private void ThrowInvalidOperandError(string operatorLexeme, ItemType firstType, ItemType secondType)
{
string message = string.Empty;
if (firstType == secondType)
{
message = string.Format(InvalidOperatorInExpression, operatorLexeme, firstType.GetFriendlyName());
}
else
{
message = string.Format(InvalidOperandsInExpression, operatorLexeme,
firstType.GetFriendlyName(), secondType.GetFriendlyName());
}
throw new ExpressionException(message);
}