Adding properties based on lambda expressions

Question

I am using C# to create a view model that I later serialize into Json for use with KnockoutJs.

Now I need to add information on a property level if a certain user has access to view and/or edit the property.

Since Javascript has little to no reflection possibilities I want to do this with C# before serializing to Json.

My view model could look like this:

class ViewModel {
    public string Name { get; set; }        
}

I want to use a User Access Service that would take the user, which has some Roles, and the view model and go through the Roles and apply the access rights. Something like this:

class AccessService {
    public void Apply(IUser user, ViewModel viewModel) {
        if(user.Roles.Contains(Roles.Admin)) {
            viewModel.AllowRead(vm => vm.Name);
        }
    }
}

The viewModel.AllowRead(..) method would be an extension method that would take an object (or maybe an interface or type if necessary) and this is where the magic would happen.

I would like the result of this operation to be that the viewModel would get a new property with the name CanRead which in turn would have a boolean property with the name Name.

The resulting viewModel would look like this:

class ViewModel {
    public string Name { get; set; }
    public object CanRead { // Could non anonymous as well.
        return new {
            Name = true;
        };
    }
}

Can this be done with dynamics or do I have to use Reflection.Emit? I'm not asking for "show me the codez". I just want to know if my idea is mental or if it's possible.

Any ideas?

No, it is used with KnockoutJs which is a MVVM thing for Javascript, hence the JSON. — Mikael Östberg
– Mikael Östberg, Commented Feb 13, 2012 at 16:48

Roy Goode · Accepted Answer · 2012-02-13 16:49:01Z

1

I think it would be possible and you could use the Lambdanator to help achieve this.

Example usage:

Lambda.Reflect<SomeClass>(x => x.AMethod());    
Lambda.Reflect<SomeClass>(x => x.AProperty);    
Lambda.Reflect(() => local);

answered Feb 13, 2012 at 16:49

Roy Goode

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3 Comments

Mikael Östberg Over a year ago

While this would help me getting existing properties from a type based on a Lambda expression it doesn't help me in the Reflection.Emit, dynamic or whatever thing that would be needed to reach my goal. Still interesting though. Thanks!

Roy Goode Over a year ago

Yes it's just the first step on the walk. I think what you're asking for is possible with Reflection & dynamics but I haven't tried.

Mikael Östberg Over a year ago

I've solved it now. It's beautiful! I used a dynamic ExpandoObject on a ViewModelBase class that is exposed as a property. Then I stole some Expression magic from Lambdanator, basically just the Expression<Func<T, object>>, and created an extension method that adds a property to the dynamic object using ((IDictionary<String, Object>)theDynamicProperty)[memberAccess.Member.Name] = true;. It works really well! Thanks for the hint with the Lambdanator.

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