How do I use MethodInvoker in C++?

Question

I've got a C++/CLI application which has a background thread. Every so often I'd like it to post it's results to the main GUI. I've read elsewhere on SO that MethodInvoker could work for this, but I'm struggling to convert the syntax from C# to C++:

    void UpdateProcessorTemperatures(array<float>^ temperatures)
    {
        MethodInvoker^ action = delegate
        {
            const int numOfTemps = temperatures->Length;
            if( numOfTemps > 0 ) { m_txtProcessor2Temperature->Text = temperatures[0]; } else { m_txtProcessor2Temperature->Text = "N/A"; }
            if( numOfTemps > 1 ) { m_txtProcessor2Temperature->Text = temperatures[1]; } else { m_txtProcessor2Temperature->Text = "N/A"; }
            if( numOfTemps > 2 ) { m_txtProcessor2Temperature->Text = temperatures[2]; } else { m_txtProcessor2Temperature->Text = "N/A"; }
            if( numOfTemps > 3 ) { m_txtProcessor2Temperature->Text = temperatures[3]; } else { m_txtProcessor2Temperature->Text = "N/A"; }
        }
        this->BeginInvoke(action);
    }

...gives me:

1>c:\projects\MyTemperatureReporter\Form1.h(217) : error C2065: 'delegate' : undeclared identifier
1>c:\projects\MyTemperatureReporter\Form1.h(217) : error C2143: syntax error : missing ';' before '{'

What am I missing here?

Hans Passant · Accepted Answer · 2010-07-14 16:22:15Z

8

C++/CLI doesn't support anonymous delegates, that's an exclusive C# feature. You need to write the delegate target method in a separate method of the class. You'll also need to declare the delegate type, MethodInvoker can't do the job. Make it look like this:

    delegate void UpdateTemperaturesDelegate(array<float>^ temperatures);

    void UpdateProcessorTemperatures(array<float>^ temperatures)
    {
        UpdateTemperaturesDelegate^ action = gcnew UpdateTemperaturesDelegate(this, &Form1::Worker);
        this->BeginInvoke(action, temperatures);
    }

    void Worker(array<float>^ temperatures) 
    {
        const int numOfTemps = temperatures->Length;
        // etc..
    }

edited Jul 14, 2010 at 16:22

answered Jul 14, 2010 at 15:44

Hans Passant

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2 Comments

Ben Voigt Over a year ago

He isn't calling the delegate's BeginInvoke method, but the form's BeginInvoke method. Which is exactly the right thing to do.

Ben Voigt Over a year ago

Also, the reason anonymous delegates haven't been supported is that the syntax for lambdas is being standardized. I would expect the first version of Visual C++ released after the C++0x standard is accepted to add support for anonymous delegates, using the C++0x syntax for lambdas and closures.

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