/// 

/// Set Unique permissions to document Library
/// 
/// 
/// 
/// possible values 'Full Control', 'Read', 'Contribute' 
public static void SetDocLibPermissions(SPWeb spWeb, SPGroup grpToGrantPermission, string roleDefinitionName)
{
 string docLibName = "MyDocumentLib";

 SPList MyDocLibrary = spWeb.Lists[docLibName];
 if (MyDocLibrary != null)
 {
  MyDocLibrary.BreakRoleInheritance(false);
  if (roleDefinitionName != null)
  {
   SPRoleAssignment roleAssignment = new SPRoleAssignment(grpToGrantPermission);
   SPRoleDefinition roleDefinition = spWeb.RoleDefinitions[roleDefinitionName];
   roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
   MyDocLibrary.RoleAssignments.Add(roleAssignment);

   MyDocLibrary.Update();
  }
 }
}