如何在dialog中获取edittext的值

代码如下，这样获取edittext.gettext（）.tostring（）值总为空，该怎么改呢？

               final View viewDialog=(View)getLayoutInflater().inflate(R.layout.ad_dialog,null);
                new AlertDialog.Builder(Ad_Delete.this).setTitle("请输入删除数量（不能大于现有本数）")
                        .setView(getLayoutInflater().inflate(R.layout.ad_dialog, null))
                        .setPositiveButton("确定", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {

                                EditText ad_delete_edit = (EditText)viewDialog.findViewById(R.id.ad_delete_number);
                                int deleteNumber = Integer.parseInt(ad_delete_edit.getText().toString());

                                if (deleteNumber > nowNumber || deleteNumber < 0) {
                                    Toast.makeText(Ad_Delete.this, "输入数量有误，请输入不大于现有数量的数目", Toast.LENGTH_LONG).show();
                                } else {
                                    sqLiteOpenHelper_ad.getReadableDatabase().execSQL("updata Administrator_inf set book_totalnumber = ?, " +
                                                    "book_nownumber = ? where _id = ?",
                                            new Integer[]{totalNumber - deleteNumber, nowNumber - deleteNumber, id});
                                    Toast.makeText(Ad_Delete.this, "删除成功！", Toast.LENGTH_LONG).show();
                                }
                            }
                        }).show();
            }
        });