public string GetFunctionName( 
   SQLFunction sqlFunction
)

public async Task UsingSqlSyntaxToFormPlatformAgnosticQueries()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("C:\\Data\\LocalGovernment.gdb"))))
    using (FeatureClass featureClass = geodatabase.OpenDataset<FeatureClass>("FacilitySite"))
    {
      SQLSyntax sqlSyntax = geodatabase.GetSQLSyntax();
      string substringFunctionName = sqlSyntax.GetFunctionName(SQLFunction.Substring);
      string upperFunctionName = sqlSyntax.GetFunctionName(SQLFunction.Upper);
      string substringfunction = string.Format("{0}({1}(FCODE, 1, 6)) = 'SCHOOL'", upperFunctionName, substringFunctionName);

      QueryFilter queryFilter = new QueryFilter
      {
        WhereClause = substringfunction
      };
      using (Selection selection = featureClass.Select(queryFilter, SelectionType.ObjectID, SelectionOption.Normal))
      {
        // work with the selection.
      }
    }
  });
}