Why am I losing object references on the postback?

问题

I am developing an asp.net (3.5) application and I am puzzled with the behavior of the postbacks.

Consider the following scenario: I have a web user control that is basically a form. However each form field is a web user control in itself.

In the click event of the save button I iterate through all controls in my form and I retrieve the field value and the field name that refers to the database field that I am saving the value to.

The click event triggers a postback and it is during the postback that I visit the controls and here is the funny thing: the property value for the database field has become null! Could anyone shed a light here?

Here is some basic code:

[Serializable]
public partial class UserProfileForm : CustomIntranetWebappUserControl
{
    protected override void OnInit(EventArgs e)
    {
        //AutoEventWireup is set to false
        Load += Page_Load;
        CancelLinkButton.Click += CancelButtonClickEvent;
        SaveLinkButton.Click += SaveButtonClickEvent;
        base.OnInit(e);
    }

    private void SaveButtonClickEvent(object sender, EventArgs e)
    {
        VisitFormFields();
    }

    private void VisitFormFields()
    {
        var userProfileVisitor = new UserProfileVisitor();

        foreach (var control in Controls)
        {
            if (control is FormFieldUserControl)
            {
                var formField = (FormFieldUserControl) control;
                formField.Visit(userProfileVisitor);
            }
        }
        userProfileVisitor.Save();
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            BindText();
        }
    }

    private void BindText()
    {
        LastNameFormLine.LabelText = string.Format("{0}:", HomePage.Localize("Last Name"));
        LastNameFormLine.InputValue = UserProfile.LastName;
        LastNameFormLine.IsMandatoryField = true;
        LastNameFormLine.IsMultilineField = false;
        LastNameFormLine.ProfileField = "UserProfile.LastName";
        //... the rest of this method is exactly like the 4 lines above.
    }
}

[Serializable]
public abstract class FormFieldUserControl : CustomIntranetWebappUserControl
{
    public string ProfileField { get; set; }
    public abstract void Visit(UserProfileVisitor userProfileVisitor);
}


[Serializable]
public partial class FormLineTextBox : FormFieldUserControl
{
//...  irrelevant code removed... 

    public override void Visit(UserProfileVisitor userProfileVisitor)
    {
        if (userProfileVisitor == null)
        {
            Log.Error("UserProfileVisitor not defined for the field: " + ProfileField);
            return;
        }
        userProfileVisitor.Visit(this);
    }
}

[Serializable]
public class UserProfileVisitor
{

    public void Visit(FormLineTextBox formLine)
    {
        // The value of formLine.ProfileField is null!!!
        Log.Debug(string.Format("Saving form field type {1} with profile field [{0}] and value {2}", formLine.ProfileField, formLine.GetType().Name, formLine.InputValue));
    }

    // ... removing irrelevant code... 

    public void Save()
    {
        Log.Debug("Triggering the save operation...");
    }
}

回答1:

Remember ASP.NET is stateless. Any properties created are destroyed after the page has been render to the browser. So you have to recreate objects on each post back or store them in View, Session, or Application State.

When you do a property you have to tell it to save the view state it does not do it automatically. Here is a sample of a view state property.

public string SomePropertyAsString
{
    get
    {
        if (this.ViewState["SomePropertyAsString"] == null)
            return string.Empty;

        return (string)this.ViewState["SomePropertyAsString"];
    }
    set { this.ViewState["SomePropertyAsString"] = value; }
}

public MyCustomType ObjectProperty
{
    get
    {
        if (this.ViewState["ObjectProperty"] == null)
            return null;

        return (MyCustomType)this.ViewState["ObjectProperty"];
    }
    set { this.ViewState["ObjectProperty"] = value; }
}

回答2:

First guess would be that BindText() shouldn't be in Page_Load, but in Page_Init, so the control state will be saved.

回答3:

@David Basarab, this is not true afaik, and was only the case in .Net 1.1, in .Net2 and up this is all handled by the framework if you do all the magic stuff in the Init.

回答4:

Your problem is that 'ProfileField' isn't available on the Postback, right?

The solution is to store the value for that in ViewState (instead of an auto-implemented property). Without that, it won't be available on the postback.

来源：https://stackoverflow.com/questions/1378924/why-am-i-losing-object-references-on-the-postback

标签

ASP.NET

postback

object-reference