How to Resolve 'Cast to Int32 Failed' Errors in LINQ Queries with Null Values?

Handling null values ​​in LINQ queries: solving "Cannot convert to Int32" error

When you encounter the "Cannot convert to value type 'Int32' because the materialized value is null" error in a LINQ query, it indicates a mismatch between the expected non-null type and the possible presence of null values ​​in the query results.

Original query:

<code class="language-csharp">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>

Question:

This query retrieves the sum of ch.Amount for users with a specific userID. However, if there is no record for that user in the CreditHistory table, the query will try to convert null values ​​in the database results to Int32, causing an error.

Solution:

To accommodate null values, solutions include introducing nullable types and handling null cases appropriately.

Use nullable types:

First, convert the expression in the select clause to a nullable type, such as int?. This will tell the compiler that the property may contain a null value:

<code class="language-csharp">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();</code>

Use the null coalescing operator (??):

Next, use the ?? operator to handle the null case. It checks if the expression is empty and returns the default value (in this case 0):

<code class="language-csharp">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>

By incorporating these modifications, queries can handle null values ​​gracefully and return appropriate results, preventing conversion errors.

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