BWPM/BlazorApp/Pages/Shared/SchuelerList.razor

@using BWPMModels;
<RadzenListBox @bind-Value=@SchuelerID
               Data=@schuelerliste TextProperty="bezeichnung" ValueProperty="id" Style="height:350px"
               Change=@(args => SchuelerChange(args, "schuelerlist")) />

@code {
    [Parameter]
    public EventCallback<string> OnSchuelerChanged { get; set; }

    public List<BWPMModels.Schueler> Schueler { get; set; }
    private Schueler _schueler { get; set; } = new BWPMModels.Schueler();
    public List<Schuelerlist> schuelerliste = new List<Schuelerlist>();
    public int SchuelerID;
    public class Schuelerlist
    {
        public int id { get; set; }
        public string bezeichnung { get; set; }
    }
    protected override async Task OnInitializedAsync()
    {

    }

    public string ReloadData(string KlasseID)
    {
        if (KlasseID == "-1")
        {
            Schueler = BlazorApp.Controller.SchuelerController.GetAllActiveData();
        }
        else{
            Schueler = BlazorApp.Controller.SchuelerController.GetByKlasseID(Convert.ToInt32(KlasseID));
}


        schuelerliste.Clear();

        int newschuelerid = 0;
        bool found = false;
        foreach (BWPMModels.Schueler item in Schueler)
        {
            Schuelerlist sl = new Schuelerlist();
            sl.id = item.ID;
            if (newschuelerid == 0) { newschuelerid = sl.id; };
            //if (SchuelerID == sl.id) { found = true; }
            sl.bezeichnung = item.name + " " + item.vorname;
            schuelerliste.Add(sl);

        };
        SelectFirst();
        return SchuelerID.ToString();

    }


    public Task SchuelerChange(object value, string name)
    {
        var str = value is IEnumerable<object> ? string.Join(", ", (IEnumerable<object>)value) : value; 
        return OnSchuelerChanged.InvokeAsync(str.ToString());
    }

    public string GetSchuelerID()
    {
        return SchuelerID.ToString();
    }

    public void SelectFirst()
    {
        try
        {
            SchuelerID = schuelerliste[0].id;
        }
        catch { SchuelerID = 0; };
    }

}