Knockout js , mvc drop down default value is not selecting

问题

it seems so simple but I am unable to do it.

I have bounded a Dropdown list with Json data from c#.

View page

<div class="col-md-6">
    <select data-bind="options: VehicleTypes, optionsText: 'Name', optionsValue:'ID', optionsCaption: 'All', value: VehicleTypeId" class="form-control"></select>
</div>

Json call

Knockout Model

var baseModel = {
    VehicleTypeId: null,
    VehicleTypes: [],
}

 var viewModel =  ko.mapping.fromJS(baseModel);

 $.get(urlPath + "/GetVehicleTypes", function (data) {     
      viewModel.VehicleTypes(data);
 });

It does fill up the drop down and when I select a value in drop down value gets updated for VehicleTypeId.

My problem is I want to set a default value for my drop down list, I have googled it but nothing is workign for me

Data I am getting back in Json is as follow.

[{
    "ID": 32,
    "Name": "Vehicle 1"
}, {
    "ID": 30,
    "Name": "Vehicle 2"
}, {
    "ID": 31,
    "Name": "Vehicle 3"
}]

回答1:

Your VehicleTypes need to be in an observableArray and make sure you apply your bindings:

var baseModel = {
                // data
                VehicleTypeId: ko.observable(null),
                VehicleTypes: ko.observableArray([{
                    "ID": 32,
                    "Name": "Vehicle 1"
                }, {
                    "ID": 30,
                    "Name": "Vehicle 2"
                }, {
                    "ID": 31,
                    "Name": "Vehicle 3"
                }]),
            }

            ko.applyBindings(baseModel);

回答2:

You will have to assign a value to the VehicleTypeId so that the Default value is set.

Here is a working example.

function VehicleTypesViewModel(){
    var self = this;
  self.VehicleTypeId = null;
  self.VehicleTypes = [];   
}    
var viewModel =  new VehicleTypesViewModel();

$.get(urlPath + "/GetVehicleTypes", function (data) {     
  viewModel.VehicleTypeId = 32;
  viewModel.VehicleTypes = data;
  ko.applyBindings(viewModel);
});

来源：https://stackoverflow.com/questions/24146318/knockout-js-mvc-drop-down-default-value-is-not-selecting