datatable-to-json.cs

StringBuilder jsonData = new StringBuilder();
jsonData.Append("[");
for (int i = 0; i < dt.Rows.Count; ++i)
{
    jsonData.Append("{");
    jsonData.Append("\"" + "DNI" + "\":" + "\"" + dt.Rows[i][0] + "\"" + "," + "\"" + "Name" + "\":" + "\"" + dt.Rows[i][1] + "\"" + ",");
    jsonData.Append("\"" + "Telephone" + "\":" + "\"" + dt.Rows[i][2] + "\"" + "," + "\"" + "Observations" + "\":" + "\"" + dt.Rows[i][3] + "\"");

    if (i == dt.Rows.Count - 1) { jsonData.Append("}"); }
    else { jsonData.Append("},"); }
}

jsonData.Append("]");
string jsonArray = jsonData.ToString();