[BUG] [Csharp] Validating DateTime format exception

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

This is my first issue opened for an open-source project, so please let me know if I'm doing something wrong. Also I'm somewhat new in C# and using the OpenApi framework, so maybe I have misunderstood some things as well.

Description

When a class is generated that contains a DateTime object, that is specified by a patten, it seems as though a validation method is created for the class, which checks if the DateTime object follows the pattern. But the pattern is a string and the object is a DateTime, so the error CS1503: Argument 1: cannot convert from 'System.DateTime' to 'string' error is thrown.

To be more precise. The errornous Json part describing the API;

"Foo":{"title":"Foo","required":["myDateTime"],"type":"object","properties":{"myDateTime":{"pattern":"^\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:00.000Z$","type":"string","description":"The time in UTC format","format":"date-time"}},"x-examples":{}}

The auto-generated validation method (using net9.0):

IEnumerable<ValidationResult> IValidatableObject.Validate(ValidationContext validationContext)
{
    if (this.myDateTime != null) {
        // myDateTime (DateTime) pattern
        Regex regexMyDateTime = new Regex(@"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:00.000Z$", RegexOptions.CultureInvariant);
        if (!regexMyDateTime.Match(this.myDateTime).Success)
        {
            yield return new System.ComponentModel.DataAnnotations.ValidationResult("Invalid value for myDateTime, must match a pattern of " + regexMyDateTime, new [] { "myDateTime" });
        }
    }
{

openapi-generator version

7.13.0

Suggest a fix

I don't know what the best solution for this would be. But I have 2 different thoughts of how to "fix" this (or atleast making it not throw and Exception);

• Do not implement validation on DateTime objects in regards to pattern.
• OR create the myDateTime object as a string instead of a DateTime object if the pattern specification is present.

Start by reproducing the supplied OpenAPI schema with the C# generator and inspect the generated IValidatableObject.Validate method for the DateTime property. Check how pattern validation is generated for date-time fields; done means the generated C# code compiles and handles the pattern without passing DateTime to a string-based regex.