Expression tree create dictionary with property values for class

Question

Essentially I'm trying to do this using expression trees

var properties = new Dictionary<string, object>();

foreach (var propInfo in objType.GetTypeInfo().GetProperties(BindingFlags.Public))
{
    var name = propInfo.Name;
    var value = propInfo.GetValue(objInstance);

    properties.Add(name, value);
}

return properties;

I.e. create a dictionary of name and value pairs where name is the name of a property for objType and value is the value of the property for the instance objInstance of objType

Now converting this to an expression should compile to a delegate that simply does

Func<T, Dictionary<string, object>> func = i =>
{
    var properties = new Dictionary<string, object>();

    properties.Add("Prop1", (object)i.Prop1);
    properties.Add("Prop2", (object)i.Prop2);
    properties.Add("Prop3", (object)i.Prop3);
    // depending upon the number of properties of T, Add will continue

    return properties;
};

I know how to perform some of this, but what I am not sure on is how to create a local instance of a dictionary and then use it (and return it) in subsequent expressions?

Halfmoon

You may use Expression.Constant(..) to produce expression from your local variable – Dodecagon

Expression and delegate are different things. What exactly are you trying to achieve? Func<object, Dictionary<string, object>> or Expression<Func<object, Dictionary<string, object>>>? – Counterman

Sorry, by local I mean local to the delegate of the compiled expression, see delegate body amendment above. – Halfmoon

By delegate I mean the result of the compiled expression – Halfmoon

Answer 1

It should be something like (comments inline):

public static Func<T, Dictionary<string, object>> GetValuesFunc<T>()
{
    Type objType = typeof(T);

    var dict = Expression.Variable(typeof(Dictionary<string, object>));
    var par = Expression.Parameter(typeof(T), "obj");

    var add = typeof(Dictionary<string, object>).GetMethod("Add", BindingFlags.Public | BindingFlags.Instance, null, new[] { typeof(string), typeof(object) }, null);

    var body = new List<Expression>();
    body.Add(Expression.Assign(dict, Expression.New(typeof(Dictionary<string, object>))));

    var properties = objType.GetTypeInfo().GetProperties(BindingFlags.Public | BindingFlags.Instance);

    for (int i = 0; i < properties.Length; i++)
    {
        // Skip write only or indexers
        if (!properties[i].CanRead || properties[i].GetIndexParameters().Length != 0)
        {
            continue;
        }

        var key = Expression.Constant(properties[i].Name);
        var value = Expression.Property(par, properties[i]);
        // Boxing must be done manually... For reference type it isn't a problem casting to object
        var valueAsObject = Expression.Convert(value, typeof(object));
        body.Add(Expression.Call(dict, add, key, valueAsObject));
    }

    // Return value
    body.Add(dict);

    var block = Expression.Block(new[] { dict }, body);

    var lambda = Expression.Lambda<Func<T, Dictionary<string, object>>>(block, par);
    return lambda.Compile();
}

use it like:

public class Test
{
    public int A { get; set; }
    public string B { get; set; }
}

and

Func<Test, Dictionary<string, object>> fn = GetValuesFunc<Test>();

var obj = new Test
{
    A = 5,
    B = "Foo"
};

var res = fn(obj);

Bethesde

Superb! Thank you so much. – Halfmoon

What is the point of using Expressions here? It seems to just overcomplicate, no? – Kordofan

@Kordofan What solution would you propose then? – Bethesde

Answer 2

You only need Expressions to cache the getters, unless you know an easier way (e.g. maybe dynamic somehow?)

public static Func<T, Dictionary<string, object>> GetToDict<T>(){
    var getters = typeof(T)
        .GetProperties(BindingFlags.Public | BindingFlags.Instance)
        .Select(p => {
            var param = Expression.Parameter(typeof(T));
            return new KeyValuePair<PropertyInfo, Func<T, object>>(p, 
                Expression.Lambda<Func<T, object>>(Expression.Convert(Expression.MakeMemberAccess(param, p), typeof(object)), param).Compile());
        })
        .ToList();
    return i => getters.ToDictionary(x => x.Key.Name, x => x.Value(i));
}

Kordofan