How do I access a Dictionary Item using Linq Expressions

I want to build a Lambda Expression using Linq Expressions that is able to access an item in a 'property bag' style Dictionary using a String index. I am using .Net 4.

    static void TestDictionaryAccess()
    {
        ParameterExpression valueBag = Expression.Parameter(typeof(Dictionary<string, object>), "valueBag");
        ParameterExpression key = Expression.Parameter(typeof(string), "key");
        ParameterExpression result = Expression.Parameter(typeof(object), "result");
        BlockExpression block = Expre开发者_运维知识库ssion.Block(
            new[] { result },               //make the result a variable in scope for the block
            Expression.Assign(result, key), //How do I assign the Dictionary item to the result ??????
            result                          //last value Expression becomes the return of the block
        );

        // Lambda Expression taking a Dictionary and a String as parameters and returning an object
        Func<Dictionary<string, object>, string, object> myCompiledRule = (Func<Dictionary<string, object>, string, object>)Expression.Lambda(block, valueBag, key).Compile();

        //-------------- invoke the Lambda Expression ----------------
        Dictionary<string, object> testBag = new Dictionary<string, object>();
        testBag.Add("one", 42);  //Add one item to the Dictionary
        Console.WriteLine(myCompiledRule.DynamicInvoke(testBag, "one")); // I want this to print 42
    }

In the above test method, I want to assign the Dictionary item value i.e. testBag["one"] into the result. Note that I have assigned the passed in Key string into the result to demonstrate the Assign call.

You can use the following to access the Item property of the Dictionary

Expression.Property(valueBag, "Item", key)

Here is the code change that should do the trick.

ParameterExpression valueBag = Expression.Parameter(typeof(Dictionary<string, object>), "valueBag");
ParameterExpression key = Expression.Parameter(typeof(string), "key");
ParameterExpression result = Expression.Parameter(typeof(object), "result");
BlockExpression block = Expression.Block(
  new[] { result },               //make the result a variable in scope for the block           
  Expression.Assign(result, Expression.Property(valueBag, "Item", key)),
  result                          //last value Expression becomes the return of the block 
);