How to use SUM aggregate function SQL Query in C# .Net

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object dd = dr["LL"];
            decimal? d = (dd is DBNull) ?null : (decimal?)(dr["LL"]);

You need to ExecuteScalar. Please use the parameterized queries.

string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
    using (SqlCommand cmd = new SqlCommand("Select Sum(Lamt) as LL from Loan$ where FingerPrintID = @FingerPrintId ", con))
    {
        using (SqlDataAdapter da = new SqlDataAdapter(cmd))
        {
            cmd.Parameters.AddWithValue("@FingerPrintId", RCBox1.Text);
            con.Open();
            object value = cmd.ExecuteScalar();
            if (value !=null)
            {
                decimal d = Convert.ToDecimal(value);

            }
            con.Close();
                
        }
    }
}