The C# Invoice OCR API for High-Accuracy Data Extraction

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using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Json;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading.Tasks;
using System.Collections.Generic;

public class InvoiceOcrExample
{
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    private const string ApiKey = "YOUR_API_KEY_HERE";
    private const string ApiEndpoint = "https://api.structocr.com/v1/invoice";

    private static readonly HttpClient client = new HttpClient();

    public static async Task Main(string[] args)
    {
        // Note: Currently supports image inputs (JPG, PNG)
        string imagePath = "invoice.jpg";

        if (!File.Exists(imagePath))
        {
            Console.WriteLine($"Error: File not found at {imagePath}");
            return;
        }

        try
        {
            // 1. Prepare Payload
            byte[] imageBytes = await File.ReadAllBytesAsync(imagePath);
            string base64Image = Convert.ToBase64String(imageBytes);
            var payload = new { img = base64Image };

            // 2. Setup Request
            // Important: Clear headers to avoid duplication if reusing client
            client.DefaultRequestHeaders.Clear();
            client.DefaultRequestHeaders.Add("x-api-key", ApiKey);

            // 3. Send POST Request
            Console.WriteLine($"Sending invoice to {ApiEndpoint}...");
            HttpResponseMessage response = await client.PostAsJsonAsync(ApiEndpoint, payload);

            // 4. Handle Response
            string responseBody = await response.Content.ReadAsStringAsync();

            if (!response.IsSuccessStatusCode)
            {
                Console.WriteLine($"API Error ({response.StatusCode}): {responseBody}");
                return;
            }

            // 5. Deserialize JSON to Objects
            var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
            var result = JsonSerializer.Deserialize<ApiResponse>(responseBody, options);

            if (result?.Success == true && result.Data != null)
            {
                var data = result.Data;
                Console.WriteLine("✅ Extraction Successful!");
                Console.WriteLine($"Invoice #: {data.InvoiceNumber}");
                Console.WriteLine($"Date:      {data.Date}");
                Console.WriteLine($"Vendor:    {data.Merchant?.Name} (Tax ID: {data.Merchant?.TaxId})");
                Console.WriteLine($"Total:     {data.Financials?.TotalAmount} {data.Currency}");

                Console.WriteLine("\n--- Line Items ---");
                if (data.LineItems != null)
                {
                    foreach (var item in data.LineItems)
                    {
                        Console.WriteLine($"- {item.Description}: {item.Quantity} x {item.UnitPrice} = {item.Amount}");
                    }
                }
            }
            else
            {
                Console.WriteLine($"Extraction Failed: {result?.Error}");
            }
        }
        catch (Exception e)
        {
            Console.WriteLine($"Unexpected Error: {e.Message}");
        }
    }
}

// --- Data Models ---
public class ApiResponse
{
    public bool Success { get; set; }
    public InvoiceData Data { get; set; }
    public string Error { get; set; }
}

public class InvoiceData
{
    [JsonPropertyName("invoice_number")]
    public string InvoiceNumber { get; set; }
    public string Date { get; set; }
    public string Currency { get; set; }
    public MerchantData Merchant { get; set; }
    public FinancialData Financials { get; set; }
    
    [JsonPropertyName("line_items")]
    public List<LineItem> LineItems { get; set; }
}

public class MerchantData
{
    public string Name { get; set; }
    [JsonPropertyName("tax_id")]
    public string TaxId { get; set; }
}

public class FinancialData
{
    [JsonPropertyName("total_amount")]
    public decimal? TotalAmount { get; set; }
    [JsonPropertyName("tax_amount")]
    public decimal? TaxAmount { get; set; }
}

public class LineItem
{
    public string Description { get; set; }
    public decimal? Quantity { get; set; }
    [JsonPropertyName("unit_price")]
    public decimal? UnitPrice { get; set; }
    public decimal? Amount { get; set; }
}