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'.
任何方式我都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:
你会建议哪个?
编辑2: 这段代码会起作用吗?
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
快速测试方法
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
答案 0 :(得分:4)
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。
这是经过充分测试和生产就绪的代码。
这些“库”功能中没有错误处理。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
答案 1 :(得分:1)
这需要一些hocus-pocus:
Public Function GetSetting(Of T As IConvertible)(ByVal SettingName As String, ByRef DefaultVal As T) As T
Dim formatter As IFormatProvider = CultureInfo.InvariantCulture
Dim targetType As Type = GetType(T)
Dim value As IConvertible = Nothing
Return If(Configuration.TryGetValue(SettingName, value), _
DirectCast(value.ToType(targetType, formatter), T), _
DefaultVal)
End Function
此代码使用反射从所有基本值类型实现的 IConvertible
接口调用适当的转换方法。可以使用DirectCast
简化:此代码使用ToType
方法,因此不需要反射。
请注意,IConvertible
类型约束在这里甚至不是必需的 - 因为我们在配置的IConvertible
返回值上调用String
方法,而不是实际类型。但是约束仍然有用,因为这可以确保存在适当的转换。
答案 2 :(得分:1)
很简单,只需先把它放到一个对象上就可以忘记它有一个字符串。
Function GetSetting(Of T)(ByVal SettingName As String, ByRef DefaultVal As T) As T
Return If(Configuration.ContainsKey(SettingName), CType(CObj(Configuration(SettingName)), T), DefaultVal)
End Function
答案 3 :(得分:0)
您需要在方法定义上设置约束,以将其约束为可以转换为字符串的类型。有关向泛型添加约束的信息,请参阅this。你只能约束一个classthough。我会约束对象(因为你仍然可以指定任何从约束类型派生的类型),然后调用ToString()而不是强制转换。
答案 4 :(得分:0)
类型Configuration
是Dictionary<string, string>
(c#)?
如果是,您不需要泛型。
编辑:如果Configuration(SettingsName)
返回一个字符串,则该容器是非泛型的&amp;因此,你不需要这里的泛型。
EDIT2:
void Main()
{
Test<bool>.trythis("false");
Test<bool>.trythis("true");
Test<int>.trythis("28");
Test<decimal>.trythis(decimal.MaxValue.ToString());
}
public class Test<T>
{
public static void trythis(string value)
{
T retVal = (T)Convert.ChangeType(value, typeof(T));
Console.WriteLine(retVal.GetType());
}
}
对不起,我不太了解vb.net
可能是Convert.ChangeType(Configuration(SettingsName), T.GetType())
&amp;然后在结果上使用DirectCast
。
希望这有帮助。
答案 5 :(得分:0)
Public Function Value(Of T)(ByVal SettingName As String, ByVal DefaultValue As T) As T
Return If(Configuration.ContainsKey(SettingName), DirectCast(System.Convert.ChangeType(Configuration(SettingName), GetType(T)), T), DefaultValue)
End Function
答案 6 :(得分:0)
我知道回答这个问题已经很晚了,但我一直在寻找类似的东西,并发现还有另一种方法可以做到。
使用IConvertible
接口,并要求T
泛型类型实现接口。那么我们就可以使用Convert.ChangeType()
方法。
代码很容易编写
Function GetSetting(Of T As IConvertible)(ByVal SettingName As String, _
Optional ByRef DefaultVal As T = Nothing) _
As T
If Configuration.ContainsKey(SettingName) Then
Return Convert.ChangeType(Configuration(SettingName), GetType(T))
End If
Return DefaultVal
End Function