using System;
using System.Data.Objects;
using System.Linq;

namespace OutOfMemory.Codes
{
    /// <summary>
    /// Adds extension methods to the <see cref="IQueryable{T}"/> class.
    /// </summary>
    public static class QueryableExtender
    {
        /// <summary>
        /// Specifies the related objects to include in the query results.
        /// </summary>
        /// <typeparam name="T"><see cref="Type"/> of query to extend.</typeparam>
        /// <param name="this">The query to extend.</param>
        /// <param name="path">
        /// Dot-separated list of related objects to return in the query results.
        /// </param>
        /// <returns>
        /// A new <see cref="IQueryable{T}"/> with the defined query path.
        /// </returns>
        /// <exception cref="ArgumentNullException">
        /// The parameter <paramref name="path"/> is null.
        /// </exception>
        /// <exception cref="ArgumentException">
        /// The parameter <paramref name="path"/> is empty.
        /// </exception>
        public static IQueryable<T> Include<T>(this IQueryable<T> @this, string path)
        {
            var objectQuery = @this as ObjectQuery<T>;

            if (objectQuery != null)
            {
                return objectQuery.Include(path);
            }
            return @this;
        }
    }
}

DbExtensions.Include<T> Method (IQueryable<T>, String)

https://msdn.microsoft.com/en-us/library/system.data.entity.dbextensions.include

Namespace:  System.Data.Entity
Assembly:  EntityFramework (in EntityFramework.dll)