public class People
{
    public int Id { get; set; }
    public string Name { get; set; }

    public void SayHi()
    {
        Console.WriteLine("morning!");
    }
}

 public class GenericConstrint
{
   public static void Show<T>(T tValue) where T : People
   {
       tValue.SayHi();
   }

   ///
 
 <summary>
   /// 值型別約束
   /// </summary>
   /// <typeparam name="T"></typeparam>
   /// <param name="t"></param>
   public static T GetValue<T>(T t) where T : struct
   {
            return default(T);
   }

   /// <summary>
   /// 引用型別約束
   /// </summary>
   /// <typeparam name="T">
 
</typeparam>
   /// <param name="t"></param>
   public static T GetRef<T>(T t) where T : class
   {
       return null;//引用型別，所以可以返回null
   }
}

使用程式碼：

GenericConstrint.GetValue<decimal>(100m);
GenericConstrint.GetValue<int>(100);
GenericConstrint.GetValue<float>(100f);
 


GenericConstrint.GetRef<string>("1234");
GenericConstrint.GetRef<People>(p);