将枚举的所有描述绑定到组合框

Question

如何将枚举的格式化字符串描述绑定到组合框？我希望组合框显示说明，而不是枚举。

我有一个这样的枚举：

Public Enum Sandwiches
<ComponentModel.Description("Ham Sandwich")> HamSandwich
<ComponentModel.Description("Reuben")> Reuben
<ComponentModel.Description("Po’ Boy")> PoBoy
<ComponentModel.Description("Grilled Cheese")> GrilledCheese
End Enum

在描述字符串已绑定到组合框后，我可以使用此函数将所选项目转换回枚举：

        Public Function GetEnumFromDescriptionAttribute(Of T)(description As String) As T
        Dim type As Type = GetType(T)
        If Not type.IsEnum Then
            Throw New InvalidOperationException()
        End If
        For Each fi As Reflection.FieldInfo In type.GetFields()
            Dim descriptionAttribute As ComponentModel.DescriptionAttribute = TryCast(Attribute.GetCustomAttribute(fi, GetType(ComponentModel.DescriptionAttribute)), ComponentModel.DescriptionAttribute)
            If descriptionAttribute IsNot Nothing Then
                If descriptionAttribute.Description <> description Then
                    Continue For
                End If
                Return DirectCast(fi.GetValue(Nothing), T)
            End If
            If fi.Name <> description Then
                Continue For
            End If
            Return DirectCast(fi.GetValue(Nothing), T)
        Next
        Return Nothing
    End Function

但我找不到一种干净的方法将所有枚举绑定到组合框。我知道一个经常建议的解决方案是将枚举和枚举描述转换为字典，然后将它们设置为 cbo DisplayMember 和 ValueMember。但我不知道如何真正做到这一点。

我已经准备好了几十个关于如何做到这一点的部分解决方案；问题是它们都是用 C# 编写的，在 Option Strict 关闭的情况下使用隐式转换，并且没有显示完整的实现。因此，我无法将任何解决方案转换为 .NET，因为我不知道已定义的类型变量是什么。

Answer 1

这是我前段时间写的一个类，用于表示一个 Enum 值及其描述：

Imports System.ComponentModel
Imports System.Reflection
 
''' <summary>
''' Contains an enumerated constant value and a friendly description of that value, if one exists.
''' </summary>
''' <typeparam name="T">
''' The enumerated type of the value.
''' </typeparam>
Public Class EnumDescriptor(Of T)
 
    ''' <summary>
    ''' The friendly description of the value.
    ''' </summary>
    Private _description As String
    ''' <summary>
    ''' The enumerated constant value.
    ''' </summary>
    Private _value As T
 
    ''' <summary>
    ''' Gets the friendly description of the value.
    ''' </summary>
    ''' <value>
    ''' A <b>String</b> containing the value's Description attribute value if one exists; otherwise, the value name.
    ''' </value>
    Public ReadOnly Property Description() As String
        Get
            Return Me._description
        End Get
    End Property
 
    ''' <summary>
    ''' Gets the enumerated constant value.
    ''' </summary>
    ''' <value>
    ''' An enumerated constant of the <b>EnumDescriptor's</b> generic parameter type.
    ''' </value>
    Public ReadOnly Property Value() As T
        Get
            Return Me._value
        End Get
    End Property
 
    ''' <summary>
    ''' Creates a new instance of the <b>EnumDescriptor</b> class.
    ''' </summary>
    ''' <param name="value">
    ''' The value to provide a description for.
    ''' </param>
    Public Sub New(ByVal value As T)
        Me._value = value
 
        'Get the Description attribute.
        Dim field As FieldInfo = value.GetType().GetField(value.ToString())
        Dim attributes As DescriptionAttribute() = DirectCast(field.GetCustomAttributes(GetType(DescriptionAttribute), _
                                                                                        False),  _
                                                              DescriptionAttribute())
 
        'Use the Description attribte if one exists, otherwise use the value itself as the description.
        Me._description = If(attributes.Length = 0, _
                             value.ToString(), _
                             attributes(0).Description)
    End Sub
 
    ''' <summary>
    ''' Overridden.  Creates a string representation of the object.
    ''' </summary>
    ''' <returns>
    ''' The friendly description of the value.
    ''' </returns>
    Public Overrides Function ToString() As String
        Return Me.Description
    End Function
 
End Class

这是我刚刚写的一个简化版本：

Imports System.ComponentModel
Imports System.Reflection

Public Class EnumDescriptor(Of T)

    Public ReadOnly Property Description As String

    Public ReadOnly Property Value As T

    Public Sub New(value As T)
        Me.Value = value

        'Get the Description attribute.
        Dim field = value.GetType().GetField(value.ToString())
        Dim attributes = DirectCast(field.GetCustomAttributes(GetType(DescriptionAttribute), False), DescriptionAttribute())

        'Use the Description attribute if one exists, otherwise use the value itself as the description.
        Description = If(attributes.Length = 0, value.ToString(), attributes(0).Description)
    End Sub

    Public Overrides Function ToString() As String
        Return Description
    End Function

End Class

这是另一个类，可用于为 Enum 中的每个值存储前一个类的实例：

''' <summary>
''' A collection of EnumDescriptors for an enumerated type.
''' </summary>
''' <typeparam name="T">
''' The type of the enumeration for which the EnumDescriptors are created.
''' </typeparam>
Public Class EnumDescriptorCollection(Of T)
    Inherits ObjectModel.Collection(Of EnumDescriptor(Of T))
 
    ''' <summary>
    ''' Creates a new instance of the <b>EnumDescriptorCollection</b> class.
    ''' </summary>
    Public Sub New()
        'Populate the collection with an EnumDescriptor for each enumerated value.
        For Each value As T In [Enum].GetValues(GetType(T))
            Items.Add(New EnumDescriptor(Of T)(value))
        Next
    End Sub
 
End Class

以下是将集合类的实例绑定到 ComboBox 的一些示例用法：

Public Class Form1
 
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        With ComboBox1
            .DisplayMember = "Description"
            .ValueMember = "Value"
            .DataSource = New EnumDescriptorCollection(Of Sandwiches)
        End With
    End Sub
 
    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
        MessageBox.Show(ComboBox1.SelectedValue.ToString(), ComboBox1.Text)
    End Sub
 
End Class

Answer 2

您的目标似乎是覆盖 Enum 值的字符串表示形式。基本表示显示 Enum 的名称，而您希望显示 Enum 的描述属性。

假设这是一个 WinForm 的组合框，标准的方法是用自定义的 TypeConverter 装饰 Enum 并覆盖 ConvertTo 方法。使用 EnumConverter Class 和自定义转换器的基础可以最大限度地减少所需的代码。

Imports System.ComponentModel
Imports System.Globalization
Imports System.Reflection

Public Class EnumDescriptionConverter(Of T As {Structure, IConvertible}) : Inherits EnumConverter

  Private enumDescriptions As New Dictionary(Of T, String)

  Public Sub New()
    MyBase.New(GetType(T))
    ' Since the ability to add an Enum constraint to the generic type does not exist in VB.
    ' this check is to ensure that it is an Enum.
    ' Note that normally, throwing an exception in a constructor is considered bad form, but
    ' there is no other option
    If GetType(T).IsEnum Then
      LoadEnumDescriptions()
    Else
      Throw New ArgumentException($"{GetType(T).Name} is not an Enum")
    End If
  End Sub

  Private Sub LoadEnumDescriptions()
    ' An Enum type comprises static (Shared) fields that represent the defined enum values and
    ' an instance field that holds thae actual value. so only retrieve the static fields
    ' to get the defined (named) enum members.
    For Each fi As FieldInfo In GetType(T).GetFields(BindingFlags.Public Or BindingFlags.Static)
      Dim description As String
      Dim descAttrib As DescriptionAttribute = fi.GetCustomAttribute(Of DescriptionAttribute)
      If descAttrib Is Nothing Then
        ' no description attribute so use the defined name
        description = fi.Name
      Else
        description = descAttrib.Description
      End If
      enumDescriptions.Add(CType(fi.GetValue(Nothing), T), description)
    Next
  End Sub

  Public Overrides Function ConvertTo(context As ITypeDescriptorContext, culture As CultureInfo, value As Object, destinationType As Type) As Object
    Dim ret As Object

    ' the purpose of this converter is to provide an enum's description attribute as the string representation
    ' instead of the field name as provided by the standard implemention.
    If destinationType Is GetType(String) AndAlso value IsNot Nothing Then
      Dim enumValue As T = DirectCast(value, T)
      If enumDescriptions.ContainsKey(enumValue) Then
        ret = enumDescriptions(enumValue)
      Else
        ret = enumValue.ToString(Nothing)
      End If
    Else
      ret = MyBase.ConvertTo(context, culture, value, destinationType)
    End If
    Return ret
  End Function
End Class

修改您的 Enum 定义以使用此自定义转换器。

<TypeConverter(GetType(EnumDescriptionConverter(Of Sandwiches)))>
Public Enum Sandwiches
  <ComponentModel.Description("Ham Sandwich")> HamSandwich
  <ComponentModel.Description("Reuben")> Reuben
  <ComponentModel.Description("Po’ Boy")> PoBoy
  <ComponentModel.Description("Grilled Cheese")> GrilledCheese
End Enum

现在您需要做的就是使用枚举值加载 ComboBox.Items。

ComboBox1.Items.AddRange(System.Enum.GetValues(GetType(Sandwiches)).Cast(Of Object).ToArray())

使用 ComboBox.SelectedItem 检索选定的值。