Send exception for UNIQUE constraint to UI in .NET core - sql

Question

When a unique constraint exception occurs, how do I message the UI in .NET Core; I want to return JSON, not MVC/Razor approach.

Answer 1

I think you have been looking at my article Catching Bad Data in Entity Framework. In there I describe a way to catch a SQL error and turn it into a validation error for EF Core.
UPDATE
In answer to your follow on questions I can point you to the code associated with the book I am writing, Entity Framework Core in Action. In this I built a method called SaveChangesSqlCheck, which contains the code to check for sql errors. You would use this method instead of calling SaveChanges, and it will return a ValidationResult.
My book has associated git repository where I have unit tests for just about everything I show in the book. Below are links to a unit test to see how you call SaveChangesSqlCheck, and then the SaveChangesSqlCheck code itself.
Unit test: https://github.com/JonPSmith/EfCoreInAction/blob/Chapter10/Test/UnitTests/DataLayer/Ch10_CatchSqlError.cs#L59-L89
The SaveChangesSqlCheck code: https://github.com/JonPSmith/EfCoreInAction/blob/Chapter10/DataLayer/EfCode/SaveChangesSqlCheck.cs
The method that catches the Unique error: https://github.com/JonPSmith/EfCoreInAction/blob/Chapter10/DataLayer/EfCode/SqlErrorFormatters.cs
Note: you need to format the unique constraint name in a special way - see https://github.com/JonPSmith/EfCoreInAction/blob/Chapter10/Test/Chapter10Listings/EfCode/Configuration/MyUniqueConfig.cs for an example.
If you plan to use the code shown in the article then all you need to do is use JsonConvert.SerializeObject(errors) to turn that into json. I have included some code so you can see what the json output would look like.
var error = new ValidationResult("error message");
var jsonList = JsonConvert.SerializeObject(error,
new JsonSerializerSettings { Formatting = Newtonsoft.Json.Formatting.Indented });
The json output of this would be
{
"MemberNames": [],
"ErrorMessage": "error message"
}
I hope that helps.