How to instantiate and initialize a dynamic object in expression tree?

Question

When working with IQuerayble<TItem> we can call Select like this:

query.Select( item => new { A=item.Prop1, B=item.Prop2});

And Select method expects Expression<Func<TItem,TResult>>

I need to use ExpandoObject instead of anonymous but statically typed class.

If it were possible it would look like:

query.Select( item => dynamic new ExpandoBoject { A=item.Prop1, B=item.Prop2});

So I want to construct expression tree Expression<Func<TItem,ExpandoObject>> where object's properties are initialized in the similar way as with anonymous type.
Dynamic functionality is only needed for initialization so it's ok that Func returns ExpandoObject instead of dynamic.

I cannot find much documentation about Expression.Dynamic and corresponding binders I should use.

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Update 1

Why do I need all this stuff?
Because I want to get primary keys.
I want to do it for any entity type.

I know how to get the list of the properties composing PK, but now I need to make a tricky projection of the entity to EntityKey. Well, may be to same equvalent of this class.

var keys = context.Set<TEntity>().Where(Expression<Func<TEntity,bool>).Select(Expression<Func<TEntity,EntityKey>>);

As I noted in the comments lambdas containing blocks cannot be converted to expression trees so i cannot simple create the dictionary and fill it. Now I'm playing with the expression tree semantically close to this code:

var dict = new Dictionary<string,object>();
dict.Add("Prop1",value1);
dict.Add("Prop2",value2);
return dict

But I doubt EF can parse the expression containing blocks. Need to check.
And I'm curious whether it will work with dynamic objects and Expression.MemberInit as it works with static objects.

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Update 2

Entity Framework does not support dictionary initialization syntax.
It throws NotSupportedException with the message: Only list initializer items with a single element are supported in LINQ to Entities.

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Update 3

EF does not support block expressions aswell.
NotSupportedException with message: Unknown LINQ expression of type 'Block'.

Answer 1

Now I'm playing with the expression tree semantically close to this code:

var dict = new Dictionary<string,object>();
dict.Add("Prop1",value1);
dict.Add("Prop2",value2);
return dict;

You can do that, because you can write that code as as a single expression like this:

new Dictionary<string, object>
{
    { "Prop1", value1 },
    { "Prop2", value2 }
};

And you can create an expression tree that contains this expression (which EF should be able to handle) like this:

var addMethod = typeof(Dictionary<string, object>).GetMethod("Add");

var expression = Expression.Lambda<Func<Dictionary<string, object>>>(
    Expression.ListInit(
        Expression.New(typeof(Dictionary<string, object>)),
        Expression.ElementInit(
            addMethod,
            Expression.Constant("Prop1"),
            value1Expression),
        Expression.ElementInit(
            addMethod,
            Expression.Constant("Prop2"),
            value2Expression)),
    itemParameterExpression);