函数返回具有参数中指定类型的对象 [英] Function returning an object with type specified in the argument

问题描述

如何创建一个函数，该函数返回特定类型的对象，该函数的用户在参数中指定该对象(使用 vb.net 2010)?

How can I create a function that returns an object of a certain type, which the user of the function specify in the argument (using 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.

The code above doesn't work, but maybe it can explain what I want to achieve.

使用此功能，我想从数据表中提取数据传输对象.客户端将调用此函数，指定客户端需要哪个 DTO，此函数将创建该 DTO 并使用反射 GetType.GetProperties() 填充属性.

Using this feature, I want to hydrate my data transfer objects from datatable. Client will just call this function, specifying which DTO the client wants, and this function will just create that DTO and populating the properties with reflection GetType.GetProperties().

推荐答案

这是一个使用泛型的非常基本的示例.方法 GetPropFromDatabase 只使用了一个 Select Case，但您显然会调用真正的数据库查找调用.

Here's a very basic example that uses generics. The method GetPropFromDatabase just uses a Select Case but instead of that you'd obviously put your real database lookup call.

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

根据对象的设置方式，您可能需要在 GetProperties() 上使用 BindingFlags.

You might have to play with BindingFlags on GetProperties() depending on how your object is setup.

编辑 2

您可能还想考虑使用自定义属性.例如，如果您的数据库中有一个名为 [First Name] 的列，由于空间原因，该列显然不能作为对象属性存在.使用自定义属性，您可以标记某些要以特殊方式忽略或解析的属性.下面的代码显示了上述代码的扩展版本.

You might also want to look into using Custom Attributes. For instance, if you've got a column in your database called [First Name] that obviously can't exist as an object property because of the space. With a custom attribute you can flag certain properties to be ignored or parsed in a special manner. The code below shows an extended version of the above code.

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

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