通过Reflection读取子类的非静态属性值

Question

I am implementing a variation of the Command design pattern. I have an abstract CodedCommand class with some subclasses, each of one with an overriden "Code" property of type char . Each subclass has then the responsibility to "encapsulate" its own Code letter to which it is associated.

The use case is: when some serial parser receive a char/byte, I want to create an instance of the appropriate subclass, to further process the command payload.

My problem is: I don't know how to get the specific value from each subclass.

My current implementation is below. The problem, highlighted with a comment, is that type.GetProperty("Code") is returning null!

internal abstract class CodedCommand
{
    internal abstract char Code { get; }
}

internal class EventA : CodedCommand
{
    internal override char Code => 'A';
}

internal class EventB : CodedCommand
{
    internal override char Code => 'B';
}

public class CommandCreator
{
    Dictionary<char, Type> _typeMap 
        = new Dictionary<char, Type>();

    public CommandCreator()
    {
        var types = GetType().Assembly.GetTypes()
                                     .Where(type => type.IsSubclassOf(typeof(CodedCommand)))
                                     .Where(type => !type.IsAbstract);

        foreach (var type in types)
        {
            var field = type.GetProperty("Code");  // returns null!!!
            var value = field.GetValue(null);
            var code = (char)value;

            _typeMap[code] = type;
        }
    }


    CodedCommand BuildCommand(char code)
    {
        _typeMap.TryGetValue(code, out Type type);

        if (type != null)
        {
            return (CodedCommand)(Activator.CreateInstance(type));
        }

        return null;
    }
}

So my question is how can I fix this design?

Answer 1

Here is complete solution for getting value of property:

var types = GetType().Assembly.GetTypes()
   .Where(type => type.IsSubclassOf(typeof(CodedCommand)) && !type.IsAbstract);

foreach (var type in types)
{
    var obj = Activator.CreateInstance(type);
    var field = type.GetProperty("Code", BindingFlags.Instance | BindingFlags.NonPublic);
    var value = field.GetValue(obj);
    var code = (char)value;

    _typeMap[code] = type;
}

Please note that field.GetValue needs object instance to work and NonPublic -BindingFlag is needed when getting PropertyInformation for internal property.

Answer 2

The problem is your property is marked as internal , not public and we cannot access the non-public properties using the overload which you are using.

You need to specify the flags parmeter by using this overload if you want properties other than public like:

type.GetProperty("Code",BindingFlags.Instance | 
                        BindingFlags.NonPublic |
                        BindingFlags.Public)

GetProperty(String) overload :

Searches for the public property with the specified name.

GetProperty(string name,BindingFlags bindingAttr) overload :

Searches for the specified property, using the specified binding constraints.