Binding Directions

You may need to customize which properties are transferred from the server to client or from client to the server when using the command bindings. In many cases, there is no need to transfer the whole viewmodel in both directions.

You can use the [Bind(...)] attribute to define which properties are transferred in which direction.

[Bind(Direction.ServerToClient)]
public string FirstName { get; set; }

Bind Attribute

The Direction is an enumeration with the following options:

• Both is the default setting. The property value is transferred in both ways. Use this setting e.g. for values the user enters in the form controls, e.g. TextBox.

• None the property value is not transferred either from the server to the client nor from the client to the server.

• ServerToClient transfers the value from the server to the client. This is useful e.g. for a property representing the list of items in the ComboBox when the collection of items can be changed depending on the other form fields.

• ServerToClientFirstRequest transfers the value from the server to the client on the initial page load, not during postbacks. This is useful e.g. for a property representing the list of items in the ComboBox when the collection is static and cannot be changed. It is transferred only once.

• ClientToServer transfers the value only from the client to the server. You can use this for the values of the form fields which can only be read on the server (e.g. value in the search box). However, this setting is not used very frequently.

• IfInPostBackPath is a special setting that allows to transfer only part of the viewmodel from the server to the client. Consider a page with a GridView control with 20 rows representing the users. The viewmodel contains a collection of 20 objects - one for each user. If you have a button in each row that can e.g. enable or disable the user, you can use this setting to transfer only the single object from the collection.

<dot:GridView DataSource="{value: Users}">
    <dot:GridViewTextColumn ValueBinding="{value: UserName}" HeaderText="User Name" />
    <dot:GridViewCheckBoxColumn ValueBinding="{value: IsEnabled}" HeaderText="Is Enabled?" />
    <dot:GridViewTemplateColumn>
        <dot:LinkButton Text="Enable / Disable" Click="{command: _parent.EnableDisableUser(_this)}" />
    </dot:GridViewTemplateColumn>
</dot:GridView>

If you use the [Bind(Direction.IfInPostBackPath)] on the Users property in the viewmodel, only the user from the row which was accessed, will be transferred. The viewmodel that will be posted to the server will look like this:

{
    "Users": [
        ...
        null,
        { 
            "UserName": "the user being edited",
            "IsEnabled": true
        },
        null,
        ...
    ]
}

Please be careful when using non-default binding directions. Some properties of the viewmodel might not be initialized and may contain null values!

Sending the entire viewmodel to the server and back may be inefficient in many real-world scenarios. DotVVM offers different ways of calling methods on the server which don't require presence of the viewmodel. See Optimizing PostBacks page for more info.