title: Set a string column to nullable in EF6
body: <p>I have a model that i created it bu EF6:</p>
  
  <pre><code>public partial class Comment
  {
      [DisplayName("شناسه نظر")]
      public int Id { get; set; }
  
      [Required(ErrorMessage = "متن نظر را وارد کنید")]
      [DisplayName("متن نظر")]
      public string CommentText { get; set; }
  
      [DisplayName("تعداد پسندیدن ")]
      public long LikeCount { get; set; }
  
      [DisplayName("تعداد نپسندیدن")]
      public long DisLikeCount { get; set; }
  
      [DisplayName("تاریخ انتشار ")]
      public System.DateTime PublishDate { get; set; }
  
      [DisplayName("وضعیت نمایش ")]
      public string Visible { get; set; }
  
      [DisplayName("نام کاربری ")]
      public Nullable&lt;string&gt; AutherUserName { get; set; }
  
      [DisplayName("شناسه نظراصلی")]
      public Nullable&lt;int&gt; CommentFKId { get; set; }
  
      [DisplayName("شناسه کاربر")]
      public Nullable&lt;int&gt; StudentId { get; set; }
  
      [DisplayName("شناسه محتوا ")]
      public Nullable&lt;int&gt; ContentId { get; set; }
  
      public virtual Comment Comments { get; set; }
      public virtual Comment Comment1 { get; set; }
      public virtual Student Student { get; set; }
      public virtual Content Content { get; set; }
  }
  </code></pre>
  
  <p>As you can see i have several <strong>Nullable</strong> int columns in my model ,But i can't set a string column to null :</p>
  
  <pre><code>public Nullable&lt;string&gt; AutherUserName { get; set; }
  </code></pre>
  
  <p>And I got this error :</p>
  
  <blockquote>
    <p>The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'</p>
  </blockquote>
  
  <p>I am working with MVC4 </p>