Dynamic Create Func C#

Question

I'm trying to create a dynamic lambda exp to filter some results on my list, but I can't figure out on how to create a dynamic func<,>

//Here I get the type of my object "Produto", but I can have other objects here...Like "Pedido" or another one.
Type type = typeof(Produto);

var param = Expression.Parameter(type, "arg");
var propName = Expression.Property(param, "Codigo");
var constP = Expression.Constant("123");

var nameStartsWith = Expression.Call(
    propName,
    typeof(string).GetMethod("StartsWith", new[] { typeof(string) }),
    constP);

//Here I have to instantiate my Func<T, bool>
//I can't instantiate it with my variable "type" to use on my Where clausule.
//I don't know how to do this in another way, to create my lambda.

var WhereExp = Expression.Lambda<Func<type, bool>>(nameStartsWith, param);

return _uow.produto.GetAll().AsQueryable().Where(WhereExp).ToList();

If you can create a function on `_uow` with a generic type it'd work better, I think. — Vagner Lucas, Nov 03 '16 at 02:09
What is preventing you from passing the type in as a generic type at compile time? Do you not know the type at compile time? — David L, Nov 03 '16 at 02:12
@Abion47 I want to instantiate my Expression.Lambda> with a generic type, for example In this case, my "type" var is a typeof(Produto), but it can be a typeof(someOtherObject) — Lucas Freitas, Nov 03 '16 at 02:13
Possible duplicate of [How to create a Expression.Lambda when a type is not known until runtime?](http://stackoverflow.com/questions/7801165/how-to-create-a-expression-lambda-when-a-type-is-not-known-until-runtime) — David L, Nov 03 '16 at 02:14
@DavidL I dunno, My method gonna receive a random type and I have to set it do my func<> — Lucas Freitas, Nov 03 '16 at 02:22
I'm suspecting that this might be an XY problem. What is your actual goal here? — Abion47, Nov 03 '16 at 02:53

score 2 · Accepted Answer · answered Nov 03 '16 at 03:15

You need to create a generic method. Something like this (untested):

public Expression<Func<T, bool>> DynamicWhere<T>(string pname, string value) where T : class
{
    var param = Expression.Parameter(typeof(T), "arg");
    var propName = Expression.Property(param, pname);
    var constP = Expression.Constant(value);

    var nameStartsWith = Expression.Call(
        propName,
        typeof(string).GetMethod("StartsWith", new[] { typeof(string) }),
        constP);

    return Expression.Lambda<Func<T, bool>>(nameStartsWith, param);
}

Then you can use it like this:

var WhereExp = DynamicWhere<Produto>("Codigo", "123");

return _uow.produto.GetAll().AsQueryable().Where(WhereExp).ToList();

Dynamic Create Func C#

1 Answers1