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.
            }
        }
    });
}