Format Of Local Variable As Input Parameter In Stored Procedure In Sql Server

Stored procedure ALTER proc [dbo].[spSearchCombo](@searchBy nvarchar(50),@searchKey nvarchar(50)) as select * from CD_DETAILS where @searchBy like '%' +@searchKey+ '%' I'm using ASP.net2.0 with c# to extract rows basedon search key from a text box and searchBy for the column to be searched. When i use column name instead of @searchBy which comes from value selected from a ddropdownlist i get the desired result . There seems to be a problem with format of @searchBy and i get a blank page.

protected void btnSearch_Click(object sender, EventArgs e)
{
string constring = ConfigurationManager.AppSettings.Get("con").ToString();
SqlConnection conn = new SqlConnection(constring);
conn.Open();
SqlCommand cmdSP = new SqlCommand("spSearchCombo", conn);
cmdSP.CommandType=CommandType.StoredProcedure;
cmdSP.Parameters.Add(new SqlParameter("@searchBy",SqlDbType.NVarChar,50));
cmdSP.Parameters["@searchBy"].Value=ddlSearchBy.SelectedValue.ToString();
cmdSP.Parameters.Add(new SqlParameter("@searchKey", SqlDbType.NVarChar, 50));
cmdSP.Parameters["@searchKey"].Value=txtSearch.Text.Trim();
SqlDataAdapter da=new SqlDataAdapter(cmdSP);
DataSet ds=new DataSet();
da.Fill(ds);
this.dgv1.DataSource=ds.Tables[0].DefaultView;
dgv1.DataBind();
}