TypeDescriptor.GetProperties(thisType) to return properties, which are write-only

Question

I'm trying to get all properties from a type, but using TypeDescriptor.GetProperties(thisType) will only supply me with properties, that has both setter and a getter. I have write-only properties. Are there a way to retrieve a PropertyDescriptorCollection including those?

/Asger

Answer 1

Write-only properties are a rare beast, and don't exist in the System.ComponentModel / PropertyDescriptor space. PropertyDescriptors are designed to be readable. I could probably hack HyperDescriptor to shim write-only properties, but it would be a hack - and it would presumably have to throw exceptions for get, which could impact calling code quite a bit.

As an aside; I generally advise against write-only properties; the text-book example that people trot out is passwords (public string Password {private get;set;}) - I'd much rather have a void SetPassword(string newPassword) method...

What is it that you actually want to do? There are a range of options here, all very achievable:

• use reflection alone (slow; maybe not an option)
• use Delegate.CreateDelegate (very easy)
• use Expression.Compile (a little harder, but not much)
• use Reflection.Emit (quite hard)
• shim write-only properties into PropertyDescriptor (quite hard)

If you let me know what you actually want to do (rather than the way you are currently trying to do it), I might be able to help more.

As an example using Delegate.CreateDelegate (note you would want to stash the delegate somewhere and re-use it lots of times):

edited to show how to do it if you don't know the specific types at runtime

using System;
using System.Reflection;

class Foo
{
    public string Bar { private get; set; }
    public override string ToString()
    {
        return Bar; // to prove working
    }
}
static class Program
{
    static void Main()
    {
        ISetter setter = Setter.Create(typeof(Foo), "Bar");
        Foo foo = new Foo();
        setter.SetValue(foo, "abc");
        string s = foo.ToString(); // prove working
    }
}
public interface ISetter {
    void SetValue(object target, object value);
}
public static class Setter
{
    public static ISetter Create(Type type, string propertyName)
    {
        if (type == null) throw new ArgumentNullException("type");
        if (propertyName == null) throw new ArgumentNullException("propertyName");
        return Create(type.GetProperty(propertyName));
    }
    public static ISetter Create(PropertyInfo property)
    {
        if(property == null) throw new ArgumentNullException("property");
        if (!property.CanWrite) throw new InvalidOperationException("Property cannot be written");
        Type type = typeof(TypedSetter<,>).MakeGenericType(
                property.ReflectedType, property.PropertyType);
        return (ISetter) Activator.CreateInstance(
            type, property.GetSetMethod());
    }
}

public class TypedSetter<TTarget, TValue> : ISetter {
    private readonly Action<TTarget, TValue> setter;
    public TypedSetter(MethodInfo method) {
        setter = (Action<TTarget, TValue>)Delegate.CreateDelegate(
            typeof(Action<TTarget, TValue>), method);
    }
    void ISetter.SetValue(object target, object value) {
        setter((TTarget)target, (TValue)value);
    }
    public void SetValue(TTarget target, TValue value) {
        setter(target, value);
    }
}

---

Or alternatively using the Expression API (.NET 3.5):

using System;
using System.Linq.Expressions;
using System.Reflection;

class Foo
{
    public string Bar { private get; set; }
    public override string ToString()
    {
        return Bar; // to prove working
    }
}
static class Program
{
    static void Main()
    {
        Action<object,object> setter = Setter.Create(typeof(Foo), "Bar");
        Foo foo = new Foo();
        setter(foo, "abc");
        string s = foo.ToString();
    }
}

public static class Setter
{
    public static Action<object,object> Create(Type type, string propertyName)
    {
        if (type == null) throw new ArgumentNullException("type");
        if (propertyName == null) throw new ArgumentNullException("propertyName");
        return Create(type.GetProperty(propertyName));
    }
    public static Action<object,object> Create(PropertyInfo property)
    {
        if(property == null) throw new ArgumentNullException("property");
        if (!property.CanWrite) throw new InvalidOperationException("Property cannot be written");

        var objParam = Expression.Parameter(typeof(object), "obj");
        var valueParam = Expression.Parameter(typeof(object), "value");
        var body = Expression.Call(
            Expression.Convert(objParam, property.ReflectedType),
            property.GetSetMethod(),
            Expression.Convert(valueParam, property.PropertyType));
        return Expression.Lambda<Action<object, object>>(
            body, objParam, valueParam).Compile();
    }
}