How to Transform API Responses to Excel Reports in C#

In today’s data-driven world, businesses rely heavily on REST APIs for data exchange. However, many stakeholders prefer to consume this data in Excel format for analysis and reporting. This guide demonstrates how to build a complete C# application that:

Connects to REST APIs
Retrieves JSON response data
Converts the data to professionally formatted Excel reports
Adds visualization with charts and tables

Business Value

This solution delivers significant value by:

Automating manual data extraction and report creation processes
Ensuring data accuracy by eliminating manual copying
Providing consistent report formatting
Enabling scheduled report generation

Step-by-Step Implementation Guide

Step 1: Create a New C# Application

Start by creating a new console application:

dotnet new console -n ApiToExcelReporter
cd ApiToExcelReporter

Step 2: Install Required Packages

Add the necessary NuGet packages:

dotnet add package Aspose.Cells
dotnet add package Newtonsoft.Json
dotnet add package RestSharp

Step 3: Implement the REST API Client

Create a service for API communication:

using RestSharp;
using Newtonsoft.Json;
using System;
using System.Threading.Tasks;

public class ApiService
{
    private readonly RestClient _client;
    
    public ApiService(string baseUrl)
    {
        _client = new RestClient(baseUrl);
    }
    
    public async Task<T> GetApiDataAsync<T>(string endpoint, string apiKey = null)
    {
        var request = new RestRequest(endpoint, Method.Get);
        
        // Add authentication if needed
        if (!string.IsNullOrEmpty(apiKey))
        {
            request.AddHeader("Authorization", $"Bearer {apiKey}");
        }
        
        var response = await _client.ExecuteAsync(request);
        
        if (response.IsSuccessful)
        {
            return JsonConvert.DeserializeObject<T>(response.Content);
        }
        
        throw new Exception($"API call failed: {response.ErrorMessage}");
    }
}

Step 4: Process JSON Response Data

Add a class to process the JSON data:

using Newtonsoft.Json.Linq;
using System.Collections.Generic;

public class DataProcessor
{
    public List<Dictionary<string, object>> FlattenJsonData(string jsonData)
    {
        var results = new List<Dictionary<string, object>>();
        JToken token = JToken.Parse(jsonData);
        
        // Handle different JSON structures
        if (token is JArray array)
        {
            foreach (var item in array)
            {
                results.Add(FlattenObject(item));
            }
        }
        else if (token is JObject obj)
        {
            // For nested data like {"data": [{...}, {...}]}
            var dataToken = obj["data"] as JArray;
            if (dataToken != null)
            {
                foreach (var item in dataToken)
                {
                    results.Add(FlattenObject(item));
                }
            }
            else
            {
                results.Add(FlattenObject(obj));
            }
        }
        
        return results;
    }
    
    private Dictionary<string, object> FlattenObject(JToken token)
    {
        var result = new Dictionary<string, object>();
        FillDictionary(result, token, "");
        return result;
    }
    
    private void FillDictionary(Dictionary<string, object> dict, JToken token, string prefix)
    {
        switch (token.Type)
        {
            case JTokenType.Object:
                foreach (var prop in token.Children<JProperty>())
                {
                    FillDictionary(dict, prop.Value, 
                        string.IsNullOrEmpty(prefix) ? prop.Name : $"{prefix}.{prop.Name}");
                }
                break;
                
            case JTokenType.Array:
                int index = 0;
                foreach (var item in token.Children())
                {
                    FillDictionary(dict, item, $"{prefix}[{index}]");
                    index++;
                }
                break;
                
            default:
                dict[prefix] = ((JValue)token).Value;
                break;
        }
    }
}

Step 5: Convert to Excel with Aspose.Cells

Implement the Excel conversion:

using Aspose.Cells;
using Aspose.Cells.Utility;
using System;
using System.Collections.Generic;
using System.IO;

public class ExcelReportGenerator
{
    public void GenerateReport(string jsonData, string outputPath)
    {
        // Create a new workbook
        Workbook workbook = new Workbook();
        Worksheet sheet = workbook.Worksheets[0];
        
        // Configure JSON layout options
        JsonLayoutOptions options = new JsonLayoutOptions
        {
            ArrayAsTable = true,
            ConvertNumericOrDate = true,
            IgnoreNull = true
        };
        
        // Make headers bold
        options.TitleStyle = new CellsFactory().CreateStyle();
        options.TitleStyle.Font.IsBold = true;
        
        // Import JSON data
        JsonUtility.ImportData(jsonData, sheet.Cells, 0, 0, options);
        
        // Auto-fit columns
        sheet.AutoFitColumns();
        
        // Save the workbook
        workbook.Save(outputPath);
    }
    
    public void GenerateReportFromObjects(List<Dictionary<string, object>> data, string outputPath)
    {
        Workbook workbook = new Workbook();
        Worksheet sheet = workbook.Worksheets[0];
        
        // Add headers if there's data
        if (data.Count > 0)
        {
            int col = 0;
            foreach (var key in data[0].Keys)
            {
                // Add header with styling
                Cell cell = sheet.Cells[0, col];
                cell.PutValue(key);
                
                Style style = cell.GetStyle();
                style.Font.IsBold = true;
                cell.SetStyle(style);
                
                col++;
            }
            
            // Add data rows
            for (int row = 0; row < data.Count; row++)
            {
                col = 0;
                foreach (var value in data[row].Values)
                {
                    sheet.Cells[row + 1, col].PutValue(value);
                    col++;
                }
            }
        }
        
        // Format as a table
        if (data.Count > 0)
        {
            int lastRow = data.Count;
            int lastCol = data[0].Count - 1;
            
            var tableRange = sheet.Cells.CreateRange(0, 0, lastRow + 1, lastCol + 1);
            sheet.ListObjects.Add(tableRange, "DataTable");
        }
        
        // Auto-fit columns
        sheet.AutoFitColumns();
        
        // Save the workbook
        workbook.Save(outputPath);
    }
}

Step 6: Add Professional Formatting

Enhance the Excel output with professional styling:

public void ApplyProfessionalFormatting(Workbook workbook)
{
    Worksheet sheet = workbook.Worksheets[0];
    
    // Create a style for the title
    Style titleStyle = workbook.CreateStyle();
    titleStyle.Font.Size = 14;
    titleStyle.Font.IsBold = true;
    titleStyle.HorizontalAlignment = TextAlignmentType.Center;
    
    // Add a title to the report
    sheet.Cells.Merge(0, 0, 1, sheet.Cells.MaxColumn + 1);
    Cell titleCell = sheet.Cells[0, 0];
    titleCell.PutValue("API Data Report - " + DateTime.Now.ToString("yyyy-MM-dd"));
    titleCell.SetStyle(titleStyle);
    
    // Insert a blank row after the title
    sheet.Cells.InsertRow(1);
    
    // Apply alternating row colors to the data
    Style evenRowStyle = workbook.CreateStyle();
    evenRowStyle.ForegroundColor = Color.FromArgb(240, 240, 240);
    evenRowStyle.Pattern = BackgroundType.Solid;
    
    int dataStartRow = 3; // Accounting for title and blank row
    int lastRow = sheet.Cells.MaxRow;
    
    for (int row = dataStartRow; row <= lastRow; row += 2)
    {
        for (int col = 0; col <= sheet.Cells.MaxColumn; col++)
        {
            sheet.Cells[row, col].SetStyle(evenRowStyle);
        }
    }
    
    // Add page header and footer
    sheet.PageSetup.SetHeader(1, "&\"Arial,Bold\"&14API Report");
    sheet.PageSetup.SetFooter(1, "Page &P of &N");
    
    // Set print options
    sheet.PageSetup.Orientation = PageOrientationType.Landscape;
    sheet.PageSetup.FitToPagesWide = 1;
    sheet.PageSetup.TopMargin = 0.5;
    sheet.PageSetup.LeftMargin = 0.5;
    sheet.PageSetup.RightMargin = 0.5;
    sheet.PageSetup.BottomMargin = 0.5;
}

Step 7: Add Data Visualization

Enhance the report with charts:

public void AddChartVisualization(Workbook workbook, int dataColumn)
{
    Worksheet sheet = workbook.Worksheets[0];
    Worksheet chartSheet = workbook.Worksheets.Add("Chart Analysis");
    
    // Get the data range (skip header row)
    int lastRow = sheet.Cells.MaxRow;
    int nameColumn = 0; // Assuming first column has names/categories
    
    // Add a chart
    int chartIndex = chartSheet.Charts.Add(ChartType.Column, 2, 2, 20, 10);
    Chart chart = chartSheet.Charts[chartIndex];
    
    // Set the data range for the chart
    chart.NSeries.Add($"Sheet1!B2:B{lastRow + 1}", true);
    chart.NSeries.CategoryData = $"Sheet1!A2:A{lastRow + 1}";
    
    // Set chart title and other properties
    chart.Title.Text = "Data Analysis";
    chart.Legend.Position = LegendPositionType.Bottom;
    
    // Additional chart customization
    chart.NSeries[0].Area.ForegroundColor = Color.FromArgb(79, 129, 189);
    chart.PlotArea.Area.ForegroundColor = Color.White;
    
    // Add data labels
    chart.NSeries[0].DataLabels.IsValueShown = true;
    chart.NSeries[0].DataLabels.Position = DataLabelPositionType.OutsideEnd;
}

Step 8: Put It All Together

Create the main program that ties everything together:

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        try
        {
            // Configure API endpoint
            string baseUrl = "https://api.example.com";
            string endpoint = "/data/endpoint";
            string apiKey = "your-api-key"; // If needed
            
            // Connect to API and get data
            Console.WriteLine("Connecting to API...");
            var apiService = new ApiService(baseUrl);
            var jsonData = await apiService.GetApiDataAsync<string>(endpoint, apiKey);
            
            Console.WriteLine("Data retrieved successfully");
            
            // Generate Excel report
            Console.WriteLine("Generating Excel report...");
            var reportGenerator = new ExcelReportGenerator();
            
            // Option 1: Direct JSON to Excel conversion
            string outputPath = "ApiReport_" + DateTime.Now.ToString("yyyyMMdd_HHmmss") + ".xlsx";
            reportGenerator.GenerateReport(jsonData, outputPath);
            
            // Option 2: Process JSON and create enhanced report
            // var processor = new DataProcessor();
            // var processedData = processor.FlattenJsonData(jsonData);
            // reportGenerator.GenerateReportFromObjects(processedData, outputPath);
            
            Console.WriteLine($"Report saved to {outputPath}");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
    }
}

Handling Different API Response Structures

REST APIs return data in various formats. Here’s how to handle common structures:

1. Array of Objects

[
  { "id": 1, "name": "Product A", "price": 29.99 },
  { "id": 2, "name": "Product B", "price": 49.99 }
]

For this structure, use:

JsonLayoutOptions options = new JsonLayoutOptions();
options.ArrayAsTable = true;
JsonUtility.ImportData(jsonData, sheet.Cells, 0, 0, options);

2. Nested Object with Data Array

{
  "status": "success",
  "data": [
    { "id": 1, "name": "Product A", "price": 29.99 },
    { "id": 2, "name": "Product B", "price": 49.99 }
  ]
}

Use the DataProcessor class to extract the “data” array, or:

// Parse with Newtonsoft.Json
JObject jsonObj = JObject.Parse(jsonData);
JArray dataArray = (JArray)jsonObj["data"];

// Convert to string and import
string dataArrayJson = dataArray.ToString();
JsonUtility.ImportData(dataArrayJson, sheet.Cells, 0, 0, options);

3. Nested Arrays and Objects

{
  "categories": [
    {
      "name": "Electronics",
      "products": [
        { "id": 1, "name": "Laptop", "price": 999.99 },
        { "id": 2, "name": "Phone", "price": 699.99 }
      ]
    },
    {
      "name": "Books",
      "products": [
        { "id": 3, "name": "Novel", "price": 14.99 }
      ]
    }
  ]
}

For complex structures, create multiple worksheets:

// Parse the JSON
JObject root = JObject.Parse(jsonData);
JArray categories = (JArray)root["categories"];

// Create a worksheet for each category
foreach (var category in categories)
{
    string categoryName = category["name"].ToString();
    Worksheet sheet = workbook.Worksheets.Add(categoryName);
    
    // Get and import the products array
    JArray products = (JArray)category["products"];
    JsonUtility.ImportData(products.ToString(), sheet.Cells, 0, 0, options);
}

Taking It to the Next Level: Scheduled Reports

To automate report generation, add scheduling capabilities:

// Install Task Scheduler package
// dotnet add package TaskScheduler

using Microsoft.Win32.TaskScheduler;

public void ScheduleDailyReportGeneration(string appPath)
{
    using (TaskService ts = new TaskService())
    {
        // Create a new task
        TaskDefinition td = ts.NewTask();
        td.RegistrationInfo.Description = "Daily API Data Report Generation";
        
        // Create a trigger that will fire daily at 7am
        td.Triggers.Add(new DailyTrigger { StartBoundary = DateTime.Today.AddHours(7) });
        
        // Create an action that will run the application
        td.Actions.Add(new ExecAction(appPath));
        
        // Register the task in the root folder
        ts.RootFolder.RegisterTaskDefinition("DailyApiReport", td);
    }
}

Advanced Features to Consider

Email delivery - Automatically send reports via email
Multi-API integration - Combine data from multiple APIs
Template-based reports - Use Excel templates for consistent branding
Dashboard creation - Generate interactive Excel dashboards
Error tracking and reporting - Log issues and report on success/failure

By following this guide, you’ve created a robust C# application that automates the process of retrieving API data and converting it to professional Excel reports—saving time, ensuring accuracy, and delivering business value.