Question

我有一个返回通用GetValueProvider委托的通用方法Func(Of...)。在其他方法中，我需要调用GetValueProvider，调用返回的委托，最后得到它的返回值。但是我无法直接调用它，因为我只将类型作为参数。通过反射，我得到GetValueProvider的输出不是问题，但我得到的只是Object。我该怎么做来调用底层委托？

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

例外：

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)
...

有什么想法吗？感谢。

Answer 1

哦，傻我。实际上，代码中使用的两种方法都有效。如果有人想知道，异常是由tokensProvider中的错误类型引起的。上面的示例应该可以正常工作，我在代码中实际拥有的是：

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

修正：

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

如何将Object强制转换为Func并调用？

1 个答案: