VSTHRD011 Use AsyncLazy<T>

The Lazy<T> type executes the value factory just once and the value factory inherits the context of the first one to request the Lazy<T>.Value property's value. This can lead to deadlocks when the value factory attempts to switch to the main thread.

Examples of patterns that are flagged by this analyzer

Using Lazy<T> where T is Task<T2>

When T is Task<T2> (because the value factory is an async method), if the first caller had no access to the main thread, and the value factory requires it, it will block. If later a second caller calls the Value property and that second caller is blocking the UI thread for its result, it will deadlock.

var lazy = new Lazy<Task<int>>(async delegate // analyzer flags this line
{
    await Task.Yield();
    return 3;
});

int value = await lazy.Value;

Using synchronously blocking methods in Lazy<T> value factories

When the value factory passed to the Lazy<T> constructor calls synchronously blocking methods such as JoinableTaskFactory.Run, only the first caller can help any required transition to the main thread.

var lazy = new Lazy<int>(delegate
{
    return joinableTaskFactory.Run(async delegate { // analyzer flags this line
        int result = await SomeAsyncMethod();
        return result + 3;
    });
});

int value = lazy.Value;

Solution

Use AsyncLazy<T> with an async value factory:

var lazy = new AsyncLazy<int>(async delegate
{
    await Task.Yield();
    return 3;
});

int value = await lazy.GetValueAsync();