As String) As String Select Case name Case "PropertyA" Return "Value1" Case "PropertyB" Return "Value2" Case "First Name" Return "Bob Dole" End Select Throw New ApplicationException(String.Format("Unknown database column : {0}", name)) End Function End Class Public Class TestClass Public Property PropertyA As String Public Property PropertyB As String <ColumnNameInDatabase("First Name")> Public Property FirstName As String End Class Public Class ColumnNameInDatabase Inherits Attribute Private _ColumnNameInDatabase As String Public ReadOnly Property ColumnNameInDatabase As String Get Return Me._ColumnNameInDatabase End Get End Property Public Sub New(ByVal columnNameInDatabase As String) Me._ColumnNameInDatabase = columnNameInDatabase End Sub End Class