WPF: C# How to get selected value of Listbox?

since you are using anonymous type to populate the listbox, so you have left with no option then an object type to cast the SelectedItem to. other approach may include Reflection to extract the field/property values.

but if you are using C# 4 or above you may leverage dynamic

    if (LstGroup1.SelectedIndex >= 0 && LstGroup2.SelectedIndex >= 0)
    {
        //use dynamic as type to cast your anonymous object to
        dynamic Booth = LstGroup1.SelectedItem as dynamic;
        //Few things to do 

        //you may use the above variable as
        string BoothID = Booth.BoothID;
        string BoothName = Booth.BoothName:

    }

this may solve your current issue, but I would suggest to create a class for the same for many reasons.

Have you tried :

var Booth = (LstGroup1.SelectedItem);
string ID=Booth.BoothID;
string Name=Booth.BoothName:

I think, you should try one of the following : //EDIT : This solution only works, if Booth is a class/struct

var myBooth = LstGroup1.SelectedItem as Booth;
String id =myBooth.BoothID;
String name=myBooth.BoothName:

or use a generic list with a different type :

public List<Booth> BoothsInGroup1 { get; set; }
....
var myBooth = LstGroup1.SelectedItem;
String id =myBooth.BoothID;
Stringname=myBooth.BoothName:

And if Booth isn't a class yet, add a new class:

public class Booth
{
    public int BoothID { get; set; }
    public String BoothName { get; set; }
}

So u can use it in your generic list, an you can fill it after databasereading

BoothsInGroup1.Add(new Booth
   { 
     BoothID = Convert.ToInt32(da["BoothID"]),
     BoothName = da["BoothName"].ToString() 
   });

You can always create a class called Booth to get the data in the Object Format. From My point of view Dynamic is not the right way of doing this. Once you have the Class Booth in the Solution you can run

Booth Booth1 = LstGroup1.SelectedItem as Booth;
string BoothID1 = Booth1.BoothID;