EF Core 8 and LINQKit: Expression Include Clauses Throw 'InvalidOperationException' for Variable of Type System.Int32 Referenced in Scope, Not Defined in DBContext (2024)

Abstract: This article discusses an issue encountered when using EF Core 8 and LINQKit, where an 'InvalidOperationException' is thrown when invoking expression include clauses with a variable of type System.Int32 that is referenced in the scope but not defined in the DBContext.

2024-05-20 by DevCodeF1 Editors

EF Core 8 LINQ Kit: Expression Include Clauses Throw Invalid Operation Exception Variable Type System.Int32 Referenced Scope, Not Defined DB Context

In this article, we will explore a common issue that developers face when working with EF Core and LINQ Kit, specifically when using expression include clauses. The error message that we are focusing on is: InvalidOperationException: The type of the specified variable is Int32, which is not a valid reference type for an expression tree. The variable must be a reference type in order to be used in an expression tree.

Understanding the Issue

The issue arises when we try to use an integer variable as a parameter in an expression include clause. This is not valid because expression trees can only contain reference types, not value types. In this case, the variable type is an integer, which is a value type, and therefore cannot be used in an expression tree.

Example Scenario

Let's consider an example scenario to understand the issue better. Suppose we have a Blog class and a Post class, where each post belongs to a single blog. We want to retrieve all the posts for a given blog using LINQ and PostgreSQL as our database provider.

public class Blog{ public int BlogId { get; set; } public string Name { get; set; } public string Url { get; set; } public IList Posts { get; set; }}public class Post{ public int PostId { get; set; } public int BlogId { get; set; } public string Title { get; set; } public string Content { get; set; } public Blog Blog { get; set; }}

We can retrieve all the posts for a given blog using the following LINQ query:

var blog = PostgreSQLDbContext.Blogs.Include(b => b.Posts).FirstOrDefault(b => b.BlogId == blogId);

However, if we try to use an expression tree to include the posts, we will encounter the aforementioned error:

var blog = PostgreSQLDbContext.Blogs.Include(_ => _.Posts).FirstOrDefault(b => b.BlogId == blogId);

Solution

The solution to this issue is to use a reference type instead of a value type as the parameter in the expression include clause. In this case, we can use a Func<Blog, IIncludable<Blog, Post>> delegate instead of an integer variable.

var includeFunc = new Func>(b => b.Posts);var blog = PostgreSQLDbContext.Blogs.Include(includeFunc).FirstOrDefault(b => b.BlogId == blogId);

By using a delegate, we can avoid the issue of using a value type as a parameter in the expression include clause, and the query will execute successfully.

In this article, we have explored a common issue that developers face when working with EF Core and LINQ Kit, specifically when using expression include clauses. We have learned that the issue arises when we try to use a value type as a parameter in an expression include clause, which is not valid. We have also learned how to solve this issue by using a reference type instead of a value type as the parameter in the expression include clause. By following these best practices, we can ensure that our EF Core queries execute successfully and efficiently.

References

• Related Data - EF Core
• Expression Trees - C# Programming Guide
• Compose Queryable - EF Core
• SQL Server Functions - EF Core

In this article, we will explore the root cause of the issue, provide a workaround, and discuss potential solutions for developers encountering similar problems in their EF Core 8 and LINQKit projects.

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