Dynamic dropdown from another spreadsheet - google-apps-script

Question

With this script I can auto populate a dropdown from a spreadsheet to another but it is limited to import in the dropdown only 500 values, I need it for more values.
In addition, when I run it, it works on the cells that I have selected in the destination sheet.
How to set up a specific range for the dropdown, for example the entire column C?
function importSheetA() {
return SpreadsheetApp.openById('xxxxx')
.getSheetByName('xxxxx')
.getRange('xxxxx')
.getValues()
.flat(); // This ensures a simple array is returned
}
function populateDropdown() {
var values = importSheetA();
var rule = SpreadsheetApp.newDataValidation()
.requireValueInList(values, true)
.setAllowInvalid(false)
.build();
var range = SpreadsheetApp.getActiveRange();
range.setDataValidation(rule);
}

Answer 1

Try this:
function importSheetA() {
const ss=SpreadsheetApp.openById('ssid');
const sh=ss.getSheetByName('Sheet1');
const rg=sh.getRange(1,3,sh.getLastRow());
return rg;//passing range now
}
function populateDropdown() {
const rg=importSheetA();
const rule=SpreadsheetApp.newDataValidation().requireValueInRange(rg).setAllowInvalid(false).build();
const range=SpreadsheetApp.getActiveRange();
range.clearDataValidations();
range.setDataValidation(rule);
}
Perhaps you could implement something like this as a workaround:
function importSheetA() {
const ss=SpreadsheetApp.openById('ssid')
const sh=ss.getSheetByName('Sheet1');
const rg=sh.getRange(1,3,sh.getLastRow());
return rg.getValues();
}
function populateDropdown() {
const ss=SpreadsheetApp.getActive();
const sh=ss.getSheetByName('validationsheet');//local sheet for storing remote validation values
sh.getDataRange().clear();//optional it is noticeably slower
const v=importSheetA();
const rg=sh.getRange(1,1,v.length,1);
rg.setValues(v);
const rule=SpreadsheetApp.newDataValidation().requireValueInRange(rg).setAllowInvalid(false).build();
const range=SpreadsheetApp.getActiveRange();
range.clearDataValidations();
range.setDataValidation(rule);
}

Answer 2

Explanation:
I would like to assume that your list does not consist of more
than 500 unique items. I might well be mistaken though.
The following script will consider the unique values in your data and if the total number of unique values is less than 500, your issue will be resolved.
To find the unique list of values you can use filter:
var uniqueValues = values.filter((v, i, a) => a.indexOf(v) === i);
Solution:
Change populateDropdown to this:
function populateDropdown() {
var values = importSheetA();
var uniqueValues = values.filter((v, i, a) => a.indexOf(v) === i); // <= New code
var rule = SpreadsheetApp.newDataValidation()
.requireValueInList(uniqueValues, true) // <= New code
.setAllowInvalid(false)
.build();
var range = SpreadsheetApp.getActiveRange();
range.setDataValidation(rule);
}