动态创建 Func<T,TR> C#

Question

我正在尝试创建一个动态 lambda exp 来过滤我列表中的一些结果，但我不知道如何创建一个动态 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();

Answer 1

您需要创建一个泛型方法。像这样的东西（未经测试）：

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);
}

那么你可以这样使用它：

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

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