dataverse_select_expand.cs

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

public class DataverseSelectExpandExample
{
    public static async Task Main()
    {
        var environmentUrl = "https://yourorg.crm.dynamics.com";
        var accessToken = "YOUR_ACCESS_TOKEN";
        var requestUrl = environmentUrl + "/api/data/v9.2/contacts?$select=fullname,emailaddress1&$expand=parentcustomerid_account($select=name,accountnumber)";

        using var client = new HttpClient();
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Add("OData-Version", "4.0");
        client.DefaultRequestHeaders.Add("OData-MaxVersion", "4.0");

        var response = await client.GetAsync(requestUrl);
        response.EnsureSuccessStatusCode();

        var content = await response.Content.ReadAsStringAsync();
        Console.WriteLine(content);
    }
}