c# - i want to get particular field value from json

You can define your custom class:

internal class Data
{
    public Rate rate;
}
internal class Rate
{
    public float combined_rate;
}

And deserialize this string to object:

var s = new JavaScriptSerializer();
var o = s.Deserialize(json);
var value = o.rate.combined_rate;

Also, you can extract other values from your string, just add new fields to Data class.