xaml - Silverlight 4.0表单初始化问题

Question

我在 Silverlight 4.0 页面之一上遇到了一个非常奇怪的错误。我有一个带有“保存”按钮的表单，默认情况下该按钮处于禁用状态。此表单由一堆用户指定的默认值填充，这些默认值来自异步服务器调用(下面的MyFacade.getFormDefaults)。当用户更改其中一个字段(填充后)时，我希望启用“保存”按钮。

我认为我的逻辑是正确的，但我遇到了一个非常奇怪的错误，我找不到太多有用的信息。错误是:System.InvalidOperationException: The initialization of an object or value resulted in an object or value being accessed recursively before it was fully initialized.

下面是我所拥有的非常简化的版本...

profile.fs:

type profile() as this =
    inherit UriUserControl("/whatever;component/profile.xaml", "profile")

    [<DefaultValue>]
    val mutable isFormLoaded : bool
    [<DefaultValue>]
    val mutable btnSave : Button
    [<DefaultValue>]
    val mutable txtEmail : TextBox

    // constructor
    do
        this.isFormLoaded <- false

        // make the "this" values point at the XAML fields
        this.btnSave <- this?btnSave
        this.txtEmail <- this?txtEmail

        // get the form defaults and send them to
        MyFacade.getFormDefaults(new Action<_>(this.populateFormDefaults))
        ()

    member this.populateFormDefaults (formDefaults : MyFormDefaultsUIVO array option) =
        // populate this.txtEmail with the default value here
        this.isFormLoaded <- true // set the form to be loaded once that's done
        ()

    // enable the "Save" button when the user modifies a form field
    member this.userModifiedForm (sender : obj) (args : EventArgs) =
        // **** EXCEPTION OCCURS ON THE LINE BELOW ****
        if this.isFormLoaded then
            this.btnSave.IsEnabled <- true
        ()

profile.xaml:

<nav:Page Name="profile" Loaded="formLoaded">
    <TextBox Name="txtEmail" TextChanged="userModifiedForm "/>
    <Button Name="btnSave" IsEnabled="False"/>
</nav:Page>

即使我摆脱了所有的isFormLoaded逻辑，只需设置 this.btnSave.IsEnabled <- true this.userModifiedForm里面， 我犯了同样的错误。任何想法将不胜感激。谢谢。

Answer 1

当在完全初始化之前访问对象时，F# 运行时会生成异常“对象或值的初始化导致对象或值在完全初始化之前被递归访问”。

访问 this - 是否在构造函数运行之前检查 isFormLoaded 或 btnSave.IsEnabled - 将导致错误。您是否已验证 userModifiedForm 仅在构造函数之后调用？