save data using dropdown in asp.net Entity Framework

Hello..

i am saving a data using dropdown but i am getting a error

"Cannot implicity convert type "long" to 'String'"

"btnSave" runat="server" Text="Save" OnClick="btnSave_Click" />

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
bindUsingCategory();
}
}
public void bindUsingCategory()
{
using (WallpaperEntities10 context = new WallpaperEntities10())
{
ddlWallCategry.DataSource = (from r in context.WallPaperCategries select new { CategoryName = r.CategoryName, Id = r.Id }).ToList();
ddlWallCategry.DataTextField = "CategoryName";
ddlWallCategry.DataValueField = "Id";
ddlWallCategry.DataBind();
ddlWallCategry.Items.Insert(0, new System.Web.UI.WebControls.ListItem("Select Option"));
}
}
protected void btnSave_Click(object sender, EventArgs e)
{
Model.WallpaperCategory newusermodel = new Model.WallpaperCategory();
string str = filePath.FileName;
filePath.PostedFile.SaveAs(Server.MapPath("~/Upload/" + str));
string Path = "~/Upload/" + str.ToString();
newusermodel.Category = ddlWallCategry.SelectedIndex;
using (var context = new WallpaperEntities10())
{
WallpaperRecord user = new WallpaperRecord();
{
string name = filePath.ToString();
user.Category = newusermodel.Category; //getting error in this line
};
var usersadd = context.Set();
var response = context.WallpaperRecords.Where(u => u.Category = newusermodel.Category).FirstOrDefault();
if (response == null)
{
usersadd.Add(user);
context.SaveChanges();
}
}
}

This code is not performing comparison

var response = context.WallpaperRecords.Where(u => u.Category = newusermodel.Category).FirstOrDefault();

It should be

var response = context.WallpaperRecords.Where(u => u.Category == newusermodel.Category).FirstOrDefault();

This might be issue due to casting to different data type