Examples of errors detected by the V3146 diagnostic

V3146. Possible null dereference. A method can return default null value.

Azure SDK for .NET

V3146 Possible null dereference of 'value'. The 'FirstOrDefault' can return default null value. BatchSharedKeyCredential.cs 127

public override Task
  ProcessHttpRequestAsync(HttpRequestMessage httpRequest, ....)
{
  ....
  foreach (string canonicalHeader in customHeaders)
  {
    string value = httpRequest.Headers.
                   GetValues(canonicalHeader).FirstOrDefault();
    value = value.Replace('\n', ' ').Replace('\r', ' ').TrimStart();
    ....
  }
  ....
}

osu!

V3146 [CWE-476] Possible null dereference of 'ruleset'. The 'FirstOrDefault' can return default null value. APILegacyScoreInfo.cs 24

public ScoreInfo CreateScoreInfo(RulesetStore rulesets)
{
  var ruleset = rulesets.GetRuleset(OnlineRulesetID);

  var mods = Mods != null ? ruleset.CreateInstance()
                                   .GetAllMods().Where(....)
                                   .ToArray() : Array.Empty<Mod>();
  ....
}

public RulesetInfo GetRuleset(int id) =>
  AvailableRulesets.FirstOrDefault(....);

RavenDB

V3146 Possible null dereference of 'mainSchema'. The 'Tables.FirstOrDefault' can return default null value. DatabaseSchema.cs(31) Raven.Server

public HashSet<string> FindSpecialColumns(string tableSchema, string tableName)
{
  var mainSchema = GetTable(tableSchema, tableName);

  var result = new HashSet<string>();
  mainSchema.PrimaryKeyColumns.ForEach(x => result.Add(x)); // <=

  foreach (var fkCandidate in Tables)
    foreach (var tableReference in fkCandidate.References.Where(
        x => x.Table == tableName && x.Schema == tableSchema
      )
    )
    {
      tableReference.Columns.ForEach(x => result.Add(x));
    }

  return result;
}

public TableSchema GetTable(string schema, string tableName)
{
  return Tables.FirstOrDefault(
    x => x.Schema == schema && x.TableName == tableName
  );
}

OpenRA

V3146 Possible null dereference of 'tooltip'. The 'FirstOrDefault' can return default null value. Disguise.cs 192

public void DisguiseAs(Actor target)
{
  ....
  var tooltip = target.TraitsImplementing<ITooltip>().FirstOrDefault();
  AsPlayer = tooltip.Owner;
  AsActor = target.Info;
  AsTooltipInfo = tooltip.TooltipInfo;
  ....
}

ONLYOFFICE Community Server

V3146 Possible null dereference. The 'listTags.FirstOrDefault' can return default null value. FileMarker.cs 299

public static void RemoveMarkAsNew(....)
{
  ....
  var listTags = tagDao.GetNewTags(userID, (Folder)fileEntry, true).ToList();
  valueNew = listTags.FirstOrDefault(tag => tag.EntryId.Equals(....)).Count;
  ....
}

Similar errors can be found in some other places:

• V3146 Possible null dereference. The 'FirstOrDefault' can return default null value. CalendarApi.cs 335

ILSpy

V3146 Possible null dereference. The 'FirstOrDefault' can return default null value. ILSpy.BamlDecompiler BamlResourceEntryNode.cs 76

bool LoadBaml(AvalonEditTextOutput output, CancellationToken cancellationToken)
{
  var asm = this.Ancestors().OfType<AssemblyTreeNode>()
                            .FirstOrDefault().LoadedAssembly;       // <=
  ....
  return true;
}