Resolving InvalidOperationException: Understanding JSON Deserialization Issues in ASP.NET Core

Discover the causes and solutions for InvalidOperationException in JSON deserialization when using ASP.NET Core MVC and Web APIs. Learn how to resolve model mismatches effectively.
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Understanding and Resolving InvalidOperationException in ASP.NET Core Projects

In the world of software development, encountering errors is a common occurrence. One particular error that can confuse many developers is the InvalidOperationException, specifically the one stating that it "Cannot get the value of a token type 'Number' as a string." This issue typically arises when there is a mismatch between the JSON data being received from an API and the model expected by the application. Let's delve into the root cause of this problem and explore how to effectively resolve it.

The Problem: What Does InvalidOperationException Mean?

You're developing an MVC project that consumes a Web API. While everything seems set up correctly, you encounter the following error when attempting to deserialize JSON data into your model:

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This error indicates that your application is trying to convert a number value from the JSON response into a string property in your model, which causes an exception due to the type mismatch.

Analyzing the Code: Where Did It Go Wrong?

To understand this issue better, let’s break down the relevant sections of your code.

The Model Definition

Your HouseModel class is defined as follows:

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

If, for instance, the JSON being returned from the API has the price as a numeric type, let’s say {"Price": 300000}, your model is expecting a string type for Price, leading to a deserialization error.

JSON Deserialization Flow

Your HouseController invokes the Find() method.

The HouseService class fetches data from the API and calls the ReadContentAsync<List<HouseModel>>() method.

In this method, the JsonSerializer.Deserialize<T> method utilizes the properties defined in the HouseModel class to map the JSON data to the model.

Common Causes of the Error

Data Type Mismatch: This is often the core issue. If the JSON property is a number but the model expects a string, the deserializer fails.

Different Model Definitions: Using different versions of the model in the API and MVC application can lead to significant issues during deserialization.

The Solution: Fixing the Mismatch

Upon investigating the issue, the author discovered that they were using two different models: one for the MVC and another for the API. These models contained differing data types and variable names, leading to the deserialization problem.

To resolve this issue, here are the recommended steps:

Step 1: Ensure Consistency in Models

Make sure that the data types and names used in your HouseModel match the data returned by the API. If an API property is of type number, consider changing your model to:

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Step 2: Validate the API Response

You can print the API response before attempting to deserialize:

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This ensures the API response matches what the model expects.

Step 3: Utilize Custom Converters if Necessary

If data types must remain different for some reason (e.g., legacy systems), consider using a custom JSON converter. This could help in converting types more gracefully.

Example of a custom converter:

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

Conclusion

The InvalidOperationException around JSON deserialization primarily stems from mismatches between the expected types in your model and the actual data returned from the API. By ensuring your models are consistent and reflective of the data structures being returned, you can avoid these issues and create a more seamless integration between your MVC application