Debugging System.FormatException when launching ASP.NET Core

I talked with a user the other day, experiencing a System.FormatException exception when trying to launch ASP.NET Core. While trying to help to figure out what went wrong, I realized that there can be a lot of different causes for this error in ASP.NET Core. This post is a summary of what I found out and actions to go through next time.

There are a number of problems that can cause a System.FormatException during startup. If you haven't already, make sure to enable the developer exception page when running locally by including the following code in the Configure method in the Startup.cs file:

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
}

This will provide you with some additional information that can be helpful when needing to find the source of this error.

Here's a couple of variants of the exception:

JsonReaderException: The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.

JsonReaderException: 'd' is an invalid start of a property name. Expected a '"'. LineNumber: 1 | BytePositionInLine: 14.

JsonReaderException: ':' is invalid after a value. Expected either ',', '}', or ']'. LineNumber: 2 | BytePositionInLine: 14.

As you can see from the list, the FormatException typically wrap a JsonReaderException meaning that something is wrong with a JSON file. Here's a checklist to go through:

The steps in the following section should be executed for all of the following files:

• appsettings.json
• appsettings.*.json (like appsettings.development.json)
• secrets.json
• global.json
• launchSettings.json
• Any custom .json files. ASP.NET Core supports including additional configuration through custom JSON files. Make sure to validate those as well.

And the steps to execute:

• Open the appsettings.json file and validate that the file contains any data. An empty file will cause a FormatException. If you don't want any config in the file, as a minimum include {}.
• Verify that the JSON content is syntactically correct. You can use the built-in validation support in Visual Studio or an external validator like this one: JSON Formatter and Validator.

Also, you can consider validating that the content of the appsettings.*.json files is valid using this Appsettings.json Validator. Invalid content usually doesn't cause a FormatException but better safe than sorry.

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