Error from System.Text.Json trying to deserialize a single string

Question

Error from System.Text.Json trying to deserialize a single string

Dennis Tabako 36

In my Blazor app, I am getting a response back from the API that consists of a single string value. I'm trying to deserialize using:

return await JsonSerializer.DeserializeAsync<T>(await response.Content.ReadAsStreamAsync(), options);

In the above case, T is string. This is producing a "System.Text.Json.JsonException: 'V' is an invalid start of a value" error (V is the first character of the string being returned from the API). Visually inspecting the response content in the debugger, it looks like a UTF-8 encoded string. Is there something I'm missing?

Thanks!
Dennis

AgaveJoe 30,126 Reputation points

2021-11-18T17:32:24.447+00:00

The JSON deserializer converters a JSON string (name/value pairs) into a C# type instance. The JSON deserializer is not needed if the response is string like "Hello World".
Dennis Tabako 36 Reputation points

2021-11-18T18:26:54.43+00:00

OK, that makes sense. Thanks! It was only really an issue because I was trying to make a generic method that would send an HTTP Post to the API to add an item of some generic type. In one case, I'm just maintaining a list of names - strings - so the response contains the added name. So how does it work for something like deserializing a list of strings, which it seems to handle OK:

await JsonSerializer.DeserializeAsync<IEnumerable<T>>(await _client.GetStreamAsync(RelativeUri), new JsonSerializerOptions() { PropertyNameCaseInsensitive = true })

Again, with T being a string.
AgaveJoe 30,126 Reputation points

2021-11-18T19:29:15.963+00:00

A collection is a valid C# type that can be deserialized. Pass a List<string> to the generic method.
Dennis Tabako 36 Reputation points

2021-11-18T19:31:19.237+00:00

Great! Thanks!

1 answer

Your answer

AgaveJoe 30,126 Reputation points

2021-11-18T17:32:24.447+00:00

The JSON deserializer converters a JSON string (name/value pairs) into a C# type instance. The JSON deserializer is not needed if the response is string like "Hello World".
Dennis Tabako 36 Reputation points

2021-11-18T18:26:54.43+00:00

OK, that makes sense. Thanks! It was only really an issue because I was trying to make a generic method that would send an HTTP Post to the API to add an item of some generic type. In one case, I'm just maintaining a list of names - strings - so the response contains the added name. So how does it work for something like deserializing a list of strings, which it seems to handle OK:

await JsonSerializer.DeserializeAsync<IEnumerable<T>>(await _client.GetStreamAsync(RelativeUri), new JsonSerializerOptions() { PropertyNameCaseInsensitive = true })

Again, with T being a string.
AgaveJoe 30,126 Reputation points

2021-11-18T19:29:15.963+00:00

A collection is a valid C# type that can be deserialized. Pass a List<string> to the generic method.
Dennis Tabako 36 Reputation points

2021-11-18T19:31:19.237+00:00

Great! Thanks!

Answer 1

Bruce (SqlWork.com) 77,686 Volunteer Moderator

Your issue is the string is not valid json. A json string has quotes around it. Ex

“Valid json string”

So your server is not returning json. The client code should check the content-type. It’s probably text, not json.

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Error from System.Text.Json trying to deserialize a single string

1 answer

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