Changing workflow state in Sitecore

I'm having an issue changing the workflow state for an item programmatically. The state isn't being changed no matter what I do to the field. I've tried using (new SecurityDisabler()){} and putting the item in editing mode then changing the field manually. I've noticed that the item itself has the Lock set to <r />, could this be causing an issue?

Here is some sample code of what I've tried to do:

        [HttpPost]
        [MultipleButton(Name = "action", Argument = "Submit")]
        public ActionResult Submit(LoI model)
        {
            if (model.Submitted || !model.Signed)
            {
                return Redirect("/Profile/LoI");
            }

            ModifyCandidateInfo(model, true);

            Session["message"] = Translate.Text("loi-submitted-message");
            Session["messageClass"] = "success";

            return Redirect("/Profile/LoI");
        }
    private static void ModifyCandidateInfo(LoI model, bool isSubmission)
    {
        using (new SecurityDisabler())
        {
            var candidateFolder = CBUtility.GetCandidateFolder();

            var loi= candidateFolder.GetChildren().SingleOrDefault(loi => loi.TemplateID == LoITemplateId);

            if (loi == null) return;

            loi.Editing.BeginEdit();

            EditFields(loi, model);

            EditChildren(loi, model);

            //Send emails upon submission
            if (isSubmission)
            {
                loi.ExecuteCommand("Submit",
                    loi.Name + " submitted for " + model.CandidateName);

                using (new SecurityDisabler())
                {
                    loi.Editing.BeginEdit();

                    loi.Fields["__Workflow state"].Value = "{F352B651-341B-4CCF-89FE-BD77F5E4D540}";
                    loi.Editing.EndEdit();
                }
            }

            loi.Editing.EndEdit();


        }

    }

I initalized the item's workflow with the following function:

public static void InitializeWorkflow(Item item, ID workflowId)
        {
            item.Editing.BeginEdit();
            var workflow =
                item.Database.WorkflowProvider.GetWorkflow(workflowId.ToString());
            workflow.Start(item);
            item.Editing.EndEdit();
        }

The item starts at the default drafting state and executed a "Submit" command that fires off emails. Through the Sitecore UI if I hit submit it'll go to the next workflow state but not programmatically when I fire off the ExecuteCommand function. Below you'll find the ExecuteCommand function.

public static WorkflowResult ExecuteCommand(this Item item, string commandName, string comment)
        {
            using (new SecurityDisabler())
            {
                var workflow = item.Database.WorkflowProvider.GetWorkflow(item);
                if (workflow == null)
                {
                    return new WorkflowResult(false, "No workflow assigned to item");
                }

                var command = workflow.GetCommands(item[FieldIDs.WorkflowState])
                    .FirstOrDefault(c => c.DisplayName == commandName);
                return command == null
                    ? new WorkflowResult(false, "Workflow command not found")
                    : workflow.Execute(command.CommandID, item, comment, false);
            }
        }

The command fires off fine and the emails are sent but I can't figure out why the state won't change. Could someone provide me with other suggestions or a solution?

Am I reading the workflow state id correctly? I'm using the item ID for the workflow state.

I think your code is really similar to my implementation. This is my code's background.

All items have the same workflow named "WF" and it has three workflow states (Working, Awaiting Approval, and Approved). One page-item having "WF" has some rendering items and those datasource items. Suppose a content editor is ready to submit and approve the item with its related items. By hitting the "Submit" and "Approval" button in the page, all page-item's related items have the same workflow state as the page-item's one.

Most code are from Marek Musielak and this code is perfectly working in my side.

public class UpdateWorkflowState
{
    // List all controls in page item
    public RenderingReference[] GetListOfSublayouts(string itemId, Item targetItem)
    {
        RenderingReference[] renderings = null;

        if (Sitecore.Data.ID.IsID(itemId))
        {
            renderings = targetItem.Visualization.GetRenderings(Sitecore.Context.Device, true);
        }
        return renderings;
    }

    // Return all datasource defined on one item
    public IEnumerable<string> GetDatasourceValue(WorkflowPipelineArgs args, Item targetItem)
    {
        List<string> uniqueDatasourceValues = new List<string>();
        Sitecore.Layouts.RenderingReference[] renderings = GetListOfSublayouts(targetItem.ID.ToString(), targetItem);

        LayoutField layoutField = new LayoutField(targetItem.Fields[Sitecore.FieldIDs.FinalLayoutField]);
        LayoutDefinition layoutDefinition = LayoutDefinition.Parse(layoutField.Value);
        DeviceDefinition deviceDefinition = layoutDefinition.GetDevice(Sitecore.Context.Device.ID.ToString());

        foreach (var rendering in renderings)
        {
            if (!uniqueDatasourceValues.Contains(rendering.Settings.DataSource))
                uniqueDatasourceValues.Add(rendering.Settings.DataSource);
        }

        return uniqueDatasourceValues;
    }

    // Check workflow state and update state
    public WorkflowResult ChangeWorkflowState(Item item, ID workflowStateId)
    {
        using (new EditContext(item))
        {
            item[FieldIDs.WorkflowState] = workflowStateId.ToString();
        }

        Sitecore.Layouts.RenderingReference[] renderings = GetListOfSublayouts(item.ID.ToString(), item);
        return new WorkflowResult(true, "OK", workflowStateId);
    }

    // Verify workflow state and update workflow state
    public WorkflowResult ChangeWorkflowState(Item item, string workflowStateName)
    {
        IWorkflow workflow = item.Database.WorkflowProvider.GetWorkflow(item);
        if (workflow == null)
        {
            return new WorkflowResult(false, "No workflow assigned to item");
        }

        WorkflowState newState = workflow.GetStates().FirstOrDefault(state => state.DisplayName == workflowStateName);

        if (newState == null)
        {
            return new WorkflowResult(false, "Cannot find workflow state " + workflowStateName);
        }

        unlockItem(newState, item);

        return ChangeWorkflowState(item, ID.Parse(newState.StateID));
    }

    // Unlock the item when it is on FinalState
    public void unlockItem(WorkflowState newState, Item item)
    {
        if (newState.FinalState && item.Locking.IsLocked())
        {
            using (new EditContext(item, false, false))
            {
                item["__lock"] = "<r />";
            }
        }
    }
}