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copied!<p>When passing the variable using the <code>ref</code> keyword, you can't use it inside a lambda expression. Try using a local variable inside the lambda and assign the <code>ref</code> variable outside it, if possible (somewhat simplified example):</p> <pre><code>private static void Main(string[] args) { int i = 0; DoSomethingThatTakesAgesAndNeedsToUpdateUiWhenFinished(ref i); Console.WriteLine(i); } public static void DoSomethingThatTakesAgesAndNeedsToUpdateUiWhenFinished(ref int i) { int temp = i; Thread t = new Thread(() => { temp = 3; // assign the captured, local variable }); t.Start(); t.Join(); i = temp; // assign the ref parameter } </code></pre> <p><strong>Update</strong><br> In response to the updated answer: your problem is that <code>_dataSet</code> inside the lambda expression is not the same variable as dataSet outside the lambda expression. What you could do is the following:</p> <pre><code>class DataSetContainer { public DataSet DataSet { get; set; } } </code></pre> <p>Now we have a reference type with a property that we can safely modify inside the lambda expression:</p> <pre><code>public static void Select(DataGridView dataGridView, DataSetContainer dataSetContainer, params object[] parameters) { AsyncCommandExecutor commandExecutor = new AsyncCommandExecutor(System.Threading.SynchronizationContext.Current); commandExecutor.ExecuteWithContinuation( () => { // this is the long-running bit dataSetContainer.DataSet = getDataFromDb(parameters); // This is the continuation that will be run on the UI thread return () => { dataGridView.DataSource = _dataSet.Tables[0].DefaultView; }; }); } </code></pre> <p>}</p> <p>In the above code, the lambda expression will update the <code>DataSet</code> property of the <code>DataSetContainer</code> instance that is passed to the <code>Select</code> method. Since you are not modifying the passed argument itself but only a member of that instance there is no need for the <code>ref</code> keyword, and we also get around the closure issue. </p> <p><strong>Update 2</strong><br> And now when I switched on my brain, I realize that the <code>Select</code> method makes an asynchronous call. It is quite likely as the code looks that the last line is the <code>Select</code> method will be executed long before <code>_dataSet</code> is being assigned, and as a result it will be <code>null</code>. To get around this you probably want to look into using some sort of signaling mechanism (such as <code>ManualResetEvent</code> or <code>AutoResetEvent</code>) to know when the assignment is done.</p>

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