Set Permission to Document Library Programmatically

SPRoleAssignment roleassignment;
SPRoleDefinitionCollection webroledefinitions = web.RoleDefinitions;

SPDocumentLibrary docLib = (SPDocumentLibrary)web.Lists[“LibraryName”];

SPUser oUser = web.EnsureUser(“domain\\loginname”);
roleassignment = new SPRoleAssignment(oUser);
roleassignment.RoleDefinitionBindings.Add(webroledefinitions[“Read”]);
docLib.RoleAssignments.Add(roleassignment);
docLib.Update();