Why Are Mutable Structs Dangerous in C#?

Understanding the Pitfalls of Mutable Structs in C#

Mutability—the capacity to alter a data structure's value—presents both advantages and significant drawbacks in programming. When dealing with structs in C#, mutability can introduce unexpected and potentially harmful behavior.

C# structs are value types. This means that when a struct is passed as an argument or assigned to a new variable, a complete copy is created. While this safeguards the original struct from modifications made to the copy, it can lead to confusion with mutable structs.

Consider this illustrative code snippet:

public struct Point
{
    public int X;
    public int Y;
}

static void AdjustPoint(Point p)
{
    p.X++;
}

public static void Main()
{
    Point pt = new Point { X = 10, Y = 15 };

    AdjustPoint(pt);

    Console.WriteLine(pt.X); // Output: 10
}

Here, a Point struct with modifiable X and Y properties is defined. The AdjustPoint function attempts to increment the X coordinate. Notice that printing pt.X in Main still yields 10. This is because the function modifies a copy of the struct, leaving the original unchanged.

This seemingly minor issue can escalate into critical problems in multithreaded applications. Multiple threads working with copies of the same mutable struct could inadvertently alter data, leading to unpredictable and difficult-to-debug errors.

To avoid these complications, it's best practice to design structs as immutable whenever feasible. Immutability ensures that all modifications are explicit and intentional, preventing accidental data corruption.

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