How Can I Use Output Parameters with Asynchronous Methods in .NET?

Asynchrony and Out Parameters: A Dilemma

Developers often encounter the need to write asynchronous methods with output parameters, but the .NET framework poses a challenge in this regard.

The Issue

The following code snippet illustrates the problem:

public async void Method1()
{
    int op;
    int result = await GetDataTaskAsync(out op);
}

This code attempts to create an async method with an output parameter, but such a feature is unavailable in the .NET Framework.

The Reasoning

According to Lucian Wischik, a Microsoft engineer, this limitation stems from the way async methods are implemented in the CLR. They are transformed by the compiler into state-machine-objects, which lack a safe mechanism for storing the address of out or reference parameters.

Workaround

A common workaround is to have the async method return a Tuple:

public async Task Method1()
{
    var tuple = await GetDataTaskAsync();
    int op = tuple.Item1;
    int result = tuple.Item2;
}

public async Task<Tuple<int, int>> GetDataTaskAsync()
{
    //...
    return new Tuple<int, int>(1, 2);
}

This approach provides a way to pass multiple values back from the async method without resorting to out parameters.

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