无法将类型类的对象强制转换为“ System.String”类型

Question

Class 1

public class KlasaBetonaaaa
     {
         public string Naziv { get; set; }
         public double _Fck { get; set; }
     }

Class 2

 public static class Database
     {
         public static KlasaBetonaaaa[] GetAllKlasaBetona()
         {
             return new KlasaBetonaaaa[]
             {
                 new KlasaBetonaaaa
                 {
                     Naziv = "C12/15",
                     _Fck = 12.0
                 };
             }
         }
     }

Class 3

public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();
            {

            var klaseBetona = Database.GetAllKlasaBetona();

            klasaBetonaComboBox.Items.AddRange(klaseBetona);

            }


private void klasaBetona_SelectedIndexChanged(object sender, EventArgs e)
        {
            var selectedBeton = klasaBetonaComboBox.SelectedItem;
            CalculateSomething((string)selectedBeton);
        }


        private void CalculateSomething(string selectedBeton)
        {
            fck.Text = selectedBeton;
        }
    }

This string selectedBeton is what i tried before with only 1 name converting to string from collection. Now i created new classes (Manual database)as and array and i dont know how to link it so when i pick "C12/15" to get _Fck value 15.0 in a label.

Answer 1

klasaBetonaComboBox.SelectedItem is of type KlasaBetonaaaa , since you add an array of that type ( klasaBetonaComboBox.Items.AddRange(klaseBetona); ). That means that casting SelectedItem to string will fail.

You should cast the selected item to KlasaBetonaaaa and cast the Fck property, which is a double , using ToString (or if you want to take the Naziv string property, you don't need ToString() , since it is one already):

KlasaBetonaaaaselectedBeton = klasaBetonaComboBox.SelectedItem;
CalculateSomething(selectedBeton.Fck.ToString());

Answer 2

It seems that you have used an IEnumerable<KlasaBetona> as datasource for the combobox. Now you're trying to cast the SelectedItem which is a single instance of KlaseBetona to string . That doesn't work since you haven't overloaded the explicit -conversion operator like this:

public static explicit operator string(KlasaBetona kl)  
{
    return kl._Fck.ToString();
}

But instead of that i suggest to cast the SelectedItem to KlasaBetona and use that property in the first place:

KlasaBetona kl = (KlasaBetona) selectedBeton;
CalculateSomething(kl._Fck.ToString());

Answer 3

You need to cast to the actual type and then show the string of _Fck , you could always override the ToString method of your class and show that instead. Alternatively, if you do calculations on the object then you can pass that into the method instead.

var selectedBeton = klasaBetonaComboBox.SelectedItem as KlasaBetonaaaa;
if(selectedBeton != null)
    CalculateSomething(selectedBeton._Fck.ToString());

Answer 4

In my case I got similar message Unable to cast object of type class to type Enum when I use JsonConverter to Serialize class.

Finally I found that I forgot to remove attribute [Newtonsoft.Json.JsonConverter(typeof(Newtonsoft.Json.Converters.StringEnumConverter))] on my string property, after remove the attribute, the problem is solved.