How to give an SPGroup permissions for an SPItem?

I tried to find a simple example how to give a certain Sharepoint group X, a permission level Y, for list item Z - but can't find example code.

The shortest code I could come up with for assigning a specific user permissions ("Reader" in this case) for the item is the following:

SPRoleDefinition spRole = spWeb.RoleDefinitions["Reader"];    
SPRoleAssignment roleAssignment= new SPRoleAssignment("//myDomain/myUser",
                                                      "none@example.org",
                                                      "Name", "Notes");    
roleAssignment.RoleDefinitionBindings.Add(spRole);

SPListItem listItem = spWeb.GetListItem("http://<URL to 开发者_运维知识库item somewhere on the Site>");
listItem.BreakRoleInheritance(true);
listItem.RoleAssegnments.Add(roleAssignment);
listItem.Update();

I know that SPRoleAssignment.Add can also take an SPPrincipal which in turn is a group - I just don't know how do write it.

Please give me some example code for how to add an existing Sharepoint group (e.g. "MyGroup") with the "Reader" permission level to my item.

Actually it was rather easy - instead of the SPRoleAssignment("user","name"...) I could just add an SPGroup to the role assignment and it worked! Full code following:

//note: using SiteGroups is "safer",
//because also groups which don't yet have any permissions are included
SPGroup spGroup = spWeb.SiteGroups["MyGroup"];
SPRoleDefinition spRole = spWeb.RoleDefinitions["Read"]; 

SPRoleAssignment roleAssignment= new SPRoleAssignment(spGroup);
roleAssignment.RoleDefinitionBindings.Add(spRole);

SPListItem listItem = spWeb.GetListItem("http://<URL to item somewhere on the Site>");
listItem.BreakRoleInheritance(true);
listItem.RoleAssignments.Add(roleAssignment);
listItem.Update();