jquery 发送post请求访问webapi，无法接到参数

前台代码：

function StopDevice(key) {
$.ajax({
type: "Post",
url: dcmApi + "/dcm/StopDevice",
//data:{key:key},
data: JSON.stringify({ "key": key }),   //这里需要转成字符串才可以
contentType: "application/json",        //这里还需要添加这句话
success: function (r) {
var result = JSON.parse(r);
if (result.IsSuccess) {
RefreshState();
} else {
layer.msg(result.Message);
}
}
})
}

后台代码：

[HttpPost]
public string SStopDevice([FromBody]string key)
{
OpResult r = new Model.OpResult();
DeviceModel device = Main.DeviceList.FirstOrDefault(p => p.Key == key);
if (device == null)
{
r.Message = "停止失败！可能原因是没有找到key为" + key + "的驱动";
}
else
{
if (device.State == "停止")
{
r.Message = "当前驱动已在停止中";
}
else
{
device.KeepState = "0";//如果停止，则不进行线程维护
Main.StopDeviceList.Add(device);

Thread.Sleep(1000);
r.IsSuccess = true;
r.Message = device.Msg;
}
}
return JsonConvert.SerializeObject(r);
}