Xamarin Android - CheckBox Introduction


A CheckBox is type of a CompoundButton that has two states: Checked or UnChecked.

CheckBoxes are a popular way of representing Boolean values in Forms. Not only do they get used in mobile applications but also in desktop as well as online forms.

A CheckBox is a CompoundButton in that it has two states. Also it does derive from the Android.Widget.CompoundButton class.

Xamarin Android CheckBox

The CompoundButton is an abstract class defined in the Android.Widget package that acts as the interface definition for a callback to be invoked when the checked state of its child is changed.

The CompoundButton itself is a Button so CheckBox itself will inherit capabilities for Butttons.

CheckBox Definition

The CheckBox resides in the Android.Widget namespace.

namespace Android.Widget

It derives from Android.Widget.CompoundButton:

public class CheckBox : Android.Widget.CompoundButton

The namespace resides in the Mono.Android assembly.

Important Properties and Methods

Here are some of the important properties of CheckBox

Property Type Description
public virtual bool Checked { set; get; } Property Get or Set checked state of the button.This actually defined in the CompoundButton class.
public virtual void Toggle() Method Changes the checked state of the view to the inverse of its current state. Also defined in the CompoundButton class.
public event System.EventHandler<CompoundButton.CheckedChangeEventArgs> CheckedChange Event Raised when the checked state of the CheckBox is detected.Also defined in the CompoundButton class.

Example

We will have an Example that will change the CheckBox Text property value based on whether our CheckBox is checked or not.

using Android.App;
using Android.Widget;
using Android.OS;

namespace XamCheckBoxHello
{
    [Activity(Label = "XamCheckBoxHello", MainLauncher = true, Theme = "@android:style/android:Theme.Holo.Light.DarkActionBar", Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        private CheckBox myCheckBox;
        /*
         * When activity is Created.
         */
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
             SetContentView (Resource.Layout.Main);

            //reference checkbox
            myCheckBox = FindViewById<CheckBox>(Resource.Id.myCheckbox);
            //register event handler
            myCheckBox.CheckedChange += myCheckBox_CheckedChange;
        }
        /*
         * CheckBox State change event handler
         */
        void myCheckBox_CheckedChange(object sender, CompoundButton.CheckedChangeEventArgs e)
        {
            myCheckBox.Text = e.IsChecked ? "CheckBox Checked" : "CheckBox unchecked";
        }
    }
}

Main.axml

This is our layout. We will add our CheckBox as well as a header label which is a TextView.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/headerLabel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="casual"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:text="CheckBox"
        android:textAllCaps="true"
        android:textSize="24sp"
        android:textStyle="bold" />
    <CheckBox
        android:id="@+id/myCheckbox"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="CheckBox unchecked"
        android:padding="5dp" />
</RelativeLayout>

NOTES

  1. We import namespaces to use into our project using the using directives.

  2. Then we define our namespace to host our class.

  3. Then we register properties of application that normally get registered in the androidmanifest.xml. In this case we register them as attributes to our class. First we have the Label for application, then set this activity as the launcher activity of our app. Then we the theme to use in our xamarin android app via the Theme attribute. Instead of using the default ugly dark theme, we will change the theme to light holo theme with dark actionbar: Theme = "@android:style/android:Theme.Holo.Light.DarkActionBar".

  4. Then set make our class derive from Activity.

  5. We'll have one instance field of type CheckBox called myCheckBox.

  6. We then override the onCreate() method of our activity.

  7. We set the Content view using the SetContentView(), passing in the layout to be inflated.

  8. We then reference the CheckBox from XML layout using FindViewById.

  9. Then listen to CheckedChange events for our CheckBox.

  10. Then define the event handler. In this case we will be changing the Text property of the CheckBox.

Good day.

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