Dictionaries are an integral part of efficient data management in C#. In this article, we’ll dive deeper into how to work with dictionaries, focusing on their unique behavior, common pitfalls, and advanced techniques. Building upon our previous work with Excel integration, we’ll also enhance our application to dynamically populate a dictionary and perform safe, user-friendly lookups of country details using three-letter codes.
Keys Are Unique
One of the fundamental differences between dictionaries and lists is that dictionary keys must be unique. This ensures data consistency but also introduces potential exceptions when trying to add duplicate keys.
Key Uniqueness Example
var countryDictionary = new Dictionary<string, string>
{
{ "NOR", "Norway" }
};
// Adding a duplicate key throws an exception
// countryDictionary.Add("NOR", "New Norway"); // InvalidOperationException
In contrast, lists allow duplicate entries without issue, as they rely on positional indices.
The Uncertainty of Dictionary Lookups
While lists and arrays offer predictable indices for data access, dictionaries rely on the presence of keys, which might not always exist. Attempting to look up a key that doesn’t exist in the dictionary throws an exception.
Invalid Lookup Example
var countryDictionary = new Dictionary<string, string>
{
{ "NOR", "Norway" }
};
// Attempting to access a non-existent key
// string country = countryDictionary["MUS"]; // KeyNotFoundException
This is where the TryGetValue method becomes invaluable.
Looking Up Items Safely with TryGetValue
The TryGetValue method allows for safe lookups, ensuring no exceptions are thrown if the key isn’t found. It returns a boolean indicating whether the key exists and outputs the value if present.
TryGetValue Example
if (countryDictionary.TryGetValue("MUS", out var country))
{
Console.WriteLine($"Country: {country}");
}
else
{
Console.WriteLine("Country not found.");
}
This pattern is especially useful in dynamic applications where the presence of keys can’t be guaranteed.
Integrating Dictionaries with Excel Data
We’ll now enhance our country lookup application to leverage dictionaries for efficient key-based retrieval. This builds on the ExcelReaderWriter class and Excel setup we’ve used previously.
Step 1: Updating the ExcelReaderWriter Class
The ExcelReaderWriter class will include a method for populating a dictionary with data from the Excel file.
Method to Populate a Dictionary
using OfficeOpenXml;
using System.Collections.Generic;
using System.IO;
public class ExcelReaderWriter
{
private readonly string _filePath;
public ExcelReaderWriter(string filePath)
{
_filePath = filePath;
}
public Dictionary<string, Country> ReadAllCountriesIntoDictionary()
{
var countries = new Dictionary<string, Country>();
using (var package = new ExcelPackage(new FileInfo(_filePath)))
{
var worksheet = package.Workbook.Worksheets["Countries"];
int row = 2;
while (worksheet.Cells[row, 1].Value != null)
{
var name = worksheet.Cells[row, 1].Value?.ToString();
var code = worksheet.Cells[row, 2].Value?.ToString();
var region = worksheet.Cells[row, 3].Value?.ToString();
var population = int.Parse(worksheet.Cells[row, 4].Value?.ToString() ?? "0");
var country = new Country(name, code, region, population);
countries.Add(code, country); // Add key-value pairs
row++;
}
}
return countries;
}
}
Step 2: The Country Class
The Country class models each country’s data.
public class Country
{
public string Name { get; }
public string Code { get; }
public string Region { get; }
public int Population { get; }
public Country(string name, string code, string region, int population)
{
Name = name;
Code = code;
Region = region;
Population = population;
}
}
Step 3: Building the Interactive Program
We’ll implement a program that:
- Populates a dictionary from the Excel file.
- Allows users to search for countries by their three-letter codes.
- Uses
TryGetValuefor safe lookups.
Interactive Program
using System;
class Program
{
static void Main(string[] args)
{
string filePath = "Countries.xlsx";
var excelHandler = new ExcelReaderWriter(filePath);
Console.WriteLine("Reading all countries into a dictionary...");
var countryDictionary = excelHandler.ReadAllCountriesIntoDictionary();
while (true)
{
Console.WriteLine("\nEnter a three-letter country code (or type 'exit' to quit):");
string inputCode = Console.ReadLine()?.ToUpper();
if (inputCode == "EXIT")
break;
if (countryDictionary.TryGetValue(inputCode, out var country))
{
Console.WriteLine($"\nCountry Details:");
Console.WriteLine($"Name: {country.Name}");
Console.WriteLine($"Region: {country.Region}");
Console.WriteLine($"Population: {PopulationFormatter.FormatPopulation(country.Population)}");
}
else
{
Console.WriteLine("\nCountry not found.");
}
}
}
}
Key Features Demonstrated
Safe Lookups: Avoid exceptions usingTryGetValue.
Dynamic Data Handling: Populate dictionaries from Excel dynamically.
User Interaction: Interactive console application for searching countries.
Efficient Retrieval: Fast lookups using dictionary keys.
Expected Output
Valid Key Lookup
Enter a three-letter country code (or type 'exit' to quit):
NOR
Country Details:
Name: Norway
Region: Europe
Population: 5,379,475
Invalid Key Lookup
Enter a three-letter country code (or type 'exit' to quit):
XYZ
Country not found.
Conclusion
This article showcased the power of dictionaries for efficient data management, emphasizing unique keys and safe lookup techniques. By combining dictionaries with dynamic Excel integration, we created a scalable, user-friendly country lookup application.
This approach is perfect for applications that require fast lookups, meaningful keys, and robust error handling. Expand further by adding features like exporting results to Excel, filtering by region, or validating user input. Happy coding!
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