权限。打破文档的安全性继承并将用户添加为阅读器

ClientContext clientContext = new ClientContext(siteUrl);
SP.List oList = clientContext.Web.Lists.GetByTitle("MyList");

int itemId = 3;
ListItem oListItem = oList.Items.GetById(itemId);

oListItem.BreakRoleInheritance(false);

User oUser = clientContext.Web.SiteUsers.GetByLoginName(@"DOMAIN\alias");

RoleDefinitionBindingCollection collRoleDefinitionBinding = new RoleDefinitionBindingCollection(clientContext);

collRoleDefinitionBinding.Add(clientContext.Web.RoleDefinitions.GetByType(RoleType.Reader));

oListItem.RoleAssignments.Add(oUser, collRoleDefinitionBinding);

clientContext.ExecuteQuery();