SelectListItemExtensions.cs

using Microsoft.AspNetCore.Mvc.Rendering;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace JetStreamCore.Web.Pages
{
    public static class SelectListItemExtensions
    {
        /// <summary>
        /// https://codereview.stackexchange.com/questions/101061/helper-for-dropdownlists-with-extension-method
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="source"></param>
        /// <param name="value">Value function</param>
        /// <param name="text">Text function</param>
        /// <param name="selected">Selected condition</param>
        /// <returns></returns>
        public static IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<T> source, Func<T, string> value, Func<T, string> text, Func<T, bool> selected = null)
        {
            return source.Select(x => new SelectListItem
            {
                Value = value(x) ?? "",
                Text = text(x),
                Selected = selected != null ? selected(x) : false
            });
        }

        public static SelectList ToSelectList(this IEnumerable items, string dataValueField = "Value", string dataTextField = "Text")
        {
            return new SelectList(items, dataValueField, dataTextField);
        }

        public static SelectList ToSelectList(this IEnumerable items, string dataValueField, string dataTextField, object selectedValue)
        {
            return new SelectList(items, dataValueField, dataTextField, selectedValue);
        }
    }
}