How do I return a value in a loop from another class C# WPF

Don't know what you want to do, but this code

public Number()
{
   for (int i = 0, i < 100000, i++)
    {
      return i;
    }
}

can be simplified to

public Number()
{
    return 0;
}

You could try creating an event that fires every iteration and then handling that in the 'listening' class.

Just make sure that if you're updated the UI you do so on the dispatcher thread or you'll get errors.

As mentioned in solution 1, the easiest way to do it would be to use yield operator.

As mentioned in solution 3 and in commented code below, there are a lot of possible improvements that should be made for code. In fact, your code would not even compile.

But since Linq already have an extension method for that, it would be even simpler to reuse that instead of reinventing the wheel.

So something like:

// Ensure that Linq is included...
using System.Linq;

// Add missing class keyword...
public class MainWindow()
{
   // Class constant should be declared as such. Avoid magic numbers in code...
   private const int itemCount = 100000;

   // It is a ggod habit to always specify access (here private)...
   private void ShowNumbers()
   {
      foreach (var item in Enumerable.Range(0, itemCount))
      {
         // By the way, you should give a better name to your 
         // text box variable...
         // You also need to convert your number to a string
         // and apply desired formatting.

         // By the way, it make no sense to update same control text 
         // multiple time in a loop as only last value would be visible!
         txtbox1.Text = item.ToString("N0");

         // Thus you maybe want a combobox instead...
         // We cannot guess from your question!
         // This is WinForms syntax. I don't know WPF syntax by heart.
         comboBox.Items.Add(item.ToString("N0"));
      }
   }
}

Alternatively, if you want your own enumerator (maybe you want something more complex), then the following code would do:

class Numbers
{
   public IEnummerable<int> MyList 
   {
      get 
      {
         // This is just to get the idea. In production code, you would never
         // have hard-coded constants in your code, you would use a function
         // with parameters so that the code can be reused elsewhere...
         for (int i = 0; i < 100000; i++)
         {
            yield return i;
         }
      }
   }
}