How to get live gold prices in .NET

dotnet new console -o GoldpriceDemo
cd GoldpriceDemo

// GoldpriceClient.cs
// curl-equivalent: https://api.goldprice.dev/v1/spot/XAU-USD-SPOT
using System.Collections.Concurrent;
using System.Net.Http.Json;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace GoldpriceDemo;

/// Converts the API's string-encoded price fields to decimal without
/// the precision loss a double round-trip would introduce.
public sealed class DecimalStringConverter : JsonConverter<decimal>
{
    public override decimal Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => decimal.Parse(reader.GetString()!);

    public override void Write(Utf8JsonWriter writer, decimal value, JsonSerializerOptions options)
        => writer.WriteStringValue(value.ToString());
}

public sealed record SpotResponse(
    [property: JsonPropertyName("symbol")] string Symbol,
    [property: JsonPropertyName("quote_currency")] string QuoteCurrency,
    [property: JsonPropertyName("price"), JsonConverter(typeof(DecimalStringConverter))] decimal Price,
    [property: JsonPropertyName("bid"), JsonConverter(typeof(DecimalStringConverter))] decimal? Bid,
    [property: JsonPropertyName("ask"), JsonConverter(typeof(DecimalStringConverter))] decimal? Ask,
    [property: JsonPropertyName("is_stale")] bool IsStale,
    [property: JsonPropertyName("computed_at")] string ComputedAt
);

public sealed class GoldpriceRateLimitedException : Exception
{
    public GoldpriceRateLimitedException() : base("goldprice: 429 rate limited") { }
}

public sealed class GoldpriceClient
{
    private const string ApiBase = "https://api.goldprice.dev";
    private static readonly TimeSpan Ttl = TimeSpan.FromSeconds(60);

    private readonly HttpClient _http;
    private readonly ConcurrentDictionary<string, (SpotResponse Data, DateTimeOffset Expires)> _cache = new();

    public GoldpriceClient(HttpClient http)
    {
        var apiKey = Environment.GetEnvironmentVariable("GOLDPRICE_API_KEY")
            ?? throw new InvalidOperationException("GOLDPRICE_API_KEY not set");
        _http = http;
        _http.BaseAddress = new Uri(ApiBase);
        _http.Timeout = TimeSpan.FromSeconds(5);
        _http.DefaultRequestHeaders.Add("X-API-Key", apiKey);
    }

    /// Fetches the live spot price. Cached 60s per symbol, retries 3x on 429.
    public async Task<SpotResponse> SpotAsync(string symbol = "XAU-USD-SPOT", CancellationToken ct = default)
    {
        if (_cache.TryGetValue(symbol, out var cached) && cached.Expires > DateTimeOffset.UtcNow)
        {
            return cached.Data;
        }

        for (var attempt = 0; attempt < 3; attempt++)
        {
            using var response = await _http.GetAsync($"/v1/spot/{symbol}", ct);

            if (response.StatusCode == System.Net.HttpStatusCode.TooManyRequests)
            {
                await Task.Delay(TimeSpan.FromSeconds(Math.Pow(2, attempt)), ct); // 1s, 2s, 4s
                continue;
            }
            if (!response.IsSuccessStatusCode)
            {
                throw new HttpRequestException($"goldprice: unexpected status {(int)response.StatusCode}");
            }

            var data = await response.Content.ReadFromJsonAsync<SpotResponse>(cancellationToken: ct)
                ?? throw new JsonException("goldprice: empty response body");
            _cache[symbol] = (data, DateTimeOffset.UtcNow.Add(Ttl));
            return data;
        }

        throw new GoldpriceRateLimitedException();
    }
}

using GoldpriceDemo;

using var http = new HttpClient();
var client = new GoldpriceClient(http);

var spot = await client.SpotAsync("XAU-USD-SPOT");
Console.WriteLine($"{spot.Symbol}-{spot.QuoteCurrency}: {spot.Price} (computed {spot.ComputedAt})");
// XAU-USD: 4726.01 (computed 2026-07-06T04:49:01.706844+00:00)

// USD spot — default
await client.SpotAsync("XAU-USD-SPOT");

// INR — Indian gold market (largest retail gold-search globally)
await client.SpotAsync("XAU-INR-SPOT");

// EUR — European pricing surfaces
await client.SpotAsync("XAU-EUR-SPOT");

// Silver in USD, available on every plan
await client.SpotAsync("XAG-USD-SPOT");