How to Avoid 'Cast Failed' Errors When Summing Nullable Integers in LINQ?

Handling null values ​​in LINQ queries: solving "Conversion failed" error

When executing a LINQ query, developers may encounter the error: "Cannot convert to value type 'Int32' because the materialized value is null." This issue occurs when trying to convert a nullable value to a non-nullable value type hour.

The following LINQ query illustrates this situation:

<code>var creditsSum = (from u in context.User
                  join ch in context.CreditHistory on u.ID equals ch.UserID                                        
                  where u.ID == userID
                  select ch.Amount).Sum();</code>

This query calculates the sum of the Amount column for a specific userID in the CreditHistory table. However, if there is no record for the specified user in the CreditHistory table, the ch.Amount expression returns null. This null value cannot be converted to the non-nullable value type Int32, resulting in an error.

Solution: Handling null values

In order to handle nullable values ​​in LINQ queries, there are two possible approaches:

1. Use DefaultIfEmpty: This method adds a default value to the query results if no rows are returned. In the given scenario, the query can be modified to:

<code>var creditsSum = (from u in context.User
                  join ch in context.CreditHistory on u.ID equals ch.UserID                                        
                  where u.ID == userID
                  select ch.Amount).DefaultIfEmpty(0).Sum();</code>

2. Using the conditional operator (??): The conditional operator (??) allows you to specify a default value to be used in the event of a null result. The query can be rewritten as:

<code>var creditsSum = (from u in context.User
                  join ch in context.CreditHistory on u.ID equals ch.UserID                                        
                  where u.ID == userID
                  select (int?)ch.Amount).Sum() ?? 0;</code>

In this version, the expression (int?)ch.Amount explicitly converts the value to int?, indicating that it can be null. The subsequent ?? operator replaces any null values ​​with the default value of 0.

By implementing either of these two methods, the query can successfully handle null values ​​and return the desired results.

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