Exception:Collection was modified; enumeration operation may not execute.

SPRoleAssignmentCollection ras = list.RoleAssignments;
foreach (SPRoleAssignment ra in ras)
{
.....
SPRoleAssignment newRoleAssignment = new SPRoleAssignment((SPPrincipal)list.ParentWeb.EnsureUser("domain\user"));
SPRoleDefinition newRoleDefinition = list.ParentWeb.RoleDefinitions["参与"];
newRoleAssignment.RoleDefinitionBindings.Add(newRoleDefinition);

list.RoleAssignments.Add(newRoleAssignment);
.......
}

解决方案：

foreach 改用 for循环