Xamarin.Essentials: MainThread

The MainThread class allows applications to run code on the main thread of execution, and to determine if a particular block of code is currently running on the main thread.

Background

Most operating systems — including iOS, Android, and the Universal Windows Platform — use a single-threading model for code involving the user interface. This model is necessary to properly serialize user-interface events, including keystrokes and touch input. This thread is often called the main thread or the user-interface thread or the UI thread. The disadvantage of this model is that all code that accesses user interface elements must run on the application's main thread.

Applications sometimes need to use events that call the event handler on a secondary thread of execution. (The Xamarin.Essentials classes Accelerometer, Compass, Gyroscope, Magnetometer, and OrientationSensor all might return information on a secondary thread when used with faster speeds.) If the event handler needs to access user-interface elements, it must run that code on the main thread. The MainThread class allows the application to run this code on the main thread.

Get started

To start using this API, read the getting started guide for Xamarin.Essentials to ensure the library is properly installed and set up in your projects.

Running Code on the Main Thread

Add a reference to Xamarin.Essentials in your class:

using Xamarin.Essentials;

To run code on the main thread, call the static MainThread.BeginInvokeOnMainThread method. The argument is an Action object, which is simply a method with no arguments and no return value:

MainThread.BeginInvokeOnMainThread(() =>
{
    // Code to run on the main thread
});

It is also possible to define a separate method for the code that must run on the main thread:

void MyMainThreadCode()
{
    // Code to run on the main thread
}

You can then run this method on the main thread by referencing it in the BeginInvokeOnMainThread method:

MainThread.BeginInvokeOnMainThread(MyMainThreadCode);

Note

Xamarin.Forms has a method called Device.BeginInvokeOnMainThread(Action) that does the same thing as MainThread.BeginInvokeOnMainThread(Action). While you can use either method in a Xamarin.Forms app, consider whether or not the calling code has any other need for a dependency on Xamarin.Forms. If not, MainThread.BeginInvokeOnMainThread(Action) is likely a better option.

Determining if Code is Running on the Main Thread

The MainThread class also allows an application to determine if a particular block of code is running on the main thread. The IsMainThread property returns true if the code calling the property is running on the main thread. A program can use this property to run different code for the main thread or a secondary thread:

if (MainThread.IsMainThread)
{
    // Code to run if this is the main thread
}
else
{
    // Code to run if this is a secondary thread
}

You might wonder if you should check if code is running on a secondary thread before calling BeginInvokeOnMainThread, for example, like this:

if (MainThread.IsMainThread)
{
    MyMainThreadCode();
}
else
{
    MainThread.BeginInvokeOnMainThread(MyMainThreadCode);
}

You might suspect that this check might improve performance if the block of code is already running on the main thread.

However, this check is not necessary. The platform implementations of BeginInvokeOnMainThread themselves check if the call is made on the main thread. There is very little performance penalty if you call BeginInvokeOnMainThread when it's not really necessary.

Additional Methods

The MainThread class includes the following additional static methods that can be used to interact with user interface elements from backgrounds threads:

Method Arguments Returns Purpose
InvokeOnMainThreadAsync<T> Func<T> Task<T> Invokes a Func<T> on the main thread, and waits for it to complete.
InvokeOnMainThreadAsync Action Task Invokes an Action on the main thread, and waits for it to complete.
InvokeOnMainThreadAsync<T> Func<Task<T>> Task<T> Invokes a Func<Task<T>> on the main thread, and waits for it to complete.
InvokeOnMainThreadAsync Func<Task> Task Invokes a Func<Task> on the main thread, and waits for it to complete.
GetMainThreadSynchronizationContextAsync Task<SynchronizationContext> Returns the SynchronizationContext for the main thread.

API