Xamarin c# - Android -防止在PositiveButton单击时关闭警报对话框

I'm new to Xamarin and I don't know how to do the following in c#. I want to prevent an alertdialog from closing when clicking on the Positive/Negative buttons. I need to do some validation on the input first. If the input is correct, the dialog can close, else I will show a message with instructions. Basically, I have the following code:I'm new to Xamarin and I don't know how to do t