using System;

using System.Linq;

using System.Collections.Generic;

public class ProblemDetailsValidation

{

    public string Details { get; set; }

    public Dictionary<string, string[]> Errors { get; set; }

}

public class Program

{

    public static void Main()

    {

        // Error Simulation

        var errors = new Dictionary<string, string[]>()

        {

            { "SupplierId", new [] { "'Supplier Id' must be 24 characters in length. You entered 26 characters.",  "'Supplier Id' is not in the correct format." } }

        };

        var details = default(string);

        var response = new ProblemDetailsValidation()

        {

            Errors = errors,

            Details = details

        };

        // Error Parsing

        if (string.IsNullOrWhiteSpace(response?.Details))

        {

            Console.WriteLine(response.Details);    // Delete this

            //return response.Details;

        }

        var errorMessage = string.Join(Environment.NewLine, response.Errors.Select(error => $"'{error.Key}': {Parse(error.Value)}"));

        Console.WriteLine(errorMessage);                // Delete this

        //return errorMessage;

    }

    public static string Parse(string[] errors)

    {

        return string.Join(",", errors);

    }

}

​