DialogActivity

DialogActivity.cs

[Activity(Label = "DialogActivity", Theme = "@style/PopupTheme")]
public class DialogActivity : Activity
{
    protected override void OnCreate (Bundle bundle)
	{
	    base.OnCreate (bundle);
	    
	    ShowAsPopup(this, Resource.Layout.Main);
	    
	}
	
	public override bool OnCreateOptionsMenu (IMenu menu)
	{
		MenuInflater.Inflate(Resource.Menu.dialog_menu, menu);
		return base.OnCreateOptionsMenu (menu);
	}
	
    public void ShowAsPopup(Activity activity, int resId)
    {
        activity.RequestWindowFeature(WindowFeatures.ActionBar);
        activity.Window.SetFlags(WindowManagerFlags.DimBehind, WindowManagerFlags.DimBehind);

        var param = activity.Window.Attributes;
        param.Height = 400;
        param.Width = 300;
        param.Alpha = 1.0f;
        param.DimAmount = 0.5f;
        activity.Window.Attributes = param;
        SetContentView(resId);
    }
}

styles.xml

<?xml version="1.0" encoding="utf-8" ?>
<resources>
  <style name="PopupTheme" parent="android:Theme.Holo.Light.Dialog">
    <item name="android:windowIsFloating">false</item>
    <item name="android:windowContentOverlay">@null</item>
    <item name="android:windowSoftInputMode">stateAlwaysHidden</item>
    <item name="android:windowActionModeOverlay">true</item>
    <item name="android:windowIsTranslucent">true</item>
  </style>
</resources>