Top 10 most commonly used .NET classes

Here’s a breakdown of the Top 10 most commonly used .NET classes (mainly from the System namespace) — with short, clear examples.

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🧩 1. System.String

Used to represent and manipulate text.

string name = "Sagar";
string greeting = $"Hello, {name.ToUpper()}!";
Console.WriteLine(greeting);  // Hello, SAGAR!

✅ Common methods:

Substring(), Replace(), Split(), Contains(), StartsWith(), Trim()

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⚙️ 2. System.Console

Used for input/output in console apps.

Console.Write("Enter name: ");
string name = Console.ReadLine();
Console.WriteLine("Welcome, " + name);

✅ Common methods:

Write(), WriteLine(), ReadLine(), ReadKey()

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🧮 3. System.Math

Contains mathematical constants and functions.

double radius = 3.5;
double area = Math.PI * Math.Pow(radius, 2);
Console.WriteLine(area);

✅ Common methods:

Math.Abs(), Math.Max(), Math.Min(), Math.Sqrt(), Math.Round()

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📅 4. System.DateTime

Represents dates and times.

DateTime now = DateTime.Now;
Console.WriteLine("Today: " + now.ToShortDateString());

DateTime future = now.AddDays(10);
Console.WriteLine("After 10 days: " + future);

✅ Common methods:

AddDays(), AddMonths(), Subtract(), ToString()

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📦 5. System.Collections.Generic.List<T>

Represents a dynamic array.

List<string> fruits = new List<string> { "Apple", "Banana" };
fruits.Add("Cherry");

foreach (var fruit in fruits)
    Console.WriteLine(fruit);

✅ Common methods:

Add(), Remove(), Sort(), Contains(), Find()

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🔑 6. System.Collections.Generic.Dictionary<TKey, TValue>

Stores key–value pairs.

var capitals = new Dictionary<string, string>
{
    {"USA", "Washington"},
    {"India", "New Delhi"}
};

Console.WriteLine(capitals["India"]);  // New Delhi

✅ Common methods:

Add(), TryGetValue(), ContainsKey(), Remove()

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⚡ 7. System.Threading.Tasks.Task

Represents asynchronous operations.

async Task<int> FetchDataAsync()
{
    await Task.Delay(1000);  // simulate delay
    return 42;
}

int result = await FetchDataAsync();
Console.WriteLine(result);

✅ Common methods:

Run(), Wait(), Result, ContinueWith()

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🧠 8. System.IO.File

Used to read/write files.

string path = "example.txt";
File.WriteAllText(path, "Hello .NET!");
string content = File.ReadAllText(path);
Console.WriteLine(content);

✅ Common methods:

ReadAllText(), WriteAllText(), AppendAllText(), Exists(), Delete()

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🧮 9. System.Linq.Enumerable

Provides LINQ query extensions.

var numbers = new List<int> { 1, 2, 3, 4, 5 };
var even = numbers.Where(n => n % 2 == 0).ToList();
Console.WriteLine(string.Join(", ", even)); // 2, 4

✅ Common methods:

Where(), Select(), OrderBy(), Sum(), Count()

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🧾 10. System.Exception

Base class for all exceptions.

try
{
    int x = 10 / 0;
}
catch (Exception ex)
{
    Console.WriteLine("Error: " + ex.Message);
}

✅ Common derived classes:

IOException, ArgumentException, NullReferenceException, InvalidOperationException

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💡 Bonus (Also Commonly Used)

• System.Random → generates random numbers
• System.Text.StringBuilder → efficient string manipulation
• System.Guid → generate unique identifiers
• System.Linq.Queryable → LINQ to Entities or LINQ to SQL