FirstOrDefault returns NullReferenceException if no match is found

Question

Here is my code   string displayName   Dictionary FirstOrDefault x   gt  x Value ID    long Parse options ID   Value DisplayName    The code works fine if x Value ID matches options ID  However  I get a NullReferenceException if it doesn t

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To add to the solutions  here is a LINQ statement that might help  Utilities DIMENSION MemTbl Where a   gt  a DIMENSION ID    format ContentBrief DimensionID  Select a  gt a DIMENSION1  DefaultIfEmpty     FirstOrDefault      The result will be an empty string if the result of the query is a null

User · Answer

Use the SingleOrDefault   instead of FirstOrDefault

User · Answer

i assume you are working with nullable datatypes  you can do something like this   var t   things Where x   gt  x  null  amp  amp  x Value ID    long Parse options ID   FirstOrDefault    var res   t    null        t Value

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Simply use the question mark trick for null checks  string displayName   Dictionary FirstOrDefault x   gt  x Value ID    long Parse options ID    Value DisplayName     quot DEFINE A DEFAULT DISPLAY NAME HERE quot

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That is because FirstOrDefaultcan return null causing your following  Value to cause the exception  You need to change it to something like   var myThing   things FirstOrDefault t   gt  t Id    idToFind    if myThing    null      return     we failed to find what we wanted var displayName   myThing DisplayName

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FirstOrDefault returns the default value of a type if no item matches the predicate  For reference types that is null   Thats the reason for the exception   So you just have to check for null first   string displayName   null  var keyValue   Dictionary      FirstOrDefault x   gt  x Value ID    long Parse options ID    if keyValue     null        displayName   keyValue Value DisplayName       But what is the key of the dictionary if you are searching in the values  A Dictionary lt tKey TValue gt  is used to find a value by the key  Maybe you should refactor it   Another option is to provide a default value with DefaultIfEmpty   string displayName   Dictionary      Where kv   gt  kv Value ID    long Parse options ID        Select kv   gt  kv Value DisplayName       not a problem even if no item matches      DefaultIfEmpty  --Option unknown--      or no argument - gt  null      First                                   cannot cause an exception

User · Answer

You can use a combination of other LINQ methods to handle not matching condition   var res   dictionary Where x   gt  x Value ID    someID                       Select x   gt  x Value DisplayName                       DefaultIfEmpty  Unknown                        First

[c#] FirstOrDefault returns NullReferenceException if no match is found

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