基于下拉框onchange事件通过.net控制器类获取和返回数据库字符串数据的代码，以便使用ajax形成页面

<select id="myDropdown" onchange="getData()">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>

<div id="result"></div>

<script>
function getData() {
  var selectedValue = document.getElementById("myDropdown").value;
  
  // 发送ajax请求
  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      document.getElementById("result").innerHTML = xhr.responseText;
    }
  };
  xhr.open("GET", "/Controller/GetData?selectedValue=" + selectedValue, true);
  xhr.send();
}
</script>

public class Controller : ApiController
{
    [HttpGet]
    public string GetData(string selectedValue)
    {
        // 在这里根据selectedValue从数据库中获取数据
        // 假设数据库连接字符串为connectionString
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            connection.Open();
            SqlCommand command = new SqlCommand("SELECT Data FROM MyTable WHERE Value = @selectedValue", connection);
            command.Parameters.AddWithValue("@selectedValue", selectedValue);
            SqlDataReader reader = command.ExecuteReader();
            
            if (reader.Read())
            {
                return reader["Data"].ToString();
            }
            else
            {
                return "No data found";
            }
        }
    }
}