Solved: “Expression evaluation failed. The message body is not a valid JSON. Error reading JObject from JsonReader”

Are you tired of encountering the frustrating error message “Expression evaluation failed. The message body is not a valid JSON. Error reading JObject from JsonReader”? You’re not alone! This error can be a real showstopper, but fear not, dear developer, for we’re about to dive into the world of JSON and C# to get to the bottom of this annoying issue.

What’s causing the error?

The error message “Expression evaluation failed. The message body is not a valid JSON. Error reading JObject from JsonReader” is typically thrown when there’s a problem with the JSON data being passed to the JsonReader. This can occur when:

• The JSON data is malformed or not well-formed.
• The JSON data contains invalid characters or encoding.
• The JsonReader is not properly configured or initialized.
• There’s a mismatch between the JSON data and the expected C# object structure.

Before we begin…

Make sure you have the following installed and configured:

• Visual Studio (any version)
• .NET Framework (any version)
• Newtonsoft.Json NuGet package (version 12.0.3 or higher)

Step 1: Verify JSON data

The first step in resolving this error is to ensure that the JSON data being passed is valid and well-formed. You can use online tools like JSONLint or CodeBeautify to validate your JSON data.

{
  "name": "John Doe",
  "age": 30,
  " occupation": "Developer"
}

In the above example, the JSON data is valid and well-formed. However, if your JSON data contains errors, such as mismatched brackets or invalid characters, it will throw the “Expression evaluation failed” error.

Step 2: Configure JsonReader

Next, ensure that the JsonReader is properly configured and initialized. In C#, you can use the following code to create a JsonReader instance:

using (JsonReader reader = new JsonTextReader(new StringReader(jsonString)))
{
    // Deserialization logic goes here
}

Make sure to replace `jsonString` with the actual JSON data being passed.

Step 3: Match JSON data to C# object structure

One of the most common causes of the “Expression evaluation failed” error is a mismatch between the JSON data and the expected C# object structure. Ensure that the C# object matches the JSON data structure:

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Occupation { get; set; }
}

In the above example, the C# object `Person` matches the JSON data structure. If the C# object does not match the JSON data, it will throw the “Expression evaluation failed” error.

Step 4: Deserialize JSON data

Now that we’ve verified the JSON data and configured the JsonReader, it’s time to deserialize the JSON data into a C# object. Use the following code to deserialize the JSON data:

Person person = JsonConvert.DeserializeObject<Person>(jsonString);

Replace `jsonString` with the actual JSON data being passed. If the deserialization is successful, the `person` object will contain the deserialized data.

Troubleshooting common issues

If you’re still encountering issues, here are some common troubleshooting steps:

1. Check for leading or trailing whitespace in the JSON data.

2. Verify that the JSON data is UTF-8 encoded.

3. Ensure that the JsonReader is properly closed after use.

4. Check for any Unicode characters in the JSON data that may cause issues.

Conclusion

In conclusion, the “Expression evaluation failed. The message body is not a valid JSON. Error reading JObject from JsonReader” error can be resolved by verifying the JSON data, configuring the JsonReader, matching the JSON data to the C# object structure, and deserializing the JSON data. By following these steps, you’ll be able to resolve this error and get back to developing your application.

Remember, a well-formed JSON data and a properly configured JsonReader are key to successful deserialization. If you’re still encountering issues, feel free to reach out in the comments below.

Common JSON Errors	Fixin’ Solutions
Malformed JSON data	Validate JSON data using online tools like JSONLint or CodeBeautify
Invalid characters or encoding	Ensure UTF-8 encoding and remove invalid characters
JsonReader configuration issues	Verify JsonReader configuration and initialization
Mismatch between JSON data and C# object structure	Match C# object structure to JSON data structure

By following these steps and troubleshooting common issues, you’ll be well on your way to resolving the “Expression evaluation failed” error and getting back to developing your application.

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