public override void ItemCheckingOut(SPItemEventProperties properties)
{
    base.ItemCheckingOut(properties);

    try
    {
        int currentUserId = properties.CurrentUserId;

        // Get the currently running workflows on the list item.
        SPWorkflowCollection col = properties.Site.WorkflowManager.GetItemActiveWorkflows(properties.ListItem);

        if (col.Count > 0)
        {
            foreach (SPWorkflow wf in col)
            {
                // If the workflow is one of the approvals (and therefore should be named as such),
                // and the initiator of the workflow (that's what the property AuthorUser is) doesn't match the current user,
                // prevent the user checking the item out.
                // If it does match, allow the checkout, as it's likely to be the workflow checking the item out.
                if ((properties.List.WorkflowAssociations[wf.AssociationId].Name.ToLower().Contains("approval")) && (wf.AuthorUser.ID != currentUserId))
                {
                    // Need to cancel the checkout as the workflow is running and the document shouldn't be editable.
                    properties.ErrorMessage = "The document is currently going through the Approval process. During this process, the document is not available for editing.";
                    properties.Status = SPEventReceiverStatus.CancelWithError;
                }
            }
        }
    }
    catch (Exception ex)
    {
        properties.Status = SPEventReceiverStatus.CancelWithError;
        properties.ErrorMessage = ex.Message;
        return;
    }
}