SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-query-forms

QueryForms_GetAllForms.cs

using System.Collections.Generic;
using System.Linq;
using Telerik.Sitefinity.Forms.Model;
using Telerik.Sitefinity.Modules.Forms;

namespace SitefinityWebApp
{
    public class QueryForms_GetAllForms
    {
        private List<FormDescription> GetAllForms()
        {
            var formManager = FormsManager.GetManager();
            return formManager.GetForms().ToList<FormDescription>();
        }
    }
}