如何将对象强制转换为函数和调用

Question

我有一个通用方法GetValueProvider，返回通用Func(Of...)委托。在其他方法中，我需要调用GetValueProvider，调用返回的委托，并最终获得其返回值。但是我不能直接调用它，因为我只得到type作为参数。经过反思，我得到GetValueProvider的输出不是问题，但我得到的是Object。如何调用底层委托?

这是我的代码的简化示例。最后，我需要填充value。

Protected Function GetValueProvider(Of T)() As Func(Of XmlNode, T, Func(Of Dictionary(Of String, String)), T)
  ' some code here
End Function
Public Function GetValue(valueType As Type) As Object
  ' value that should be set
  Dim value As Object
  ' init some variables
  Dim node As XmlNode = GetNode()
  Dim defaultValue As Object = GetDefaultValue() ' type of defaultValue is always valueType
  Dim tokensProvider As Func(Of Dictionary(Of String, String)) = GetTokensProvider()
  Dim methodInfo = Me.GetType().GetMethod("GetValueProvider", Reflection.BindingFlags.Public Or Reflection.BindingFlags.NonPublic Or Reflection.BindingFlags.Instance Or Reflection.BindingFlags.InvokeMethod)
  Dim genericMethodInfo = methodInfo.MakeGenericMethod(valueType)
  Dim valueProvider = genericMethodInfo.Invoke(Me, Nothing)
  ' following doesn't work and calling DynamicInvoke ends with an exception:
  Dim valueProviderDelegate = DirectCast(valueProvider, [Delegate])
  value = valueProviderDelegate.DynamicInvoke(node, defaultValue, tokensProvider)
  ' another approach that doesn't work (Invoke ends with the same exception):
  Dim invokeMethodInfo = valueProvider.GetType().GetMethod("Invoke")
  value = invokeMethodInfo.Invoke(valueProvider, New Object() {node, defaultValue, tokensProvider})
  Return value
End Function

异常:

System.ArgumentException: Object of type 'VB$AnonymousDelegate_2`1[System.Collections.Generic.Dictionary`2[System.String,System.String]]' cannot be converted to type 'System.Func`1[System.Collections.Generic.Dictionary`2[System.String,System.String]]'.
at System.RuntimeType.TryChangeType(Object value, Binder binder, CultureInfo culture, Boolean needsSpecialCast)
at System.RuntimeType.CheckValue(Object value, Binder binder, CultureInfo culture, BindingFlags invokeAttr)
at System.Reflection.MethodBase.CheckArguments(Object[] parameters, Binder binder, BindingFlags invokeAttr, CultureInfo culture, Signature sig)
at System.Reflection.RuntimeMethodInfo.InvokeArgumentsCheck(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.Delegate.DynamicInvokeImpl(Object[] args)
at System.Delegate.DynamicInvoke(Object[] args)
...

任何想法?谢谢。

Answer 1

我真笨。实际上，代码中使用的两种方法都是有效的。如果有人想知道，异常是由tokensProvider中错误的类型引起的。上面的例子应该可以正常工作，我实际上在我的代码中是这样的:

Dim tokensProvider As = Function() As Dictionary(Of String, String)
                          ' some code here
                        End Function

修复:

Dim tokensProvider As Func(Of Dictionary(Of String, String)) = Function() As Dictionary(Of String, String)
                                                                 ' some code here
                                                               End Function