That seems to be a custom message box. Hence in C# (if you are using Windows Forms), you could use MessageBox.Show as a replacement for ConfirmDeleteDialog.showMessage. The return value in C# is a DialogResult.
Thanks for replaying to my java question. I'm using WPF Window Form (not the Windows Forms). Is there anyway you could help me to convert that java code I'm very new to programming and I don't have a clue how to do it honestly...
If you are new to programming then why are you concerned about trying to convert Java to C# when creating a WPF program? Forget Java and spend some time studying how to do things properly in WPF. There are lots of useful samples in the WPF articles section[^].
One of these days I'm going to think of a really clever signature.