In C#, struct (short for structure) is a value type used to store related data under a single unit. It's similar to a class, but with some key differences in behavior and use cases.

What is a Structure?

A structure is a user-defined type that groups different variables under one name. Structs are best used for small data containers that don’t require inheritance or complex behaviors.

struct StructName
{
    // Fields
    public int field1;
    public string field2;

    // Constructor
    public StructName(int f1, string f2)
    {
        field1 = f1;
        field2 = f2;
    }

    // Methods
    public void Display()
    {
        Console.WriteLine($"Field1: {field1}, Field2: {field2}");
    }
}

Example: Defining and Using a Struct

using System;

struct Employee
{
    public int ID;
    public string EmployeeName;

    public Employee(int id, string name)
    {
        ID = id;
        EmployeeName = name;
    }

    public void Display()
    {
        Console.WriteLine($"ID: {ID}, Name: {EmployeeName}");
    }
}

class Program
{
    static void Main()
    {
        Employee emp1 = new Employee(101, "Alice");
        emp1.Display();
    }
}

Output: 

ID: 101, Name: Alice

Struct vs Class

When to Use Struct?

Use a struct when:

1. You’re representing simple objects (like a point, color, coordinate).
2. You want value semantics (copying data instead of references).
3. You don’t need inheritance or polymorphism.

Additional Notes

1. Structs can implement interfaces, but cannot inherit from another struct or class.
2. You can use Nullable<T> or T? with structs if you want to allow null values.
3. Structs are copied by value, not by reference. So changes made to one copy don't affect the original.

Mini Quiz

struct Point {
    public int X, Y;
}

Point p1 = new Point();
p1.X = 5;
Point p2 = p1; // Now it assign 
p2.X = 10;
Console.WriteLine(p1.X);

Summery

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Structure types - C# reference