JsonStringValidator.cs

using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using Microsoft.OpenApi.Models;
using Newtonsoft.Json.Linq;

namespace Swashbuckle.AspNetCore.ApiTesting
{
    public class JsonStringValidator : IJsonValidator
    {
        public bool CanValidate(OpenApiSchema schema) => schema.Type == JsonSchemaTypes.String;

        public bool Validate(
            OpenApiSchema schema,
            OpenApiDocument openApiDocument,
            JToken instance,
            out IEnumerable<string> errorMessages)
        {
            if (instance.Type is not JTokenType.Date and not JTokenType.Guid and not JTokenType.String)
            {
                errorMessages = [$"Path: {instance.Path}. Instance is not of type 'string'"];
                return false;
            }

            var stringValue = instance.Value<string>();
            var errorMessagesList = new List<string>();

            // maxLength
            if (schema.MaxLength.HasValue && (stringValue.Length > schema.MaxLength.Value))
            {
                errorMessagesList.Add($"Path: {instance.Path}. String length is greater than maxLength");
            }

            // minLength
            if (schema.MinLength.HasValue && (stringValue.Length < schema.MinLength.Value))
            {
                errorMessagesList.Add($"Path: {instance.Path}. String length is less than minLength");
            }

            // pattern
            if ((schema.Pattern != null) && !Regex.IsMatch(stringValue, schema.Pattern))
            {
                errorMessagesList.Add($"Path: {instance.Path}. String does not match pattern");
            }

            errorMessages = errorMessagesList;
            return !errorMessages.Any();
        }
    }
}