包含字符串的结构与包含字符串的类的 C# 性能

Question

Please check out these two small examples:

public struct Struct
{
    public readonly string StringValue;
    public readonly int IntValue;

    public Struct(string stringValue)
    {
        this.StringValue = stringValue;
        this.IntValue = this.StringValue.GetHashCode(); // Just ignore this assignment, please :).
    }
}

public class Class
{
    public readonly string StringValue;
    public readonly int IntValue;

    public Class(string stringValue)
    {
        this.StringValue = stringValue;
        this.IntValue = this.StringValue.GetHashCode(); // Just ignore this assignment, please :).
    }
}

From what I know if I pass struct as a parameter it will make a copy of it someObject.Method(this.cachedStruct); - which in turn will make a copy of a StringValue as string s are immutable it will allocate array in memory for this string which is quite costly operation [pardon me if I made a mistake, please correct me in such a case].

So in this instance I assume it will be faster to use instances of the Class rather than copying StringValue each time. QUESTION: Are my assumptions correct and Class is a better choice for this kind of a situation or is it still more beneficial and faster to use Struct ?

Since String and string are the same. ref - What is the difference between String and string in C#? . I cannot use String in struct to prevent it from creating a new string object when struct has been passed as a parameter.

To better address the issue, here is another example. Sorry if it's a bit rough example.

public struct Struct
{
    public string StringValue;
    public int IntValue;

    public Struct(string stringValue)
    {
        this.StringValue = stringValue;
        this.IntValue = this.StringValue.GetHashCode(); // Just ignore this assignment, please :).
    }
}

public class Class
{
    public string StringValue;
    public int IntValue;

    private Struct _niceStruct;

    public void Accept(Struct someStruct)
    {
        someStruct.StringValue = "Something new"; // In this case even if I don't change the value, it was already allocated.
        // Because of copying of struct when it was passed as a parameter.

        Console.WriteLine(this._niceStruct.StringValue); // "Something new".
    }

    public Class(string stringValue)
    {
        this.StringValue = stringValue;
        this.IntValue = this.StringValue.GetHashCode(); // Just ignore this assignment, please :).

        this._niceStruct = new Struct("Old value");

        this.Accept(this._niceStruct);

        Console.WriteLine(this._niceStruct.StringValue); // "Old value".

        // String value was left unaffected.
        // Which [even in the case I wouldn't change the value in the method] means that a new `string` object was allocated when a copy of `Struct` was created.
        // When using `Class` in the same manner - it would change the `string`.
        // New `string` object won't be allocated if value is unchanged.

        // I assume that `string` allocation might be more costly than `Class` allocation in this instance.
    }
}

My understanding is that if I use Struct without changing string value it will still allocate memory for StringValue each time I pass it as a parameter. While when using Class and passing its instance as a parameter - it will pass Class instance as a reference type without copying values, thus not allocating any memory for StringValue .

My question might seem similar to C#, struct vs class, faster? [duplicate] and other questions (I believe I have gone through them all (the ones that struct vs class performance)) but none of them have given me an answer.

Answer 1

The string is for example at address 0x0110 and called "hello" . The struct does not store the string itself, it only stores the reference to this string. Thus the struct will only store "0x0110" . When you copy the struct, the struct copy will store the same reference "0x0110" . Not the string, just the reference. No new string object will be created. But when you change the string in the struct copy, for example from "hello" to "bla" , a new string will be created at a new address. For example "bla" will be at address 0x0220 . the struct copy will then store "0x0220" instead of "0x0110" . The original struct will still store "0x0110" . You are now having 2 strings in memory at different adresses.

0x0110 "hello"
0x0220 "bla"

PS: that is a simplification of what really happens.

Answer 2

You seem to not know exactly how reference and value types work.

You have variable a of a type that is either class or a struct. This variable holds a value , thats all a variable is. What the value is depends one whether the type is a reference type or a value type:

• If its a reference type, the value of a is a reference , that is, a memory address where the referenced object is found.
• If its a value type, the value of a is the value type itself.

In C#, by default, variables are passed by value. That is, a copy of the variable's value is made:

SomeType a = new SomeType();
var b = a;
SomeMethod(b)

void SomeMethod(SomeType t) { }

b holds a copy of a . And t holds a copy of b .

If SomeType is a reference type (for example string ) then all three variables a , b and t hold the same value; an address to the string instance originally assigned to a .

If SomeType is a value type, then all three variables hold three identical but distinct instances of the value type returned by new SomeType() . The variables that make up the inner state of said value type are themselves copied in the same manner: their value is copied.

Does that make it clearer?