How to Generate Random Number in C# - Simple Guide

In C#, you generate random numbers using the Random class. Create an instance like var rnd = new Random(); and call rnd.Next() for integers or rnd.NextDouble() for decimals.

Syntax

The Random class is used to generate random numbers. You create an object with new Random(). Then use methods like Next() to get a random integer or NextDouble() for a random decimal between 0 and 1.

• Random rnd = new Random(); - creates a random number generator
• rnd.Next() - returns a random integer
• rnd.Next(min, max) - returns a random integer between min (inclusive) and max (exclusive)
• rnd.NextDouble() - returns a random double between 0.0 and 1.0

csharp

Random rnd = new Random();
int number = rnd.Next(1, 10); // random number from 1 to 9
double decimalNumber = rnd.NextDouble(); // random decimal between 0.0 and 1.0

Example

This example shows how to generate a random integer between 1 and 100 and a random decimal between 0 and 1. It prints both values to the console.

csharp

using System;

class Program
{
    static void Main()
    {
        Random rnd = new Random();
        int randomInt = rnd.Next(1, 101); // 1 to 100 inclusive
        double randomDouble = rnd.NextDouble(); // 0.0 to 1.0

        Console.WriteLine($"Random integer: {randomInt}");
        Console.WriteLine($"Random decimal: {randomDouble}");
    }
}

Output

Random integer: 57
Random decimal: 0.374829374

Common Pitfalls

A common mistake is creating a new Random object inside a loop or repeatedly in a short time. This can cause the same random numbers to appear because Random uses the system clock as a seed.

To avoid this, create one Random instance and reuse it.

csharp

/* Wrong way: creates new Random each loop, may repeat numbers */
for (int i = 0; i < 5; i++)
{
    Random rnd = new Random();
    Console.WriteLine(rnd.Next(1, 10));
}

/* Right way: create Random once and reuse */
Random rnd = new Random();
for (int i = 0; i < 5; i++)
{
    Console.WriteLine(rnd.Next(1, 10));
}

Quick Reference

• Random(): Creates a random number generator.
• Next(): Returns a non-negative random integer.
• Next(min, max): Returns a random integer between min (inclusive) and max (exclusive).
• NextDouble(): Returns a random double between 0.0 and 1.0.

Key Takeaways

Use the Random class to generate random numbers in C#.

Create one Random instance and reuse it to avoid repeated values.

Use Next(min, max) for integers in a range and NextDouble() for decimals.

Random.Next(max) returns numbers from 0 up to max-1.

Avoid creating Random objects repeatedly in quick succession.