Add an existing site collection Group to sub site with modified Roles

OBJECTIVE

The site collection contains a group named “All Members” with “Contributor” privilege.

I want to add this group programmatically to a sub site and modify its roles from “Contributor” to 

“Read” without modifying the original group.

The following are the variables used in the code:

       -          site: the SPSite object of the site collection being used

-          web: the SPWeb object of the sub site you want to add the group to.

SOLUTION:

SPGroup group = site.RootWeb.SiteGroups["All Members"];
SPRoleAssignment roleAssignment = new SPRoleAssignment(group);
SPRoleDefinition roleDefinition = site.RootWeb.RoleDefinitions["Read"];
roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
if (!web.HasUniqueRoleAssignments)
     web.BreakRoleInheritance(true);
web.RoleAssignments.Add(roleAssignment);

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Working with Group: quick review:

      1)       BREAK INHERITANCE  
         2)       ADD NEW GROUP TO SITE COLLECTION 
         3)       ASSOCIATE THE NEW GROUP TO A WEB 
         4)       ASSIGNMENT OF THE ROLES
         5)       ADD USERS TO THE GROUP 
         6)       REMOVE ALL ROLES
     

1) BREAK INHERITANCE

if (!web.HasUniqueRoleAssignments)

            web.BreakRoleInheritance(true);

2) ADD NEW GROUP TO SITE COLLECTION:

web.SiteGroups.Add("MyNewGroup", web.AssociatedOwnerGroup, null, "MyNewGroup description");

3) ASSOCIATE THE NEW GROUP TO A WEB:

web.AssociatedGroups.Add(group);

web.Update();

4) ASSIGNMENT OF THE ROLES:

SPGroup group = web.SiteGroups["MyNewGroup"];

SPRoleAssignment roleAssignment = new SPRoleAssignment(group);

SPRoleDefinition roleDefinition = site.RootWeb.RoleDefinitions["Read"];

roleAssignment.RoleDefinitionBindings.Add(roleDefinition);

web.RoleAssignments.Add(roleAssignment);

5) ADD USERS TO THE GROUP:

SPUser user = web.EnsureUser("domain\MyUser"); 
SPGroup group = web.Groups["MyNewGroup"];                        
if(user!=null && group!=null)
     group.AddUser(user);

6) REMOVE ALL ROLES:

SPRoleAssignmentCollection SPRoleAssColn = web.RoleAssignments;
for (int i = SPRoleAssColn.Count - 1; i >= 0; i--)
{
      SPRoleAssColn.Remove(i);
}