如果委托类型是 internal ,我在为F#中的属性getter编译lambda表达式时遇到了一些麻烦。这就是函数的样子:
// if I omit the 'internal' here everything works as expected
module internal ReflectionHelpers =
open System
open System.Linq.Expressions
open System.Reflection
// it makes no difference if this delegate type is defined outside
// of the module and marked as 'internal'
type GetterFunc<'T> = delegate of 'T -> obj
/// Build a getter expression function for the
/// specified PropertyInfo
let getGetter<'a> (p : PropertyInfo) =
let inst = Expression.Parameter(p.DeclaringType, "i")
let prop = Expression.Property(inst, p)
let conv = Expression.Convert(prop, typeof<obj>)
Expression.Lambda<GetterFunc<'a>>(conv, inst).Compile()
如果我用公共类调用此方法,我会得到一个像这样的异常(仅当ReflectionHelpers模块定义为'internal'时):
System.ArgumentNullException was unhandled by user code
Message=Value cannot be null.
Parameter name: key
Source=mscorlib
ParamName=key
StackTrace:
at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
at System.Dynamic.Utils.CacheDict`2.TryGetValue(TKey key, TValue& value)
at System.Dynamic.Utils.TypeExtensions.GetParametersCached(MethodBase method)
at System.Linq.Expressions.Expression.ValidateLambdaArgs(Type delegateType, Expression& body, ReadOnlyCollection`1 parameters)
at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, String name, Boolean tailCall, IEnumerable`1 parameters)
at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, Boolean tailCall, IEnumerable`1 parameters)
at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, ParameterExpression[] parameters)
...
可能我在这里错过了一个重要的观点,但目前我看不出为什么会发生这种情况。
答案 0 :(得分:5)
F#和C#使用不同的策略为委托发出Invoke方法。如果您尝试在C#中定义类似的委托 - 委托类型将是内部但具有公共Invoke方法。 F#定义了与声明类型具有相同可见性的Invoke方法。当Expression.Lambda中的代码尝试在给定委托类型中查找Invoke方法时,它仅查找公共方法,假设所有编译器的行为类似于C#