class MyClass {
    @member
    arrayProperty: string[];

    @member
    mappingProperty: {
        [key:string]: MyClass
    };

}

function member(prototype, memberName: string){
    let designType = Reflect.getMetadata('design:type', prototype, memberName);

    // How to know the element type of an array property?
    // How to know the key type of an mapping property?
    // How to know the value type of an mapping property?
    // if there is no way of knowing this, Reflec is very partial reflection mechanism.
}

// complier options: ---emitDesign

class MyClass {
    arrayProperty: string[];
    mappingProperty: {
        [key:string]: MyClass
    };
}

function designUserFunction(type: Function, memberName: string){
    let classDesign = Design.get(type);

   let memberInfo = classDesign.members[memberName];

   if( memberInfo.kind == 'mapping' || memberInfo.isMapping ) {
       let mappingDesign: Design = memberInfo.value;
       let mappingKeyDesign: Design = memberInfo.value.key;
       let mappingKeyType = memberInfo.value.key.type; // String
       let mappingElementDesign: Design = memberInfo.value.element;
       let mappingElementType = memberInfo.value.element.type; // MyClass
   }
   else  if( memberInfo.kind == 'array' || memberInfo.isArray ) {
       let arrayDesign: Design = memberInfo.value;
       let arrayKeyDesign: Design = memberInfo.value.key;
       let arrayKeyType = memberInfo.value.key.type; // Number
       let arrayElementDesign: Design = memberInfo.value.element;
       let arrayElementType = memberInfo.value.element.type; // String
   }
   // ...
   // All type constructions of the language must be supported by Design,
   // this includes function design and tuples.
}