Why "is a" Undefined

[barski]

Let's say i have a class that is derived from PropertyDescriptor

 

public class MyProp : PropertyDescriptor
{
   //implement all abstract methods      
}

 

why would something like the following return an undefined value or need to be cast

in the first place since MyProp "is a" PropertyDescriptor and PropertyDescriptorCollections contains a collection of PropertyDescriptors

public class SomeClass
{
       private int id=0;
       private string customer="";
       
       public SomeClass()
       {
             foreach(PropertyDescriptor p in TypeDescriptor.GetProperties(this.GetType()))
             {
                      //MyProp mP = p;//this won't compile type conversion
                        MyProp mP = p;//mp is undefined
                        PropertyDescriptor prD = p//this works
                      
             }
       }

       public int ID{get{return id;}set{id=value;}}
       public string Customer{get{return customer;}set{customer=value;}}

}

 

if MyProp is a PropertyDescriptor how can this be happening. Any help would be greatly appreciated.

[PlausiblyDamp]

The "is a" only works one way round, i.e. MyProp is a PropertyDescriptor but there no guarentee than a PropertyDescriptor is a MyProp.

 

In other words you can always safetly assign a MyProp object to a variable of type PropertyDescriptor because that is always going to be a safe assignment, there is no way the compiler can verify that the reverse conversion will be true - hence the cast is required.

 

try using

foreach(PropertyDescriptor p in TypeDescriptor.GetProperties(this.GetType()))
{
MyProp mP =(MyProp)  p;
}

[barski]

thanks for the reply.

 

Error Specified Cast invalid

[PlausiblyDamp]

The TypeDescriptor.GetProperties function probably returns multiple PropertyDescriptors - not all of them are of type MyProp though.

 

You will need to check the type of each p before casting to a MyProp variable.

[barski]

thanks for the reply. figured it out. the answer was in your first reply i just needed to think on it a little more.