Operation is not valid due to the current state of the object when I select a dropdown list

Question

I have radcombo boxes on aspx page  and when I select any option from then it gives error Server Error in     Application  Operation is not valid due to the current state of the object  Description  An unhandled exception occurred during the execution of the current web request  Please review the stack trace for more information about the error and where it originated in the code   Exception Details  System InvalidOperationException  Operation is not valid due to the current state of the object   Source Error   An unhandled exception was generated during the execution of the current web request  Information regarding the origin and location of the exception can be identified using the exception stack trace below   Stack Trace    InvalidOperationException  Operation is not valid due to the current state of the object   System Web HttpValueCollection ThrowIfMaxHttpCollectionKeysExceeded    2692302    System Web HttpValueCollection FillFromEncodedBytes Byte   bytes  Encoding encoding   61 System Web HttpRequest FillInFormCollection    148  HttpException  0x80004005   The URL-encoded form data is not valid   System Web HttpRequest FillInFormCollection    206 System Web HttpRequest get Form    68 System Web HttpRequest get HasForm    8735447 System Web UI Page GetCollectionBasedOnMethod Boolean dontReturnNull   97    System Web UI Page DeterminePostBackMode    63    System Web UI Page ProcessRequestMain Boolean includeStagesBeforeAsyncPoint  Boolean includeStagesAfterAsyncPoint   133    Version Information  Microsoft  NET Framework Version 2 0 50727 3634  ASP NET Version 2 0 50727 3634

User · Answer

I know an answer has already been accepted for this problem but someone asked in the comments if there was a solution that could be done outside the web config  I had a ListView producing the exact same error and setting EnableViewState to false resolved this problem for me

User · Answer

From http   codecorner galanter net 2012 06 04 solution-for-operation-is-not-valid-due-to-the-current-state-of-the-object-error      Issue happens because Microsoft Security Update MS11-100 limits number   of keys in Forms collection during HTTP POST request  To alleviate   this problem you need to increase that number       This can be done in your application Web Config in the  lt appSettings gt    section  create the section directly under  lt configuration gt  if it   doesn   t exist   Add 2 lines similar to the lines below to the    section    lt add key  aspnet MaxHttpCollectionKeys  value  2000    gt   lt add key  aspnet MaxJsonDeserializerMembers  value  2000    gt        The above example set the limit to 2000 keys  This will lift the   limitation and the error should go away

User · Answer

This can happen if you call    SingleOrDefault      on an IEnumerable with 2 or more elements

[c#] Operation is not valid due to the current state of the object, when I select a dropdown list

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