Untitled

Here is the equivalent C# code for the provided Perl `check_required` subroutine:

```csharp
using System;
using System.Collections.Generic;
using System.Collections.Specialized;

class Program
{
    static void Main()
    {
        // Example usage
        NameValueCollection config = new NameValueCollection();
        NameValueCollection form = new NameValueCollection();
        List<string> fieldOrder = new List<string>();

        ParseForm(config, form, fieldOrder);

        CheckRequired(config, form);
    }

    static void CheckRequired(NameValueCollection config, NameValueCollection form)
    {
        List<string> error = new List<string>();

        if (string.IsNullOrEmpty(config["recipient"]))
        {
            if (form.Count > 0)
            {
                Console.WriteLine("has hash members");
            }
            else
            {
                Console.WriteLine("no recipient");
            }
        }

        foreach (string require in config.GetValues("required"))
        {
            // If the required field is the email field, check the syntax of the email address.
            if (require == "email" && !CheckEmail(config[require]))
            {
                error.Add(require);
            }
            // If it is a configuration field and it has no value or has been filled in with a space, send an error.
            else if (!string.IsNullOrEmpty(config[require]))
            {
                if (string.IsNullOrWhiteSpace(config[require]))
                {
                    error.Add(require);
                }
            }
            // If it is a regular form field which has not been filled in or filled in with a space, flag it as an error field.
            else if (string.IsNullOrEmpty(form[require]))
            {
                error.Add(require);
            }
        }

        // If any error fields have been found, send an error message to the user.
        if (error.Count > 0)
        {
            Error("missing_fields", error.ToArray());
        }
    }

    static bool CheckEmail(string email)
    {
        // Implement your email validation logic here.
        // Return true if the email is valid; otherwise, return false.
        // Example validation: return email.Contains("@");
        return true;
    }

    static void ParseForm(NameValueCollection config, NameValueCollection form, List<string> fieldOrder)
    {
        // The ParseForm function as provided in the previous response.
        // ...
    }

    static void Error(string errorType, params string[] errorFields)
    {
        // Implement your error handling logic here.
        // Output an error message to the user based on the error type and fields.
        // Example: Console.WriteLine($"Error: {errorType} - {string.Join(", ", errorFields)}");
    }
}
```

This C# code includes the `CheckRequired` function, which performs the equivalent logic of the Perl `check_required` subroutine. The `CheckEmail` function is a placeholder for email validation; you should implement your specific email validation logic there. The `ParseForm` function is assumed to be the same as provided in the previous response. The `Error` function is a placeholder for your error handling logic.

Please customize the email validation and error handling logic based on your specific requirements.