无法在静态上下文中访问非静态方法？

Question

Given this code....

public class CalibrationViewModel : ViewModelBase
{
    private FileSystemWatcher fsw;

    public CalibrationViewModel(Calibration calibration)
    {
        fsw = new FileSystemWatcher
            {
                Path = @"C:\Users\user\Desktop\Path\ToFile\Test_1234.txt",
                Filter = @"Test_1234.txt",
                NotifyFilter = NotifyFilters.LastWrite
            };

        fsw.Changed += (o, e) =>
            {
                var lastLine = File.ReadAllLines(e.FullPath).Last();
                Dispatcher.BeginInvoke((Action<string>) WriteLineToSamplesCollection, lastLine); //line that cites error
            };
    }

    private void WriteLineToSamplesCollection(string line)
    {
        // do some work
    }
}

Why am I getting the error, 'Cannot access non-static method BeginInvoke in static context'?

I have looked at several other examples on SE and most cite trying to use a field before the object is created as if they were trying to use a non-static field in a static manner, but I don't understand what it is about my code that is invoking the same error.

Lastly, what can I do to fix this specific issue/code?

Update: Fixed title to reflect issue with a 'method' and not a 'property'. I also added that the class implements ViewModelBase.

Answer 1

If this is WPF, System.Windows.Threading.Dispatcher does not have a static BeginInvoke() method.

If you want to call that statically (this is, without having a reference to the Dispatcher instance itself), you may use the static Dispatcher.CurrentDispatcher property:

Dispatcher.CurrentDispatcher.BeginInvoke(...etc);

Be aware though, that doing this from a background thread will NOT return a reference to the "UI Thread"'s Dispatcher, but instead create a NEW Dispatcher instance associated with the said Background Thread.

A more secure way to access the "UI Thread"'s Dispatcher is via the use of the System.Windows.Application.Current static property:

Application.Current.Dispatcher.BeginInvoke(...etc);

Answer 2

Change this:

Dispatcher.BeginInvoke

to this:

Dispatcher.CurrentDispatcher.BeginInvoke

the issue is BeginInvoke is an instance method and needs an instance to access it. However, your current syntax is trying to access BeginInvoke in a static manner off the class Dispatcher and that's what's causing this error:

Cannot access non-static method BeginInvoke in static context

Answer 3

It's because Dispatcher is a class not a property. Shouldn't you be making your CalibrationViewModel class a subclass of some other class which has a Dispatcher property?