C# – Async and await in MVC 4 Controller

asp.net-mvcasp.net-mvc-4async-awaitcnet

Every time I try to use the new Async and Await operators and return a collection of objects from a database I get an Invalid Operation exception. When I use it to only return a single Item it works fine.

Controller Code:

public async Task<ActionResult> EnvironmentList()
{
    EfEnvironmentDataAccess dataAccess = new EfEnvironmentDataAccess();
    ICollection<Environment> environments = await dataAccess.GetAllEnvironmentsAsync();
    return PartialView(environments);
}

View Code:

<div class="ECURightCol">
<h3>Table Dumps</h3>
@Html.Action("EnvironmentList", "Environment")
@Html.Action("ComputerList", "Computer")
@Html.Action("ProductList", "Product")
@Html.Action("InstanceList", "Instance")
@Html.Action("ProfileList", "Profile")

The Data Access Code:

public ICollection<Environment> GetAllEnvironments()
{
    using (EcuWebDataContext db = new EcuWebDataContext())
    {
        return db.Environments.OrderBy(e => e.Name).ToList();
    }
}

public async Task<ICollection<Environment>> GetAllEnvironmentsAsync()
{
    return await Task.Run(() => GetAllEnvironments());
}

The Error I get is:

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: HttpServerUtility.Execute blocked while waiting for an asynchronous operation to complete.

Best Answer

First of all, you cannot use asynchronous processing with child actions and I suppose this is what you are trying to do.

Secondly, you are not doing any asynchronous processing here by spinning up another thread to execute your code with the below line of code:

Task.Run(() => GetAllEnvironments());

It will block a thread at the end of the day and you will have nothing but a context switch overhead. EF6 will have support for asynchronous processing. For asynchronous queries with pure ADO.NET, have a look:

Asynchronous Database Calls With Task-based Asynchronous Programming Model (TAP) in ASP.NET MVC 4

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