C＃Func Delegate引发线程异常

Question

So, maybe I misunderstood the usage of Func but

Func<ComboBox, string> getCurrentValue = s => s.SelectedValue.ToString();

Creates a Thread Error when calling getCurrentValue(cb_message_type) from my Workerthread. Whats the best solution to get the Selected Value of the Combobox?

Many thanks,
rAyt

Edit
MSDN

"The underlying type of a lambda expression is one of the generic Func delegates. This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate."

Answer 1

Since windows controls have thread affinity, you have 2 options:

1. query this data before doing your threading code, for example passing it in as the state to the worker
2. query it in the worker via Control.Invoke

Since the first is trivial, I'll give an example of the second using captured variables:

object value = null;
yourCombo.Invoke((MethodInvoker) delegate {value=yourCombo.SelectedValue;});
string s = value.ToString();

Here the bits inside delegate {...} happen on the UI thread, even if the code around it is on the worker thread. You can mix the above either inside your func, or call the entire func once you've switched threads.

Answer 2

You need to call Control.Invoke with that delegate - or make the delegate itself call it. Using a lambda expression doesn't change the threading requirements of Windows Forms - it just makes it easier to create a delegate.

You might want to make a convenience method to do this:

// (Untested)
public static Func<TControl, TResult> WrapInvocation(Func<TControl,TResult> func)
    where TControl : Control
{
    return control => {
        return (TResult) control.Invoke(func);
    };
}

Use as:

Func<ComboBox, string> getCurrentValue = s => s.SelectedValue.ToString();
getCurrentValue = WrapInvocation(getCurrentValue);

Then you can call getCurrentValue(comboBox) from any thread.

Answer 3

The problem is that UI controls can only be used on the UI thread,

You need to call the Invoke method within the other thread, like this:

Func<ComboBox, string> getCurrentValue =
    s => s.Invoke(new Func<object>(() => s.SelectedValue)).ToString();

The Invoke method takes a delegate and executes it on the UI thread.

Answer 4

Generally speaking, you cannot access UI controls from a thread other than that they were created on. To overcome that, you'll either have to check ISynchronizeInvoke.InvokeRequired on the control in question and branch, invoke a delegate, etc. or use a SynchronizationContext . First option is very cumbersome, whereas second one is pretty elegant:

var synchronizationContext = WindowsFormsSynchronizationContext.Current;
string currentValue = "";    

synchronizationContext.Send(
    () => currentValue = getCurrentValue(comboBox), 
    null);

Answer 5

如果线程将仅读取ComboBox，则可行的最佳选择是在线程上有一个事件处理程序，该事件处理程序在更改时随时获取ComboBox值，然后通过可从任何线程读取的属性公开该值。