How to Fix System.NullReferenceException in ASP.NET RazorPages

Discover the solution to the common `System.NullReferenceException` in ASP.NET RazorPages. Understand the error, its causes, and how to fix it step by step.
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Understanding System.NullReferenceException in ASP.NET RazorPages

If you're new to ASP.NET and working on creating dynamic web applications, hitting a System.NullReferenceException can be frustrating. This error typically occurs when your code attempts to access a property or method on an object that is null—meaning it hasn't been instantiated or it doesn't point to any actual data.

In the context of ASP.NET RazorPages, this can happen for several reasons, particularly concerning incorrect data bindings or misconfigured model classes. In this post, we'll take a closer look at a common scenario that leads to this error and how you can easily resolve it.

The Scenario

In this example, a user is trying to build a simple dashboard using RazorPages that shows statistics like online players and errors. However, upon running the application, they encounter a System.NullReferenceException.

Here's a brief look at the relevant code:

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Controller (HomeController)

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ViewModel (DashboardViewModel)

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Solution Breakdown

Based on the provided code, there are a couple of common issues that could cause the System.NullReferenceException error. Here’s how to fix them step by step.

1. Remove the Semicolons in ViewModel

One of the common pitfalls that leads to errors in C# is the inclusion of unnecessary semicolons (;) after property definitions in a class. The properties of your DashboardViewModel should be defined without a semicolon at the end.

Before:

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After:

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2. Ensure Proper Data Model Instantiation

The method Index in your HomeController seems fine as it appears to instantiate the DashboardViewModel and populate its properties with values correctly. Just be sure that the model is being passed correctly to the view.

3. Update Razor Page Settings

Update Example

After applying the necessary changes and ensuring your data model is being populated and passed accurately, your code should look like this:

DashboardViewModel corrected:

[[See Video to Reveal this Text or Code Snippet]]

Once you adjust your code accordingly and verify the routing and linking, the System.NullReferenceException should no longer occur.

Conclusion

Encountering System.NullReferenceException can happen to anyone, especially when diving into ASP.NET RazorPages for the first time. However, by carefully checking your property definitions and ensuring that your models are instantiated correctly, you can effectively avoid these pitfalls.

Feel free to reach out if you have further questions or need additional help! Happy coding!