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Errors when Page Async="true" for the async-await pattern

Problem

Using the async-await pattern (setting the Async attribute of the Page directive to true) causes errors (example stack traces and error messages are available below). For example, calling asynchronous APIs in NeedDataSource event of a Grid, ListView, etc.

Description

The Telerik UI for ASP.NET AJAX controls do not support the async-await pattern for WebForms. As complex server controls they rely heavily on the page lifecycle of the WebForm and the state of various global objects that is set by the standard flow of events in the form. Asynchronous tasks change this order and also require a different approach to writing code (namely, minimizing the use of global objects, which is impossible in WebForms).

Sample Exceptions and Stack Traces

Here is an example of the error you will receive if you attempt to bind a grid to an async operation

Sample code:

protected async void RadGrid1_NeedDataSourceAsync(object sender, GridNeedDataSourceEventArgs e)
{
    DataTable data = await GetData();
    (sender as RadGrid).DataSource = data;
}

Error:

An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked <%@ Page Async="true" %>. This exception may also indicate an attempt to call an "async void" method, which is generally unsupported within ASP.NET request processing. Instead, the asynchronous method should return a Task, and the caller should await it. 

grid-async-binding-error

Stack trace:

[InvalidOperationException: An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked <%@ Page Async="true" %>. This exception may also indicate an attempt to call an "async void" method, which is generally unsupported within ASP.NET request processing. Instead, the asynchronous method should return a Task, and the caller should await it.]
   System.Web.AspNetSynchronizationContext.OperationStarted() +3468985
   System.Runtime.CompilerServices.AsyncVoidMethodBuilder.Create() +45
   Default4.RadGrid1_NeedDataSourceAsync(Object sender, GridNeedDataSourceEventArgs e) +81
   Telerik.Web.UI.RadGrid.OnNeedDataSource(GridNeedDataSourceEventArgs e) +133
   Telerik.Web.UI.RadGrid.ObtainDataSource(GridRebindReason rebindReason, Boolean IsBoundUsingDataSourceId) +337
   Telerik.Web.UI.RadGrid.AutoDataBind(GridRebindReason rebindReason) +101
   Telerik.Web.UI.RadGrid.OnLoad(EventArgs e) +127
   System.Web.UI.Control.LoadRecursive() +59
   System.Web.UI.Control.LoadRecursive() +131
   System.Web.UI.<LoadRecursiveAsync>d__246.MoveNext() +549
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +99
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +58
   System.Web.Util.WithinCancellableCallbackTaskAwaiter.GetResult() +31
   System.Web.UI.<ProcessRequestMainAsync>d__523.MoveNext() +4680

Here are sample stack traces of other errors:

System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
  at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
  at Telerik.Web.UI.RadButton.LoadClientState(Dictionary`2 clientState)
  at Telerik.Web.UI.RadButton.LoadPostData(String postDataKey, NameValueCollection postCollection)
  at Telerik.Web.UI.RadWebControl.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection)
  at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad)
  at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  at System.Web.UI.Page.HandleError(Exception e)
  at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  at System.Web.UI.Page.ProcessRequest()
  at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
  at System.Web.UI.Page.ProcessRequest(HttpContext context)
  at ASP.edms_projectexplorer_aspx.ProcessRequest(HttpContext context)
  at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
  at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
  at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

and

System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Telerik.Web.UI.RadButton.LoadClientState(Dictionary`2 clientState)
   at Telerik.Web.UI.RadButton.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at Telerik.Web.UI.RadWebControl.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at System.Web.UI.Page.<LoadPostDataAsync>d__394.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.UI.Page.<ProcessPostDataAsync>d__393.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Util.WithinCancellableCallbackTaskAwaitable.WithinCancellableCallbackTaskAwaiter.GetResult()
   at System.Web.UI.Page.<ProcessRequestMainAsync>d__523.MoveNext()
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.<ProcessRequestMainAsync>d__523.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Util.WithinCancellableCallbackTaskAwaitable.WithinCancellableCallbackTaskAwaiter.GetResult()
   at System.Web.UI.Page.<ProcessRequestAsync>d__515.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.UI.Page.<ProcessRequestAsync>d__554.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.TaskAsyncHelper.EndTask(IAsyncResult ar)
   at System.Web.UI.Page.AsyncPageEndProcessRequest(IAsyncResult result) 
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Solution

The solution to using asynchronous API is to wrap the asynchronous method call in a Task.Run, configureAwait to be false(to avoid potential deadlocks), and finally get the .Result of the wrapped task, which will be the item you need


protected void RadGrid1_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
{
    Task<DataTable> taskOfDataTable = Task.Run(async () => await GetGridSourceAsDataTable().ConfigureAwait(false));
    DataTable dataTableResults = taskOfDataTable.Result;
    (sender as RadGrid).DataSource = dataTableResults;

    //Task<IEnumerable<OrderModel>> taskOfList = Task.Run(async () => await GetGridSourceAsIEnumerable().ConfigureAwait(false));
    //IEnumerable<OrderModel> listResults = taskOfList.Result;
    //(sender as RadGrid).DataSource = listResults;
}

// dummy async method for demo purposes. Replace with your async method
private async Task<DataTable> GetGridSourceAsDataTable()
{
    await Task.Delay(3000);
    DataTable dataTable = new DataTable();

    dataTable.Columns.Add(new DataColumn("OrderID", typeof(int)));
    dataTable.Columns.Add(new DataColumn("OrderDate", typeof(DateTime)));
    dataTable.Columns.Add(new DataColumn("Freight", typeof(decimal)));
    dataTable.Columns.Add(new DataColumn("ShipName", typeof(string)));
    dataTable.Columns.Add(new DataColumn("ShipCountry", typeof(string)));

    dataTable.PrimaryKey = new DataColumn[] { dataTable.Columns["OrderID"] };

    for (int i = 0; i < 70; i++)
    {
        // or use the following one-liner with the following arguments
        // OrderID, OrderDate, Freight, ShipName, ShipCountry
        //dataTable.Rows.Add(i + 1, DateTime.Now, (i + 1) + (i + 1) * 0.1 + (i + 1) * 0.01, "Name " + (i + 1), "Country " + (i + 1));

        DataRow row = dataTable.NewRow();
        row["OrderID"] = i + 1;
        row["OrderDate"] = DateTime.Now;
        row["Freight"] = (i + 1) + (i + 1) * 0.1 + (i + 1) * 0.01;
        row["ShipName"] = "Name " + (i + 1);
        row["ShipCountry"] = "Country " + (i + 1);

        dataTable.Rows.Add(row);
    }

    return dataTable;
}

public class OrderModel
{
    public int OrderID { get; set; }
    public DateTime OrderDate { get; set; }
    public decimal Freight { get; set; }
    public string ShipName { get; set; }
    public string ShipCountry { get; set; }
}

// dummy async method for demo purposes. Replace with your async method
private async Task<IEnumerable<OrderModel>> GetGridSourceAsIEnumerable()
{
    await Task.Delay(3000);

    return Enumerable.Range(1, 100).Select(x => new OrderModel
    {
        OrderID = x,
        OrderDate = DateTime.Now.AddHours(x),
        Freight = x * 0.33M,
        ShipCountry = "Ship Country " + (x % 7),
        ShipName = "Ship Name " + x
    });
}

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