How to implement a custom AlertDialog View

You are correct, it's because you didn't manually inflate it. It appears that you're trying to "extract" the "body" id from your Activity's layout, and that won't work.

You probably want something like this:

LayoutInflater inflater = getLayoutInflater();
FrameLayout f1 = (FrameLayout)alert.findViewById(android.R.id.body);
f1.addView(inflater.inflate(R.layout.dialog_view, f1, false));

You can create your view directly from the Layout Inflater, you only need to use the name of your layout XML file and the ID of the layout in file.

Your XML file should have an ID like this:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:id="@+id/dialog_layout_root"
              android:orientation="vertical"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:padding="10dp"
              />

And then you can set your layout on the builder with the following code:

LayoutInflater inflater = getLayoutInflater();
View dialoglayout = inflater.inflate(R.layout.dialog_layout, null);
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setView(dialoglayout);
builder.show();