Returning a json string

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C#

RestAPI

I have this in c#

string jsonResultado = await Clases.AccesoDatos.JsonData("SELECT * FROM vistatransacciones for json path, ROOT('product.json')");

where method jsonData is:
  public static async Task<string> JsonData(string SQL)
        {
            string json = "";
            using (SqlConnection con = new SqlConnection(cadenaConexion))
            {
                SqlCommand cmd = new SqlCommand(SQL, con);
                await con.OpenAsync();
                SqlDataReader dr = await cmd.ExecuteReaderAsync();                
                if (dr.HasRows)
                {
                    DataTable jsondt = dr.GetSchemaTable();                    
                    json= JsonConvert.SerializeObject(jsondt);
                }
                
                dr.Close();
            }
            
            return json;
        }

In fact, after the if(dr.hasrows) i do not know what to do, i just want to read following result in my program:

{"Product.json":
[{"idRegistro":1,
"idUsuario":"1",
"FechaRegistro":"2023-01-18T11:29:23.030",
.....

But i do not know how to return the value

What I have tried:

codeproject paper, manuals, microsoft