using System;
using System.Text.Json;
using Json.Schema;

var schema = @"{
  ""$schema"": ""http://json-schema.org/draft-07/schema#"",
  ""additionalProperties"": false
}";

var data = @"{
  ""prop1"":""val""
}";

var jsonSchema = JsonSchema.FromText(schema);
var validationResult = jsonSchema.Validate(JsonDocument.Parse(data).RootElement, new ValidationOptions { OutputFormat = OutputFormat.Basic });
Console.WriteLine(JsonSerializer.Serialize(validationResult));

//outputs
{
    "valid": false,
    "keywordLocation": "#/additionalProperties/$false",
    "instanceLocation": "#/prop1",
    "error": "All values fail against the false schema"
}