为什么我的F＃函数的返回类型为bool？

Question

I am writing a function that takes a PropertyInfo object, fetches MyAttribute and returns a MyAttribute object:

let getparamattribute(p : PropertyInfo) = 
    let attr = p.GetCustomAttribute (typeof<MyAttribute>, true) 
    attr :? MyAttribute 

However FSI shows the return type as bool:

val getparamattribute : (PropertyInfo -> bool)

Why?

Answer 1

That's exactly what :? operator does:

Returns true if the value matches the specified type; otherwise, returns false (type test operator).

What you're looking for is :?> downcast operator:

The :?> operator performs a dynamic cast, which means that the success of the cast is determined at run time. A cast that uses the :?> operator is not checked at compile time; but at run time, an attempt is made to cast to the specified type. If the object is compatible with the target type, the cast succeeds. If the object is not compatible with the target type, the runtime raises an InvalidCastException .

from Symbol and Operator Reference