用于选择多个字段的C＃Lambda表达式

Question

Trying to encapsulate some logic in an extension method that will provide a ToSelectList method on a given IEnumerable, but I've hit a snag and I'm hoping someone can help to get me on the right track.

Essentially, I have one override that's fine. I have known fields that I use from a given base class, and the code looks as follows:

public static IEnumerable<SelectListItem> ToSelectList(this IEnumerable<BaseModel> list)
{
    return list.Select(t => new SelectListItem { Text = t.Description, Value = t.Id.ToString() });
}

Not a problem. Known fields. The problem comes when I want to use specific fields from classes derived from BaseModel, but want to specify those from the calling code to make a more generic version of this method.

Essentially, I'm looking to use lambdas in the calling code to specify the two properties that I want to retrieve from the items in the list like so:

myEnumerable.ToSelectList(textSelector => textSelector.Id.ToString(), valueSelector => valueSelector.DisplayName);

So I'd have a method that looks like the following:

public static IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<BaseModel> list, [DON'T KNOW WHAT GOES HERE] textExpression, [DON'T KNOW WHAT GOES HERE] valueExpression)
{
   return list.Cast<T>().Select(t => new SelectListItem { Text = textExpression(t), Value = valueExpression(t).ToString() });
}

I'm trying to work out what I need to do to enable me to then use those within the expression used for the Select to use those fields as the values to assign to the properties of the SelectListItem . I'm guessing there must be something similar underneath the LINQ IEnumerable<T>.ToDictionary method where there's an expression for the key and another for the value.

Is this even possible? Happy to cut my losses if it's not. At the moment, however, I'm struggling to get my head around what I'd need to pass to my ToSelectList method as parameters - is it an Expression<T> , a Func<T> , an Expression<Func<T>> ? I don't mind admitting that I get quite lost in the expression evaluation stuff.

Answer 1

You could try the following

   public static IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<BaseModel> list, Expression<Func<T, string>> textExpression, Expression<Func<T, string>> valueExpression)
        {
            var textFunc = textExpression.Compile();
            var valueFunc = valueExpression.Compile();

            return list.Select(t => new SelectListItem
                   {
                       Text = textFunc(t),
                       Value = valueFunc(t)
                   });
    }

or

   public static IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<BaseModel> list, Func<T, string> textFunc, Func<T, string> valueFunc)
        {
            return list.Select(t => new SelectListItem
                       {
                           Text = textFunc(t),
                           Value = valueFunc(t)
                       });
        }

Answer 2

You don't need Expression<> here. Plain Func<T, string> will do just fine.

Now, I can't really understand the this IEnumerable<BaseModel> list part, especially since you later do list.Cast<T>() . It would be much cleaner to do this:

IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<T>,
     Func<T, string> textProvider, 
     Func<T, string> idProvider)
     where T : BaseModel