Question

This foreach loop works fine while testing and only returning 5 rows of data, but I am well aware of how porly it is written , is there a better way, possibly using stringbuilder to re-write this more efficiently?

               SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["connstring"].ConnectionString);
    SqlCommand comm = new SqlCommand("SELECT Title, StartDate FROM tblEvents JOIN eo_UserEventWatch ON eo_UserEventWatch.EventID=tblEvents.ID WHERE eo_UserEventWatch.UserID = @GUID ;", conn);
    comm.Parameters.AddWithValue("GUID", userID);
    conn.Open();
    SqlDataAdapter da = new SqlDataAdapter(comm);
    DataTable dt = new DataTable();
    da.Fill(dt);
    string result ="{ \"event\" :[";
    foreach (DataRow dr in dt.Rows)
    {
        result += "{\"title\" : \"" + dr[0].ToString() + "\" , \"start\" : \"" + dr[1].ToString() +"\"} ,";
    }
    result = result.TrimEnd(',');
    result += "] }";
    return result;

Answer 1

Well yes, it would be more efficient to use StringBuilder:

StringBuilder builder = new StringBuilder("{ \"event\" :[");
foreach (DataRow row in dt.Rows)
{
    // Alternatively, use AppendFormat
    builder.Append("{\"title\" :\"");
           .Append(row[0])
           .Append("\", \"start\" : \"")
           .Append(row[1])
           .Append("\"} ,");
}
if (builder[builder.Length - 1] == ',')
{
    builder.Length -= 1;
}
builder.Append("] }");
string result = builder.ToString();

However, it’s still not nice code – because you’ve got all that horrible literal JSON. I would suggest using Json.NET or another JSON library. You can probably use LINQ at that point, e.g.

var result = new { event = dt.AsEnumerable()
                             .Select(r => new { 
                                 title = r.Field<string>(0),
                                 start = r.Field<DateTime>(1))
                             .ToArray() };
// Or whatever, depending on the library you use
var json = JsonSerializer.ToJson(result);

Aside from anything else, now you don’t need to worry about the format of the start value, or whether the title contains quotes etc.

(EDIT: As noted, you should absolutely have using statements for SQL connections etc. That’s outside the main point of the question, which is why I didn’t mention it here.)