Xamarin.Android applications use a linker in order to reduce the size of the application. The linker employes static analysis of your application to determine which assemblies are actually used, which types are actually used, and which members are actually used. The linker then behaves like a garbage collector, continually looking for the assemblies, types, and members that are referenced until the entire closure of referenced assemblies, types, and members is found. Then everything outside of this closure is discarded.

For example, the Hello, Android sample:

Linking results in a package that is 30% the size of the original (unlinked) package in 1.2.0, and 18% of the unlinked package in 4.0.1.

Control

Linking is based on static analysis. Consequently, anything that depends upon the runtime environment won't be detected:

// To play along at home, Example must be in a different assembly from MyActivity.
public class Example {
    // Compiler provides default constructor...
}

[Activity (Label="Linker Example", MainLauncher=true)]
public class MyActivity {
    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

        // Will this work?
        var o = Activator.CreateInstance (typeof (ExampleLibrary.Example));
    }
}

Linker Behavior

The primary mechanism for controlling the linker is the Linker Behavior drop-down within the Project Options dialog box. There are three options:

1. Don't Link
2. Link SDK Assemblies
3. Link All Assemblies

The Don't Link option turns off the linker; the above "Release without Linking" application size used this behavior. This is useful for troubleshooting runtime failures, to see if the linker is responsible.

The Link SDK Assemblies option only links assemblies that come with Xamarin.Android. All other assemblies are not linked.

The Link All Assemblies option links all assemblies.

The above example will work with the Don't Link and Link SDK Assemblies options, and will fail with the Link All Assemblies behavior, generating the following error:

E/mono    (17755): [0xafd4d440:] EXCEPTION handling: System.MissingMethodException: Default constructor not found for type ExampleLibrary.Example.
I/MonoDroid(17755): UNHANDLED EXCEPTION: System.MissingMethodException: Default constructor not found for type ExampleLibrary.Example.
I/MonoDroid(17755): at System.Activator.CreateInstance (System.Type,bool) <0x00180>
I/MonoDroid(17755): at System.Activator.CreateInstance (System.Type) <0x00017>
I/MonoDroid(17755): at LinkerScratch2.Activity1.OnCreate (Android.OS.Bundle) <0x00027>
I/MonoDroid(17755): at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_ (intptr,intptr,intptr) <0x00057>
I/MonoDroid(17755): at (wrapper dynamic-method) object.95bb4fbe-bef8-4e5b-8e99-ca83a5d7a124 (intptr,intptr,intptr) <0x00033>
E/mono    (17755): [0xafd4d440:] EXCEPTION handling: System.MissingMethodException: Default constructor not found for type ExampleLibrary.Example.
E/mono    (17755):
E/mono    (17755): Unhandled Exception: System.MissingMethodException: Default constructor not found for type ExampleLibrary.Example.
E/mono    (17755):   at System.Activator.CreateInstance (System.Type type, Boolean nonPublic) [0x00000] in <filename unknown>:0
E/mono    (17755):   at System.Activator.CreateInstance (System.Type type) [0x00000] in <filename unknown>:0
E/mono    (17755):   at LinkerScratch2.Activity1.OnCreate (Android.OS.Bundle bundle) [0x00000] in <filename unknown>:0
E/mono    (17755):   at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_ (IntPtr jnienv, IntPtr native__this, IntPtr native_savedInstanceState) [0x00000] in
<filename unknown>:0
E/mono    (17755):   at (wrapper dynamic-method) object:95bb4fbe-bef8-4e5b-8e99-ca83a5d7a124 (intptr,intptr,intptr)

PreserveAttribute

The Android.Runtime.PreserveAttribute can be used to explicitly add members to be preserved by the linker, and should be placed on a member when the linker would otherwise remove it. Thus, for the above example to work, we could either place [Preserve] on the constructor:

public class Example {
    [Android.Runtime.Preserve]
    public Example ()
    {
    }
}

Or we could use [Preserve(AllMembers=true)] on the type to preseve all members. This is also useful for XML serialization:

[Android.Runtime.Preserve(AllMembers=true)]
class Example {
    // Compiler provides default constructor...
}

falseflag

If the [Preserve] attribute can't be used, it is often useful to provide a block of code so that the linker believes that the type is used, while preventing the block of code from being executed at runtime. To make use of this technique, we could do:

[Activity (Label="Linker Example", MainLauncher=true)]
class MyActivity {

#pragma warning disable 0219, 0649
    static bool falseflag = false;
    static MyActivity ()
    {
        if (falseflag) {
            var ignore = new Example ();
        }
    }
#pragma warning restore 0219, 0649

    // ...
}

linkskip

It is possible to specify that a set of user-provided assemblies should not be linked at all, while allowing other user assemblies to be skipped with the Link SDK Assemblies behavior by using the AndroidLinkSkip MSBuild property:

<PropertyGroup>
    <AndroidLinkSkip>Assembly1;Assembly2</AndroidLinkSkip>
</PropertyGroup>

Custom Linking

If the default set of options is not enough, you can drive the linking process with an XML file that describes what you want from the linker.

You can provide extra definitions to the linker to ensure the type, methods and/or fields are not eliminated from your application. In your own code the preferred way is to use the [Preserve] custom attribute. However if you need some definitions from the SDK or product assemblies then using an XML file might be your best solution (versus adding code that will ensure the linker won't eliminate what you need).

To do this, you define an XML file with the top-level element <linker> which contains assembly nodes which in turn contain type nodes, which in turn contain method and field nodes.

Once you have this linke description file, add it to your project and set the Build action to LinkDescription.

The following example shows what the XML file looks like:

<linker>
        <assembly fullname="mscorlib">
                <type fullname="System.Environment">
                        <field name="mono_corlib_version" />
                        <method name="get_StackTrace" />
                </type>
        </assembly>
        <assembly fullname="My.Own.Assembly">
                <type fullname="Foo" preserve="fields" />
                        <method name=".ctor" />
                </type>
                <type fullname="Bar" />
                        <method signature="System.Void .ctor(System.String)" />
                        <field signature="System.String _blah" />
                </type>
        </assembly>
</linker>

In above example, the linker will read and apply the instructions on the mscorlib.dll (shipped with Mono for Android) and My.Own.Assembly (user code) assemblies.

The first section, for mscorlib.dll, will ensure that the System.Environment type will preserve it's field named mono_corlib_version and it's get_StackTrace method. Note the getter and/or setter method names must be used as the linker works on IL and does not understand C# properties.

The second section, for My.Own.Assembly.dll, will ensure that the Foo type will preserve all it's fields (i.e. the preserve="fields" attribute) and all it's constructors (i.e. all the methods named .ctor in IL). Also the Bar type will preserve specific signatures (not names) for one constructor (that accept a single string parameter) and for a specific string field _blah.