vb.net - 将泛型类型对象传递给受约束的泛型方法 vb.net

Question

我有以下函数(删除了 try-catch):

    Friend Shared Function ConvertOrDefault(Of T As {Structure, IConvertible})(convertFrom As Object, ignoreCase As Boolean) As T
        Dim retVal As T
        If Not GetType(T).IsEnum Then
            Throw New ArgumentException("Type must be enum")
        ElseIf convertFrom Is Nothing OrElse Not TypeOf convertFrom Is String Then
            Return New T
        ElseIf [Enum].TryParse(convertFrom.ToString(), ignoreCase, retVal) Then
            Return retVal
        Else
            Return New T
        End If
End Function

它将给定类型转换为枚举(因此有约束)(如果它是一个)。

那很好，但是我有另一个方法(下面简化)可以进行更一般的转换，并且如果传入的类型是枚举，我希望它使用该方法:

Friend Shared Function Convert(Of T)(value as Object) As T
    If GetType(T).IsEnum Then
         Return Enums.ConvertOrDefault(Of T)(value, True)
    Else : return DirectCast(value, T)
    End If
End Function

对于 Enums.ConvertOrDefault 的调用，会出现错误:

Type argument 'T' does not inherit from or implement the constraint type 'System.IConvertible'
Type argument 'T' does not satisfy the 'Structure' constraint for type parameter 'T'

我怎么能说“没关系，我知道它是一个枚举，所以没关系”？

--- 编辑 ---

一种(非常丑陋)的方法如下:

Dim type As Type = GetType(T)

If type.IsEnum Then
    Select Case type.Name
        Case "EnumTypeOne"
            Return DirectCast(DirectCast(Enums.ConvertOrDefault(Of EnumTypeOne)(value, True), Object), T)
         ' ...

但这太可怕了。当然有一种方法可以概括这一点吗？

-- 编辑 2:目的 --

我正在从 Oracle 数据库读取数据，该数据库将Enums(我有几个)存储为字符串；以及以各种格式存储其他数据(Byte() 作为 RAW、TimeSpan 作为 IntervalDS 等) 。然后，我使用 Convert 函数作为通用函数，根据 datareader(column) 的结果，我可以将该对象转换为适当的类型。

所有Oracle.DataAccess.Client.OracleDataReader 的.Get... 函数都采用索引而不是列名；由于我无法保证列的顺序，并且为了可读性，使用列名称更有意义 - 但随后我必须自己解析输出。

所以我的代码正在做类似的事情:

Dim id as Byte() = Convert(dataReader("id_column"))
Dim something as SomeEnum = Convert(dataReader("somethingCol"))
'...

当我期待 Enum 时，我可以故意调用 Enum.ConvertOrDefault 而不是 Convert，但这似乎违反了一个通用方法，我认为这更有意义......并且还允许我在其他上下文中重用该方法。

希望这有助于澄清一点。

--- 编辑 3 ---

我尝试了这个想法，来自评论:

friend 共享函数 Convert(Of T As {New})(value as Object) as T

和

Friend Shared Function ConvertOrDefault(Of T As{New}) convertFrom As Object, ignoreCase As Boolean) As T
    If Not GetType(T).IsEnum Then
        Throw New ArgumentException("Type must be enum")
    ElseIf convertFrom Is Nothing OrElse Not TypeOf convertFrom Is String Then
        Return New T
    End If
    Try
        Return CType([Enum].Parse(GetType(T), convertFrom.ToString(), ignoreCase), T)
    Catch ex As Exception
    End Try

    ' default
    Return New T
End Function

但是当我调用 String 或 Byte() 等类型的 Convert 方法时，会出现错误，说

"Type argument 'String' must have a public parameterless instance constructor to satisfy the 'New' constraint for type parameter 'T'

Answer 1

您可能需要考虑使用不同类型的枚举值集。您也许可以改用 polymorphic/class/subclassable 枚举模式。

我使用的方法通常有一个 TrySelect 方法，用于将基础值解析为枚举。此外，您可以为每个枚举值支持不同类型的多个基础值。例如:

public class BoolEnum
{
    private static Dictionary<bool, BoolEnum>   allValuesByNaturalValue = new Dictionary<bool, BoolEnum>();
    private static Dictionary<string, BoolEnum> allValuesByTextValue    = new Dictionary<string, BoolEnum>();
    private static Dictionary<int, BoolEnum>    allValuesByInteger      = new Dictionary<int, BoolEnum>();

    private string boolText;
    private int    integerValue;
    private bool   naturalValue;

    public static readonly BoolEnum True  = new BoolEnum(true, "Is True", 1);
    public static readonly BoolEnum False = new BoolEnum(false, "Is False", 0);

    private BoolEnum(bool naturalValue, string boolText, int integerValue)
    {
        this.naturalValue = naturalValue;
        this.boolText     = boolText;
        this.integerValue = integerValue;
        allValuesByNaturalValue.Add(naturalValue, this);
        allValuesByTextValue.Add(boolText, this);
        allValuesByInteger.Add(integerValue, this);
    }

    public static BoolEnum TrySelect(bool naturalValue, BoolEnum defaultValue)
    {
        BoolEnum returnValue;
        if (allValuesByNaturalValue.TryGetValue(naturalValue, out returnValue)) return returnValue;
        return defaultValue;
    }

    public static BoolEnum TrySelect(string boolText, BoolEnum defaultValue)
    {
        BoolEnum returnValue;
        if (allValuesByTextValue.TryGetValue(boolText, out returnValue)) return returnValue;
        return defaultValue;
    }

    public static BoolEnum TrySelect(int integerValue, BoolEnum defaultValue)
    {
        BoolEnum returnValue;
        if (allValuesByInteger.TryGetValue(integerValue, out returnValue)) return returnValue;
        return defaultValue;
    }

    public static implicit operator bool(BoolEnum boolEnum)
    {
        return boolEnum != null ? boolEnum.naturalValue : false;
    }

    public static implicit operator string(BoolEnum boolEnum)
    {
        return boolEnum != null ? boolEnum.boolText : "Is False";
    }

    public static implicit operator int(BoolEnum boolEnum)
    {
        return boolEnum != null ? boolEnum.integerValue : 0;
    }

    public bool   NaturalValue { get { return this.naturalValue; } }
    public string BoolText     { get { return this.boolText; } }
    public int    IntegerValue { get { return this.integerValue; } }

    public static IReadOnlyCollection<BoolEnum> AllValues        { get { return allValuesByNaturalValue.Values.ToList().AsReadOnly(); } }
    public static IReadOnlyCollection<bool>     AllBooleanValues { get { return allValuesByNaturalValue.Keys.ToList().AsReadOnly(); } }
    public static IReadOnlyCollection<string>   AllTextValues    { get { return allValuesByTextValue.Keys.ToList().AsReadOnly(); } }
    public static IReadOnlyCollection<int>      AllIntegerValues { get { return allValuesByInteger.Keys.ToList().AsReadOnly(); } }

    public override string ToString()
    {
        return "[" + this.naturalValue.ToString() + ", \"" + this.boolText.ToString() + "\", " + this.integerValue.ToString() + "]";
    }

}

然后您可以向枚举添加方法以进行更专业的操作。您可以使用枚举构建映射，将列映射到索引位置等。您还可以使用 All* 属性 (BoolEnum. AllValues、BoolEnum.AllBooleanValues、BoolEnum.AllTextValues、BoolEnum.AllIntegerValues)。

仅供引用> 使用泛型实现这一点并不困难，因此大部分样板文件都被干掉了。 subclassable 示例(免责声明:这是我关于此的文章)基于通用基枚举类的使用。

这是一个 dotnetfiddle，显示了上面示例枚举的实际效果:https://dotnetfiddle.net/O5YY47

这是上述内容的 VB.Net 版本:

Public Class BoolEnum
    Private Shared allValuesByNaturalValue As New Dictionary(Of Boolean, BoolEnum)()
    Private Shared allValuesByTextValue As New Dictionary(Of String, BoolEnum)()
    Private Shared allValuesByInteger As New Dictionary(Of Integer, BoolEnum)()

    Private m_boolText As String
    Private m_integerValue As Integer
    Private m_naturalValue As Boolean

    Public Shared ReadOnly [True] As New BoolEnum(True, "Is True", 1)
    Public Shared ReadOnly [False] As New BoolEnum(False, "Is False", 0)

    Private Sub New(naturalValue As Boolean, boolText As String, integerValue As Integer)
        Me.m_naturalValue = naturalValue
        Me.m_boolText = boolText
        Me.m_integerValue = integerValue
        allValuesByNaturalValue.Add(naturalValue, Me)
        allValuesByTextValue.Add(boolText, Me)
        allValuesByInteger.Add(integerValue, Me)
    End Sub

    Public Shared Function TrySelect(naturalValue As Boolean, defaultValue As BoolEnum) As BoolEnum
        Dim returnValue As BoolEnum
        If allValuesByNaturalValue.TryGetValue(naturalValue, returnValue) Then
            Return returnValue
        End If
        Return defaultValue
    End Function

    Public Shared Function TrySelect(boolText As String, defaultValue As BoolEnum) As BoolEnum
        Dim returnValue As BoolEnum
        If allValuesByTextValue.TryGetValue(boolText, returnValue) Then
            Return returnValue
        End If
        Return defaultValue
    End Function

    Public Shared Function TrySelect(integerValue As Integer, defaultValue As BoolEnum) As BoolEnum
        Dim returnValue As BoolEnum
        If allValuesByInteger.TryGetValue(integerValue, returnValue) Then
            Return returnValue
        End If
        Return defaultValue
    End Function

    Public Shared Widening Operator CType(boolEnum As BoolEnum) As Boolean
        Return If(boolEnum IsNot Nothing, boolEnum.naturalValue, False)
    End Operator

    Public Shared Widening Operator CType(boolEnum As BoolEnum) As String
        Return If(boolEnum IsNot Nothing, boolEnum.boolText, "Is False")
    End Operator

    Public Shared Widening Operator CType(boolEnum As BoolEnum) As Integer
        Return If(boolEnum IsNot Nothing, boolEnum.integerValue, 0)
    End Operator

    Public ReadOnly Property NaturalValue() As Boolean
        Get
            Return Me.m_naturalValue
        End Get
    End Property
    Public ReadOnly Property BoolText() As String
        Get
            Return Me.m_boolText
        End Get
    End Property
    Public ReadOnly Property IntegerValue() As Integer
        Get
            Return Me.m_integerValue
        End Get
    End Property

    Public Shared ReadOnly Property AllValues() As IReadOnlyCollection(Of BoolEnum)
        Get
            Return allValuesByNaturalValue.Values.ToList().AsReadOnly()
        End Get
    End Property
    Public Shared ReadOnly Property AllBooleanValues() As IReadOnlyCollection(Of Boolean)
        Get
            Return allValuesByNaturalValue.Keys.ToList().AsReadOnly()
        End Get
    End Property
    Public Shared ReadOnly Property AllTextValues() As IReadOnlyCollection(Of String)
        Get
            Return allValuesByTextValue.Keys.ToList().AsReadOnly()
        End Get
    End Property
    Public Shared ReadOnly Property AllIntegerValues() As IReadOnlyCollection(Of Integer)
        Get
            Return allValuesByInteger.Keys.ToList().AsReadOnly()
        End Get
    End Property

    Public Overrides Function ToString() As String
        Return "[" + Me.m_naturalValue.ToString() + ", """ + Me.m_boolText.ToString() + """, " + Me.m_integerValue.ToString() + "]"
    End Function

End Class

这是 VB.Net 版本的 dotnetfiddle:https://dotnetfiddle.net/HeCA5r