operator += cannot be applied with string and void asp.net

Question

FOLLOWING ERROR IS COMING

 

operator += cannot be applied with string and void asp.net

 

on this below line

ResultValue +=Convert.ToString(this.InsertRows(s_No, v_Name, rate, s_liter, entryDate);
below is whole code
DataTable dt = (DataTable)ViewState["dt"];
string s_No, v_Name, rate, s_liter, entryDate, ResultValue = Convert.ToString(0);
foreach (DataRow row in dt.Rows)
{
s_No = (row["S_No"].ToString());
v_Name = (row["V_Name"].ToString());
rate = (row["Rate"].ToString());
s_liter = (row["S_liter"].ToString());
entryDate = (row["EntryDate"].ToString());
ResultValue +=Convert.ToString(this.InsertRows(s_No, v_Name, rate, s_liter, entryDate);
}
if (ResultValue == dt.Rows.Count)
{
// Response.Redirect("BalePack.aspx");
Response.Redirect("Packrpt.aspx?PID=" + I_ID + "");
}
else
{
Response.Write("<script>alert('Not Save'</script>");
}
}
}
private void InsertRows(string s_No, string v_Name, string rate, string s_liter, string entryDate)
{
//con.Open();
using (SqlCommand cmd = new SqlCommand("Sp_InsertInvM", con))
{
cmd.Parameters.AddWithValue("@Action", "INSERTD");
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@I_ID", I_ID);
cmd.Parameters.AddWithValue("@S_Date", entryDate);
cmd.Parameters.AddWithValue("@S_No", s_No);
cmd.Parameters.AddWithValue("@P_Price", rate);
cmd.Parameters.AddWithValue("@S_Liter", s_liter);
con.Open();
cmd.ExecuteNonQuery();
ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", " alert('Duplicate Order Save SuccessFully.'); window.open('SOgvud.aspx');", true);
con.Close();