How to get ActionParameters in OnResultExecuted action filter

DORSETRIGS

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07-10-2024

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Unlocking Action Parameters in ASP.NET Core's OnResultExecuted

In ASP.NET Core, action filters are invaluable tools for adding cross-cutting concerns to your controllers. One common need is to access the parameters passed to the action method within an action filter. This can be useful for logging, auditing, or performing post-execution tasks that rely on the data sent with the request.

However, retrieving ActionParameters within the OnResultExecuted method of an action filter presents a unique challenge. This is because, by the time OnResultExecuted is called, the action method has already been executed, and the parameters are no longer readily available.

Let's break down the problem and explore a solution.

The Scenario:

Imagine you have an API endpoint that accepts a user ID as a parameter:

[HttpGet("{userId}")]
public IActionResult GetUser(int userId)
{
    // Logic to retrieve user data based on userId
    return Ok(user);
}

You want to log the userId in your OnResultExecuted method of an action filter for auditing purposes.

Original Code:

public class LoggingFilter : IActionFilter
{
    public void OnActionExecuted(ActionExecutedContext context)
    {
        // Unable to access userId here
    }

    public void OnActionExecuting(ActionExecutingContext context)
    {
        // userId available here
    }
}

As you can see, accessing the userId within OnActionExecuted is not straightforward.

Solution:

The key to accessing action parameters in OnResultExecuted lies in leveraging the HttpContext object.

1. Store parameters in the HttpContext: During OnActionExecuting, we can store the action parameters in the HttpContext using a custom key.

2. Retrieve parameters in OnResultExecuted: In OnResultExecuted, we can retrieve the stored parameters from the HttpContext using the same key.

Code Example:

public class LoggingFilter : IActionFilter
{
    private const string UserIdKey = "UserId";

    public void OnActionExecuting(ActionExecutingContext context)
    {
        if (context.ActionArguments.ContainsKey("userId"))
        {
            context.HttpContext.Items[UserIdKey] = context.ActionArguments["userId"];
        }
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        if (context.HttpContext.Items.ContainsKey(UserIdKey))
        {
            var userId = context.HttpContext.Items[UserIdKey];
            // Log the userId here
        }
    }
}

Explanation:

• In OnActionExecuting, we check if the userId parameter exists in the ActionArguments collection. If found, we store it in the HttpContext.Items dictionary under the key UserIdKey.
• In OnResultExecuted, we retrieve the stored userId from HttpContext.Items using the UserIdKey and perform our logging operation.

Additional Considerations:

• You can adapt this approach to store and retrieve multiple action parameters as needed.
• Consider using a more descriptive key to avoid conflicts with other filters or applications using the HttpContext.Items dictionary.
• For complex scenarios, consider using a dedicated service to manage the storage and retrieval of action parameters.

Conclusion:

Accessing action parameters in OnResultExecuted requires a slightly different approach than within OnActionExecuting. By storing the necessary data in the HttpContext during OnActionExecuting and retrieving it later in OnResultExecuted, we can effectively work with action parameters across different stages of the request lifecycle. This technique enables us to implement powerful action filters that can perform diverse tasks based on the request data.