How to Flatten JSON Array for Excel Conversion with C#

When working with modern APIs and web services, developers frequently encounter complex JSON structures with deeply nested arrays and objects. Converting these hierarchical structures into flat Excel tables presents a significant challenge. This guide demonstrates how to effectively convert nested JSON arrays to Excel using Aspose.Cells for .NET.

The Challenge: Complex Nested JSON Structures

Consider this typical JSON response from a web API:

{
  "company": "Acme Corp",
  "departments": [
    {
      "name": "Engineering",
      "employees": [
        {
          "id": 101,
          "name": "John Smith",
          "skills": ["C#", "ASP.NET", "Azure"]
        },
        {
          "id": 102,
          "name": "Jane Doe",
          "skills": ["JavaScript", "React", "Node.js"]
        }
      ]
    },
    {
      "name": "Marketing",
      "employees": [
        {
          "id": 201,
          "name": "Michael Johnson",
          "skills": ["Content Strategy", "SEO", "Analytics"]
        }
      ]
    }
  ]
}

Converting this hierarchical data to a flat Excel table creates several challenges:

How to handle multiple nested arrays (departments, employees, skills)
How to maintain relationships between parent and child items
How to create a readable spreadsheet structure

Step-by-Step Solution

Step 1: Install Aspose.Cells

First, install Aspose.Cells for .NET:

dotnet add package Aspose.Cells

Step 2: Set Up JsonLayoutOptions

Create properly configured JsonLayoutOptions to handle arrays:

using Aspose.Cells;
using Aspose.Cells.Utility;

// Create JsonLayoutOptions with array handling
JsonLayoutOptions options = new JsonLayoutOptions();
options.ArrayAsTable = true;  // Crucial for proper flattening
options.ConvertNumericOrDate = true;
options.IgnoreNull = true;

Step 3: Load Complex JSON Data

Load your complex nested JSON data:

// Sample JSON with nested arrays
string jsonData = File.ReadAllText("complex_data.json");

// Initialize workbook and worksheet
Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets[0];

Step 4: Configure Advanced Flattening

For advanced nested structures, implement a custom flattening solution:

// Define starting cell position
int startRow = 0;
int startColumn = 0;

// Import the JSON data with our configured options
JsonUtility.ImportData(jsonData, worksheet.Cells, startRow, startColumn, options);

Step 5: Handle Multi-level Nested Arrays

For complex multi-level arrays, we need additional processing:

// Create a second worksheet for detailed employee data
Worksheet employeeSheet = workbook.Worksheets.Add("Employees");
int empRow = 0;

// Add headers for the employee sheet
string[] headers = { "Department", "Employee ID", "Employee Name", "Skills" };
for (int i = 0; i < headers.Length; i++)
{
    employeeSheet.Cells[empRow, i].PutValue(headers[i]);
}
empRow++;

// Parse JSON to extract and flatten employee data
// Note: This would require a JSON parsing library like Newtonsoft.Json
// JObject root = JObject.Parse(jsonData);
// foreach (var dept in root["departments"]) 
// {
//     string deptName = dept["name"].ToString();
//     foreach (var emp in dept["employees"])
//     {
//         employeeSheet.Cells[empRow, 0].PutValue(deptName);
//         employeeSheet.Cells[empRow, 1].PutValue((int)emp["id"]);
//         employeeSheet.Cells[empRow, 2].PutValue(emp["name"].ToString());
//         employeeSheet.Cells[empRow, 3].PutValue(string.Join(", ", emp["skills"].ToObject<string[]>()));
//         empRow++;
//     }
// }

Step 6: Apply Professional Formatting

Enhance readability with proper formatting:

// Format both worksheets as tables with headers
worksheet.ListObjects.Add(0, 0, worksheet.Cells.LastCell.Row, worksheet.Cells.LastCell.Column, true);
employeeSheet.ListObjects.Add(0, 0, empRow - 1, 3, true);

// Auto-fit columns for better readability
worksheet.AutoFitColumns();
employeeSheet.AutoFitColumns();

Step 7: Save the Result

Export the workbook with flattened data:

// Save as Excel file
workbook.Save("flattened_data.xlsx");

A Simplified Solution Using Aspose.Cells

For many scenarios, Aspose.Cells provides a simpler approach using its built-in JSON handling:

// Initialize workbook
Workbook workbook = new Workbook();
Worksheet sheet = workbook.Worksheets[0];

// Configure JSON import options
JsonLayoutOptions options = new JsonLayoutOptions
{
    ArrayAsTable = true,
    ConvertNumericOrDate = true,
    IgnoreNull = true,
    TitleStyle = new CellsFactory().CreateStyle(),
    NestedArrayAsTable = true  // Important for nested arrays
};

// Set title style for better readability
options.TitleStyle.Font.IsBold = true;

// Import JSON
JsonUtility.ImportData(jsonData, sheet.Cells, 0, 0, options);

// Save result
workbook.Save("flattened_output.xlsx");

Key Insights and Best Practices

Use ArrayAsTable = true - This is essential for proper representation of arrays
Consider creating multiple worksheets for complex hierarchical data
Apply formatting to make the output more readable
Use NestedArrayAsTable = true when available for better handling of nested structures
For extremely complex structures, consider pre-processing the JSON before import

Common Issues and Solutions

Issue	Solution
Nested arrays appear as single cells	Enable `ArrayAsTable` and `NestedArrayAsTable` options
Data relationships are lost	Create multiple tables/sheets with relationship columns
Column names are incorrect	Use the `DateTimeGroupSeparator` option to customize naming
Memory issues with large files	Process the JSON in chunks or use streaming approaches