Update SPListItem in workflow SharePoint

Problem:

In workflow SharePoint context

SPListItem itemAssociateWithWorkflow =  this.workflowProperties.Item;

itemAssociateWithWorkflow["Your field name"] = "your value";

itemAssociateWithWorkflow.Update();

After update successfully, that item will be modified by “System Account” that means field Modified By has value System Account

Solve:

SPUser currentUser = GetCurrentUserInWorkflow(this.workflowProperties);

////impersonate real current user

using (SPSite site = new SPSite(this.workflowProperties.Web.Site.ID, currentUser.UserToken))

{

    using (SPWeb webCurrentUser = site.OpenWeb(this.workflowProperties.Web.ID))

    {

     SPListItem itemCurrentUser = webCurrentUser.Lists  [this.workflowProperties.ListId].GetItemById(this.workflowProperties.ItemId);

        itemCurrentUser["your field name"] = "your value";

        itemCurrentUser.Update();

    }

}