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Editorial Team
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Asked: 2026-06-01T00:37:00+00:00

When running one of my Visual Studio web performance tests, I noticed intermittent exceptions

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When running one of my Visual Studio web performance tests, I noticed intermittent exceptions when trying to decode viewstate:

Message: Invalid length for a Base-64 char array.

Call Stack: at System.Convert.FromBase64String(String s) at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) at System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) at System.Web.UI.HiddenFieldPageStatePersister.Load()

This only happens on one test at a single spot. Doing some research I noticed that the viewstate sent by the client and the viewstate received by the server differed only in that all plus (+) characters became spaces ( ).

What is causing this?

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1 Answer

  1. Editorial Team

    ASP.NET expects that the _VIEWSTATE hidden form field is URL-encoded before it reaches the server. When ASP.NET receives a request, it URL-decodes the _VIEWSTATE parameter, which is known to turn pluses into spaces.

    Setting the URL Encode property to true on the _VIEWSTATE hidden form field in the webtest caused the bug to go away.

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