<table id="tblPost" width="100%">
<tr>
    <td class="colleft">
        <asp:TextBox ID="txtTitle" runat="server" ToolTip="Enter Title" Width="100%" Height="25px"></asp:TextBox>  
        <asp:RequiredFieldValidator ID="reqtxtTitle" ErrorMessage="Post Title can't be empty" 
            ControlToValidate="txtTitle" runat="server" Display="Dynamic" ForeColor="Red" />  
    </td>
    <td class="colright">
                    <asp:Button ID="btnPost" runat="server" Text="Post" onclick="btnPost_Click" />
    </td>
    
</tr>
<tr>
    <td class="colleft">
        <asp:TextBox runat="Server" ID="txtBody" CssClass="post" Width="300px" Height="350px" TextMode="MultiLine" />
        <asp:RequiredFieldValidator ID="reqtxtBody" ErrorMessage="Post Description can't be empty" ControlToValidate="txtBody" runat="server" ForeColor="Red" Display="Dynamic" />
    </td>
    <td class="colright">
        <div class="head">Category</div>
        <asp:RadioButtonList ID="chkCats" runat="server"></asp:RadioButtonList>
        <asp:RequiredFieldValidator ID="reqchkCat" ErrorMessage="Select atlest one category" ControlToValidate="chkCats" ForeColor="Red"
                                                 Display="Dynamic" runat="server" />
            
    </td>
</tr>
</table>

public partial class Management_AddEditPost : System.Web.UI.Page
{
    long _id = -1;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!string.IsNullOrEmpty(Request.QueryString.Get("Id")))
        {
            _id = Convert.ToInt64(Request.QueryString.Get("Id"));
        }
        if (!Page.IsPostBack && _id > -1)
        {
            using (DatabaseEntities myEntities = new DatabaseEntities())
            {
                var post = (from p in myEntities.Posts
                              where p.Id == _id
                              select p).SingleOrDefault();
                if (post != null)
                {
                    txtTitle.Text = post.Title;
                    txtBody.Text = post.Body;
                    chkCats.DataBind();
                    ListItem myItem = chkCats.Items.FindByValue(post.CategoryId.ToString());
                    if (myItem != null)
                    {
                        myItem.Selected = true;  // binding radiobuttonlist here, but not getting assigned with category
                    }
        
                }
            }
        }
        
    protected void btnPost_Click(object sender, EventArgs e)
    {
        using (DatabaseEntities myEntities = new DatabaseEntities())
        {
            Post myPost;
            if (_id == -1) // Insert new item
            {
                myPost = new Post();
                myPost.CreateDateTime = DateTime.UtcNow;
                myPost.UpdateDateTime = myPost.CreateDateTime;
                myEntities.AddToPosts(myPost);
            }
            else  // Update existing item
            {
                myPost = (from p in myEntities.Posts
                          where p.Id == _id
                          select p).Single();
                myPost.UpdateDateTime = DateTime.UtcNow;
            }

            myPost.Title = txtTitle.Text;
            myPost.Body = txtBody.Text;
            foreach (ListItem li in chkCats.Items)
            {
                if (li.Selected)
                {
                    myPost.CategoryId = Convert.ToInt64(chkCats.SelectedValue); 
                }
            }

            myEntities.SaveChanges();
            Response.Redirect("AddEditPost.aspx");


        }
    }