Скачать или смотреть Understanding Dependency Injection Issues in Partial Classes in .NET Core

Discover solutions to the common challenges of using `dependency injection` with partial classes in .NET Core 3.1. Learn how to resolve `null` references effectively.
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Solving Dependency Injection Issues in Partial Classes in .NET Core 3.1

When developing applications with .NET Core 3.1, developers often take advantage of partial classes to organize service implementations across multiple files. While this approach can improve code readability and maintainability, it can introduce certain complications, particularly concerning dependency injection (DI). In this article, we’ll delve into a specific issue regarding null references in partial classes when using DI and how to troubleshoot it.

The Problem: Null References in Partial Classes

Imagine you have your AccountsService divided into two partial classes: AccountsService.cs and AccountsService.Roles.cs. You expect to access your DbContext in both partial classes after injecting it through the constructor. However, you encounter a scenario where calling one method results in _dbContext being null:

Method Get(String username) works as expected and utilizes _dbContext correctly.

Method HasRole(String username, int roleId) fails, showing _dbContext as null.

The reason seems to be that the constructor from AccountsService.cs is never invoked when Test2 (which calls HasRole) is executed.

Unpacking the Issue

The problems you are facing could stem from several root causes. Based on community comments and best practices, here are the potential explanations:

Constructor Called with Null Argument:
If _dbContext was instantiated with a null argument, the constructor would not work correctly. However, since you note that the constructor is "never hit" during Test2, this possibility can be ruled out.

Different Constructor Invoked:
It’s possible that there’s a different constructor being invoked—perhaps a default constructor (a parameterless constructor) is being used instead of the one that takes MyDbContext. This could happen if there exists an unexamined partial class.

Failure in Class Merging:
C# compiles partial classes into a single type. If the partial classes reside in different namespaces or assemblies, the compiler treats them as separate types. This behavior would lead to C# generating a default constructor for the partial class that calls the DI container’s default constructor instead, causing _dbContext to be null.

Troubleshooting Steps

To resolve these issues, follow these structured steps:

Step 1: Check for Additional Partial Classes

Review All Files: Make sure to inspect your project for any additional partial class definitions that may not share the same namespace or that define different constructors.

Step 2: Verify Namespaces

Ensure Same Namespace: Confirm all parts of the AccountsService are declared within the same namespace. If they are in different namespaces, the compiler will not merge them properly.

Step 3: Inspect Assemblies

Check Assembly Declarations: If you are using partial classes spread across different assemblies, this could also lead to compiler issues. Make sure all related code is compiled together.

Step 4: Testing DI Parameters

Add Unit Tests: Implement unit tests to validate that the correct constructors are called with expected parameters. Use mocking frameworks to simulate injection dependencies.

Conclusion

While utilizing partial classes in .NET Core offers significant organizational benefits, it’s important to be mindful of how dependency injection works with these structures. By understanding the merging behavior of partial classes and ensuring that they are correctly defined, you can troubleshoot and solve null reference issues effectively.

With careful examination and following best practices, you'll be able to harness the full power of dependency injection without running into the pitfalls presented by partial classes.