September 25th, 2019
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How do I create a Windows Runtime method that accepts a lambda?

Raymond Chen
Raymond Chen

A customer wanted to create a Windows Runtime method that accepted a lambda. But lambdas aren’t valid Windows Runtime types.

// C++/WinRT IDL
runtimeclass MyClass
{
  template<typename TLambda>
  void ApplyFilter(TLambda&& filter); // Does not compile
}

// C++/CX
public ref class MyClass
{
public:
  template<typename TLambda>
  void ApplyFilter(TLambda&& filter); // Does not compile
};

What can we use instead of lambdas?

The Windows Runtime doesn’t support lambdas, but it supports what you typically use the lambda for, which is providing a callback. The thing that represents a function is a delegate.

You can think of a delegate as the Windows Runtime version of a std::function. It’s an object that can hold a lambda or a static function or an object combined with a member function.

// C++/WinRT IDL
delegate Boolean ThingFilter(Thing thing);

runtimeclass MyClass
{
  void ApplyFilter(ThingFilter filter);
}

// C++/CX
public delegate bool ThingFilter(Thing^ thing);

public ref class MyClass
{
public:
  void ApplyFilter(ThingFilter^ filter);
};

You can use the delegate like a function object.

// C++/WinRT
void MyClass::ApplyFilter(ThingFilter const& filter)
{
  things.erase(std::remove_if(things.begin(), things.end(),
    [&](auto&& thing) { return !filter(thing); }),
    things.end());
}

// C++/CX
void MyClass::ApplyFilter(ThingFilter^ filter)
{
  things.erase(std::remove_if(things.begin(), things.end(),
    [&](Thing^ thing) { return !filter(thing); }),
    things.end());
}

Most language projections let you pass a lambda directly. C++/CX is the outlier here. It requires you to wrap it inside an explicit ThingFilter. The other languages will do the wrapping automatically.

// C++/WinRT
c.ApplyFilter([=](auto&& thing) { return IsOkay(thing); });

// C++/CX
c->ApplyFilter(
    ref new ThingFilter([=](Thing^ thing) { return IsOkay(thing); }));

// C#
c.ApplyFilter(thing => IsOkay(thing));

// JavaScript
c.applyFilter(thing => isOkay(thing));

// Visual Basic
c.ApplyFilter(Function (thing As Thing) IsOkay(thing))

 

Category
Old New Thing
Topics
Code

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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  • cheong00

    Much like in C# we could write event handler as lambda, but we used to create event handlers as delegates.

    1) this.Click += (s, arg) =>
    {

    };

    2) this.Click += Form1_Click;

    3) this.Click += new EventHandler(Form1_Click);

    private void Form1_Click(object sender, EventArgs e)
    {

    }

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