使用以下代码：
Function GetSetting(Of T)(ByVal SettingName As String,ByRef DefaultVal As T) As T
Return If(Configuration.ContainsKey(SettingName),CType(Configuration(SettingName),T),DefaultVal)
End Function
产生以下错误：
Value of type 'String' cannot be converted to 'T'.
无论如何,我可以指定在所有情况下,转换确实是可能的(我基本上得到整数,布尔值,双精度和字符串).
编辑：现在似乎有三种解决方案：
>使用AMissico提供的`ValueAs`功能
>转换为`object`,然后转换为`T`,并检查空值
>在Convert.ChangeType上使用`DirectCast`
你会建议哪个？
编辑2：
这段代码有用吗？
Function GetSetting(Of T)(ByVal SettingName As String,Optional ByRef DefaultVal As T = Nothing) As T
Return If(Configuration.ContainsKey(SettingName),ConvertTo(Of T)(Configuration(SettingName)),DefaultVal)
End Function
Function ConvertTo(Of T)(ByVal Str As String) As T
Return If(Str Is Nothing Or Str = "",Nothing,CType(CObj(Str),T))
End Function
编辑3：[AMJ]工作代码
Function GetSetting(Of T)(ByVal SettingName As String) As T
Return GetSetting(Of T)(SettingName,Nothing)
End Function
Function GetSetting(Of T)(ByVal SettingName As String,ByVal DefaultVal As T) As T
Dim sValue As String = Configuration(SettingName)
If Len(sValue) = 0 Then
Return DefaultVal
Else
Return CType(CObj(sValue),T)
End If
End Function
Public Sub DoIt()
Me.Configuration.Add("KeyN",Nothing)
Me.Configuration.Add("KeyE",String.Empty) '""
Me.Configuration.Add("Key1","99")
Me.Configuration.Add("Key2","1/1/2000")
Me.Configuration.Add("Key3","True")
Me.Configuration.Add("Key4","0")
Dim o As Object 'using object in order to see what type is returned by methods
o = Value(Of Integer)("KeyN",10) '10
o = Value(Of Integer)("KeyE",10) '10
o = Value(Of Integer)("Key1",10) '99
o = Value(Of Date)("KeyN",#11/11/2010#)
o = Value(Of Date)("KeyE",#11/11/2010#)
o = Value(Of Date)("Key2",#11/11/2010#)
o = GetSetting(Of Integer)("KeyN",10) '10
o = GetSetting(Of Integer)("KeyE",10) '10
o = GetSetting(Of Integer)("Key1",10) '99
o = GetSetting(Of Date)("KeyN",#11/11/2010#)
o = GetSetting(Of Date)("KeyE",#11/11/2010#)
o = GetSetting(Of Date)("Key2",#11/11/2010#)
Stop
End Sub
Value(Of T)和ValueAs方法支持可空类型.我使用Microsoft .NET 2.0源代码作为参考.
原文链接：https://www.f2er.com/vb/255480.html这是经过充分测试和生产就绪的代码.
这些“库”函数中没有错误处理.调用者有责任处理发生的任何转换错误.生成的唯一转换错误是明显的错误,例如尝试将字符串“abc”转换为Integer.
Public Sub DoIt()
Dim o As Object
o = Value(Of Integer)("foo",10)
o = Value(Of DateTime)("xxx",#1/1/2000#)
o = Value(Of Boolean?)("nop",True)
Stop
End Sub
Public Function GatherTag(ByVal tag As String) As String
If tag = "foo" Then
Return "99"
Else
Return String.Empty
End If
End Function
''' <summary>
''' Provides strongly-typed access to the tag values. The method also supports nullable types.
''' </summary>
''' <typeparam name="T">A generic parameter that specifies the return type.</typeparam>
''' <param name="tag">The ExifTool Tag Name,</param>
''' <returns>The value,of type T,of the tag.</returns>
Public Function Value(Of T)(ByVal tag As String,ByVal defaultValue As T) As T
Return DirectCast(ValueAs(GetType(T),tag,defaultValue),T)
End Function
''' <summary>
''' Returns the tag's value as the specified type. The method also supports nullable types.
''' </summary>
''' <param name="type">The type to return the tag value as.</param>
''' <param name="tag">The ExifTool Tag Name,</param>
''' <returns>The value of the tag as the type requested.</returns>
Public Function ValueAs(ByVal type As System.Type,ByVal tag As String,ByVal defaultValue As Object) As Object
Dim oResult As Object = Nothing
Dim oTag As String = GatherTag(tag)
If Len(oTag) = 0 Then
'use specified default value
oResult = defaultValue
Else
'is requested type a generic type?
If type.IsGenericType AndAlso type.GetGenericTypeDefinition Is GetType(Nullable(Of )) Then
Dim oUnderlyingType As Type = Nullable.GetUnderlyingType(type)
Dim oConstructed As Type = type.GetGenericTypeDefinition.MakeGenericType(oUnderlyingType)
Dim oValue As Object
oValue = System.Convert.ChangeType(oTag,oUnderlyingType)
If oValue IsNot Nothing Then
oResult = Activator.CreateInstance(oConstructed,oValue)
End If
Else
'non-generic type
oResult = System.Convert.ChangeType(oTag,type)
End If
End If
Return oResult
End Function