How to store an object in the viewstate?

问题:

I am using EWS to develop my email client. I found that if I store ItemId in viewstate it will cause an exception says:

Type 'Microsoft.Exchange.WebServices.Data.ItemId' in Assembly 'Microsoft.Exchange.WebServices, Version=14.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' is not marked as serializable.

If I store ItemId as string like:

ViewState["itemId"] = id.ToString();

and then try to cast back,

ItemId id = (ItemId)ViewState["itemId"];

it says I can not convert from string to ItemId. Any idea?

回答1:

As the error message suggests, you can't store an object in viewstate unless it's marked as serializable.

Looking at the documentation here, it seems that the ItemId class has a UniqueId property, which is a string, and a constructor which takes a string 'uniqueId' parameter.

So, can you store the uniqueId in viewstate, and regenerate the object using the constructor?

回答2:

You are storing string and expecting ItemId. You should store as

ItemId itemId = new ItemId();
ViewState["itemId"] = itemId;

However, since ItemId is not seriaziable, it cannot be stored. To store it make your serializable class inherited from ItemId and override all the members and store it in ViewState

[Serializable]
public class MyItemId: ItemId {
 // override all properties 
}

Store like this

MyItemId itemId = new MyItemId();
ViewState["itemId"] = itemId;

and retrieve

MyItemId id=(MyItemId)ViewState["itemId"];

回答3:

Did you try:

ItemId id = new ItemId( ViewState["itemId"] );

http://msdn.microsoft.com/en-us/library/microsoft.exchange.webservices.data.itemid.itemid(EXCHG.80).aspx

回答4:

You cannot store it in ViewState.

However you could probably store it in Session, since it uses the binary formatter.

ViewState is serialized with the LosFormatter class.