Workflow

Set Item State and Executes actions in that state

public void MoveToStateAndExecuteActions(Item item, ID workflowStateId)
{
    Sitecore.Workflows.IWorkflowProvider workflowProvider = Item.Database.WorkflowProvider;
    Sitecore.Workflows.IWorkflow workflow = workflowProvider.GetWorkflow(item);

    // if item is in any workflow
    if (workflow != null)
    {
        using (new Sitecore.Data.Items.EditContext(item))
        {
            // update item's state to the new one
            item[Sitecore.FieldIDs.WorkflowState] = workflowStateId.ToString();
        }

        Item stateItem = ItemManager.GetItem(workflowStateId, 
            Language.Current, Sitecore.Data.Version.Latest, item.Database, SecurityCheck.Disable);

        // if there are any actions for the new state
        if (!stateItem.HasChildren)
            return;

        WorkflowPipelineArgs workflowPipelineArgs = new WorkflowPipelineArgs(item, null, null);

        // start executing the actions
        Pipeline pipeline = Pipeline.Start(stateItem, workflowPipelineArgs);
        if (pipeline == null)
            return;
        WorkflowCounters.ActionsExecuted.IncrementBy(pipeline.Processors.Count);
    }
}

Thanks to this thread

Executing workflow command to change the workflow state

Thanks to this great post

If we want to mimic the Sitecore UI behavior and execute the command which will change the workflow state, we need to use WorkflowProvider to get an instance of the workflow assigned to the given item and call Execute method with a chosen command ID. This will fire all the actions which are defined under the command item node, change the state of the item and fire all the auto-actions defined below the new state item node:

public static WorkflowResult ExecuteCommand(Item item, string commandName, string comment)
{
    IWorkflow workflow = item.Database.WorkflowProvider.GetWorkflow(item);
 
    if (workflow == null)
    {
        return new WorkflowResult(false, "No workflow assigned to item");
    }
 
    WorkflowCommand command = workflow.GetCommands(item[FieldIDs.WorkflowState])
        .FirstOrDefault(c => c.DisplayName == commandName);
 
    if (command == null)
    {
        return new WorkflowResult(false, "Workflow command not found");
    }
 
    return workflow.Execute(command.CommandID, item, comment, false, new object[0]);
}