How to programmatically add delete modify item permissions in SharePoint

Note: SPPrincipal can be either an SPUser or SPGroup Object

Breaking Inheritance

//specifies whether the item has unique security 
//or inherits its role assignments from a parent object.
item.HasUniqueRoleAssignments

//Stops inheriting permissions from parent object
// if true, it will keep all existing users
// false, to remove all users
item.BreakRoleInheritance(true);

//Removes the local role assignments 
//and reverts to role assignments from the parent object.
item.ResetRoleInheritance();

Adding Permissions to an item

//SPGroup group = web.Groups[0];
//SPUser user = web.Users[0];
//SPUser user2 = web.EnsureUser("mangaldas.mano");
//SPUser user3 = web.EnsureUser("Domain Users"); ;
//SPPrincipal[] principals = { group, user, user2, user3 };
public static void SetPermissions(this SPListItem item, IEnumerable principals, SPRoleType roleType)
{
 if (item != null)
 {

  foreach (SPPrincipal principal in principals)
  {
   SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);
   SetPermissions(item, principal, roleDefinition);
  }
 }
}


public static void SetPermissions(this SPListItem item, SPUser user, SPRoleType roleType)
{
 if (item != null)
 {
  SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);
  SetPermissions(item, (SPPrincipal)user, roleDefinition);
 }
}

public static void SetPermissions(this SPListItem item, SPPrincipal principal, SPRoleType roleType)
{
 if (item != null)
 {
  SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);
  SetPermissions(item, principal, roleDefinition);
 }
}

public static void SetPermissions(this SPListItem item, SPUser user, SPRoleDefinition roleDefinition)
{
 if (item != null)
 {
  SetPermissions(item, (SPPrincipal)user, roleDefinition);
 }
}

public static void SetPermissions(this SPListItem item, SPPrincipal principal, SPRoleDefinition roleDefinition)
{
 if (item != null)
 {
  SPRoleAssignment roleAssignment = new SPRoleAssignment(principal);

  roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
  item.RoleAssignments.Add(roleAssignment);
 }
}

Deleting all user Permissions from an item

public static void RemovePermissions(this SPListItem item, SPUser user)
{
 if (item != null)
 {
  RemovePermissions(item, user as SPPrincipal);
 }
}

public static void RemovePermissions(this SPListItem item, SPPrincipal principal)
{
 if (item != null)
 {
  item.RoleAssignments.Remove(principal);
  item.SystemUpdate();
 }
}

Removing specific roles from an item

public static void RemovePermissionsSpecificRole(this SPListItem item, SPPrincipal principal, SPRoleDefinition roleDefinition)
{
 if (item != null)
 {
  SPRoleAssignment roleAssignment = item.RoleAssignments.GetAssignmentByPrincipal(principal);
  if (roleAssignment != null)
  {
   if (roleAssignment.RoleDefinitionBindings.Contains(roleDefinition))
   {
    roleAssignment.RoleDefinitionBindings.Remove(roleDefinition);
    roleAssignment.Update();
   }
  }
 }
}

public static void RemovePermissionsSpecificRole(this SPListItem item, SPPrincipal principal, SPRoleType roleType)
{
 if (item != null)
 {
  SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);
  RemovePermissionsSpecificRole(item, principal, roleDefinition);
 }
}

Updating or Modifying Permissions on an item

public static void ChangePermissions(this SPListItem item, SPPrincipal principal, SPRoleType roleType)
{
 if (item != null)
 {
  SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);
  ChangePermissions(item, principal, roleDefinition);
 }
}

public static void ChangePermissions(this SPListItem item, SPPrincipal principal, SPRoleDefinition roleDefinition)
{
 SPRoleAssignment roleAssignment = item.RoleAssignments.GetAssignmentByPrincipal(principal);
 if (roleAssignment != null)
 {
  roleAssignment.RoleDefinitionBindings.RemoveAll();
  roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
  roleAssignment.Update();
 }
}