How to select values inside multiselect?

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C#

ASP.NET

Razor

ASP.NET-Core

How to select values inside MultiSelect (Bootstrap)? I want to sellect values inside Colors_SELECT by using a string result.

Issue: I wote the following code but it doesnt select anything. Why `Colors.Add(TempArray[i]);` is not working?

Form

<select asp-for="Colors" asp-items="@Model.Colors_SELECT " class="MultiSelect" multiple>
     ....
</select>

back-end variables

[BindProperty(SupportsGet = true)]
public List<string>? Colors { get; set; }
public SelectList? Colors_SELECT { get; set; }


    public async Task<IActionResult> OnPostAsync()
    {
        var CurrentFilters = new Dictionary<string, string>();
        var ColorsTemp = string.Join(",", Colors);
        CurrentFilters.Add("Colors", ColorsTemp);

        string query = "";
        foreach (var p in CurrentFilters)
        {
            query += $"{p.Key}={p.Value}&";
        }
        query = query.TrimEnd(query[query.Length - 1]); //remove last '&'
        var url = $"{HttpContext.Request.Path}?{query}"; // create URL
        return Redirect(url); // Send new url
    }



public async Task OnGetAsync()
{
    string result = Request.Query["Colors"];
    if (!string.IsNullOrEmpty(result))
    {
        string[] TempArray = result.Split(",");
        for (int i = 0; i < TempArray.Length; i++)
            {
                Colors.Add(TempArray[i]);
            }
    }
}

What I have tried:

following code works but idk understand why cant I add `Colors.add()` inside if statment?

 public async Task OnGetAsync()
    {
-       Colors.Add("Black");

        string result = Request.Query["Colors"];
        if (!string.IsNullOrEmpty(result))
        {
            string[] TempArray = result.Split(",");
            for (int i = 0; i < TempArray.Length; i++)
                {
                    //Colors.Add(TempArray[i]);
                }
        }
    }