Helper methods for Xamarin.Android to check if IJavaObjects are still alive in the Java VM. Useful for MVVM.

MonoDroidHelper.cs

public static class MonoDroidHelper {
    /// <summary>
    ///     Returns true if the current <paramref name="javaObj"/>
    ///     does not have a <see cref="IJavaObject.Handle"/> to the
    ///     object in the Java VM.
    /// </summary>
    public static bool IsInMonoLimbo<T>(this T javaObj) where T : class, IJavaObject {
        return javaObj.Handle == IntPtr.Zero;
    }

    /// <summary>
    ///     Helper method that checks whether the current <paramref name="javaObj"/>
    ///     has a <see cref="IJavaObject.Handle"/> to the object in the Java VM, returning
    ///     null if it does not.
    /// </summary>
    public static T SafeGet<T>(this T javaObj) where T : class, IJavaObject {
        return IsInMonoLimbo(javaObj) ? null : javaObj;
    }

    /// <summary>
    ///     Helper method that checks whether the current <paramref name="javaObj"/>
    ///     has a <see cref="IJavaObject.Handle"/> to the object in the Java VM.
    /// </summary>
    /// <remarks>
    ///     Use to resolve exceptions (ArgumentException: 'jobject' must not be IntPtr.Zero.
    ///     Parameter name: jobject) that commonly occur when view model properties
    ///     are modified between the Java VM disposing the object and the mono runtime has
    ///     a chance to disposing the current <paramref name="javaObj"/>.
    /// </remarks>
    /// <returns>
    ///     The result of <see cref="getter"/> or null if <see cref="IJavaObject.Handle"/> is
    ///     equal to <see cref="IntPtr.Zero"/>.
    /// </returns>
    public static T SafeGet<TJavaObject, T>(this TJavaObject javaObj, Func<TJavaObject, T> getter)
        where TJavaObject : class, IJavaObject
        where T : class {
        return IsInMonoLimbo(javaObj) ? null : getter(javaObj);
    }
}