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From: https://sharepoint.stackexchange.com/questions/194197/how-to-manipulate-likeby-nooflikes-ratedby-averagerating-userrating-using-cs

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Some of the important aspects covered are described breifly as follows.

To rate an Item for the first time by a user the following fields should be modified with the respective field values:

1. item["RatingCount"]:

   This field stores the number of users who have rated the item its value should be incremented.

2. item["Ratings"]:

   A field which stores the user ratings in CSV string format should add the new users rating like.

   int NewRating = 4; //any desired number from 1 to 5.

   item["Ratings"] += NewRating + ",";

3. item["AverageRating"]:

   This field stores the average of the ratings given by all the rated users. Simple weighted average calculation formulas can be implemented which is

   float OldAverage = item["AverageRating"] == null ? 0 : float.Parse(item["AverageRating"].ToString());

   int OldNumberOfRatings = item["RatingCount"] == null ? 0 : int.Parse(item["RatingCount"].ToString());

   int NewNumberOfRatings = OldNumberOfRatings + 1;

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   float NewAverage = OldAverage + ( ( NewRating - OldAverage ) / NewNumberOfRatings);

   //Or.

   float NewAverage = ( ( OldAverage * OldNumberOfRatings ) + NewRating ) / NewNumberOfRatings;

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   item["AverageRating"] = NewAverage;

4. item["RatedBy"]:

   This field stores the user information in form of array of?FieldUserValue?which can be modified using following code

   FieldUserValue[] ratedUsers = item["RatedBy"] as FieldUserValue[];

   //where ratedUsers becomes an array of users who have already rated.

   List<FieldUserValue> newUsersRated = new List<FieldUserValue>();

   if (ratedUsers != null)

   foreach (FieldUserValue ratedUser in ratedUsers)

   newUsersRated.Add(ratedUser);

   newUsersRated.Add(FieldUserValue.FromUser(user.LoginName));

   item["RatedBy"] = newUsersRated;

To Rerate the item for a user who has already rated it before you need to first find the user index (say?int userIndex) in the?item["RatedBy"]?field then change the following fields:

1. item["Ratings"]:

   Change the user rating in this CSV string by use of?userIndex.

   FieldUserValue[] ratedUsers = item["RatedBy"] as FieldUserValue[];

   for (int i = 0; i < ratedUsers.Length; i++)

   if (ratedUsers[i].LookupValue == user.Title)

   int userIndex = i;

   string[] userRatings = item["Ratings"].split(‘,‘);

   int OldRating = int.Parse(userRatings[userIndex]);

   int NewRating = 3; //any desired number from 1 to 5.

   userRatings[userIndex] = NewRating.ToString();

   string ratings = userRatings.Join(‘,‘, userRatings);

   item["Ratings"] = ratings;

2. item["AverageRating"]:

   change the average value using formula

   float OldAverage = item["AverageRating"];

   int NumberOfRatings = item["RatingCount"];

   float NewAverage = OldAverage + ( ( NewRating - OldRating ) / NumberOfRatings );

   item["AverageRating"] = NewAverage;

SharePoint中Rating相关的字段。