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Custom Linker Configuration

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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, as discussed in the Linking on iOS and Linking on Android guides. 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 linker description file, add it to your project and:

  • For Android : set the Build Action to LinkDescription
  • For iOS : 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>
                <namespace fullname="My.Own.Namespace" />
                <type fullname="My.Other*" />
        </assembly>
</linker>

In the 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 its field named mono_corlib_version and its 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 its fields (i.e. the preserve="fields" attribute) and all its constructors (i.e. all the methods named .ctor in IL). The Bar type will preserve specific signatures (not names) for one constructor (that accepts a single string parameter) and for a specific string field _blah. The My.Own.Namespace namespace will preserve all the types it contains. Lastly, any type whose full name (including the namespace) matches the wildcard pattern "My.Other*" will preserve all of its fields and methods. The wildcard character * can be included multiple times within a "type fullname" pattern.