class Program

{

    static void Main(string[] args)

    {

        dynamic e = new ExpandoObject();

        e.Name = "John";


        Console.WriteLine(HasProperty("Name", e));

        Console.WriteLine(HasProperty("Test", e));

    }


    private static bool HasProperty(string name, IDynamicMetaObjectProvider dyn)

    {

        return ((IDictionary

<string,> )dyn).ContainsKey(name);

    }

}

This seems to work for me. Not really done much stuff with dynamic however - so it might be silly!

private static bool HasProperty(string name, IDynamicMetaObjectProvider dyn)

{

return dyn.GetMetaObject(Expression.Constant(dyn)).GetDynamicMemberNames().Contains(name);

}

The only problem is that dynamic providers have to fill the GetDynamicMemberNames() themself. So it's possible that the original code will correctly return true while your code will return false.

The method will just return new string[0] by default, and it's ExpandoObject that just returns all known keys there.

I'm guessing this is a "fix" to the patch I just submitted ;-)

Is the try/catch method not a workable solution?

Sounds so simple to be silly, but waht about TryGetMember ?

I think the point was to avoid putting it in a Try/Catch. I'm guessing this is for RavenDB, try/catch constructs are expensive.

@Ayende,

By the way, I saw that RavenDB uses CodeDom to compile its indexes. My guess is that you chose this approach due to lack of good alternatives of code generation with 'dynamic' and expando objects.

What do you think about the fact that with each compilation CodeDom generates assemblies that remain loaded in the app domain?

Thanks,

T

@Justin,

I had a couple of issues with your code, I have developed it further into this gist: http://gist.github.com/478382 . Based on reflection it can now also check existence of methods and property setters.

What about this? Dirty, but fully functional draft.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Dynamic;

using Microsoft.CSharp.RuntimeBinder;

using System.Linq.Expressions;

using System.Runtime.CompilerServices;

namespace DynamicCheckPropertyExistence

{

class Program

{

    static void Main(string[] args)

    {

        dynamic e = new ExpandoObject();

        e.Name = "Ayende";


        Console.WriteLine(HasProperty("Name", e));

        Console.WriteLine(HasProperty("Id", e));

        Console.ReadLine();

    }


    private static bool HasProperty(string name, IDynamicMetaObjectProvider dyn)

    {




        var binder = Binder.GetMember(CSharpBinderFlags.None, name, typeof(Program),

                         new[]

                                 {

                                     CSharpArgumentInfo.Create(

                                     CSharpArgumentInfoFlags.None, null)

                                 }) as GetMemberBinder;



        var callSite =

                    CallSite

<func<callsite,>

.Create(new SpecialBinderMember(name, false, binder));

        var result = callSite.Target(callSite, dyn);


        if (Object.ReferenceEquals(result, NoThrowExpressionVisitor.DUMMY_RESULT))

        {

            return false;

        }


        return true;


    }

}


class SpecialBinderMember : GetMemberBinder

{

    private GetMemberBinder m_innerBinder;        

    public SpecialBinderMember(string name, bool ignoreCase, GetMemberBinder innerBinder) : base(name, ignoreCase)

    {

        m_innerBinder = innerBinder;            

    }


    public override DynamicMetaObject FallbackGetMember(DynamicMetaObject target, DynamicMetaObject errorSuggestion)

    {



        var retMetaObject = m_innerBinder.Bind(target, new DynamicMetaObject[] {});


        var resultExpression = new NoThrowExpressionVisitor().Visit(retMetaObject.Expression);


        var finalMetaObject = new DynamicMetaObject(resultExpression, retMetaObject.Restrictions);

        return finalMetaObject;


    }


}


class NoThrowExpressionVisitor : ExpressionVisitor

{        

    public const string DUMMY_RESULT = "

<undefined";

    public NoThrowExpressionVisitor()

    {


    }


    protected override Expression VisitConditional(ConditionalExpression node)

    {


        if (node.IfFalse.NodeType != ExpressionType.Throw)

        {

            return base.VisitConditional(node);

        }


        Expression

<func<object> dummyFalseResult = () => DUMMY_RESULT;

        var invokeDummyFalseResult = Expression.Invoke(dummyFalseResult, null);                                    

        return Expression.Condition(node.Test, node.IfTrue, invokeDummyFalseResult);

    }


}

}

Constant DUMMY_RESULT has value "undefined"

public const string DUMMY_RESULT = "<undefined>";

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