Sequence contains no matching element

Question

I have an asp net application in which I am using linq for data manipulation  While running  I get the exception  Sequence contains no matching element    if   lstAcl Documents Count  gt  0        for  i   0  i  lt    lstAcl Documents Count - 1  i                  string id    lstAcl Documents i  ID ToString                                       var documentRow    dsACL Documents First o   gt  o ID    id           if  documentRow   null                          lstAcl Documents i  Read   documentRow Read               lstAcl Documents i  ReadRule   documentRow ReadRule                lstAcl Documents i  Create  documentRow Create               lstAcl Documents i  CreateRule   documentRow CreateRule                lstAcl Documents i  Update   documentRow Update               lstAcl Documents i  UpdateRule   documentRow UpdateRule                lstAcl Documents i  Delete   documentRow Delete               lstAcl Documents i  DeleteRule   documentRow DeleteRule

User · Answer

For those of you who faced this issue while creating a controller through the context menu  reopening Visual Studio as an administrator fixed it

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Well  I d expect it s this line that s throwing the exception   var documentRow    dsACL Documents First o   gt  o ID    id    First   will throw an exception if it can t find any matching elements  Given that you re testing for null immediately afterwards  it sounds like you want FirstOrDefault    which returns the default value for the element type  which is null for reference types  if no matching items are found   var documentRow    dsACL Documents FirstOrDefault o   gt  o ID    id    Other options to consider in some situations are Single    when you believe there s exactly one matching element  and SingleOrDefault    when you believe there s exactly one or zero matching elements   I suspect that FirstOrDefault is the best option in this particular case  but it s worth knowing about the others anyway   On the other hand  it looks like you might actually be better off with a join here in the first place  If you didn t care that it would do all matches  rather than just the first  you could use   var query   from target in  lstAcl Documents             join source in  dsAcl Document             where source ID ToString   equals target ID             select new   source  target    foreach  var pair in query        target Read   source Read      target ReadRule   source ReadRule         etc     That s simpler and more efficient IMO   Even if you do decide to keep the loop  I have a couple of suggestions    Get rid of the outer if  You don t need it  as if Count is zero the for loop body will never execute Use exclusive upper bounds in for loops - they re more idiomatic in C    for  i   0  i  lt   lstAcl Documents Count  i     Eliminate common subexpressions   var target    lstAcl Documents i      Now use target for the rest of the loop body  Where possible use foreach instead of for to start with   foreach  var target in  lstAcl Documents

User · Answer

Maybe using Where   before First   can help you  as my problem has been solved in this case     var documentRow    dsACL Documents Where o   gt  o ID    id  FirstOrDefault

User · Answer

From the MSDN library   The First lt TSource gt  IEnumerable lt TSource gt   method throws an exception if source contains no elements  To instead return a default value when the source sequence is empty  use the FirstOrDefault method

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Use FirstOrDefault  First will never return null - if it can t find a matching element it throws the exception you re seeing    dsACL Documents FirstOrDefault o   gt  o ID    id

[c#] Sequence contains no matching element

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