ref, in, out keyword in Csharp

Basically like ref keyword. Instead of copying like normally for stack-values a reference gets passed.

ref-keyword

• ref doesnt enforce any constraints on what to do with x in the below example

int value = 99;
Example(ref value);
Console.WriteLine(value);   // -> 11

void Example(ref int x){
    x = 11;  // mutates dereferenced original 'value'
}

out-keyword

var couldParse = int.TryParse("123", out int parsedValue);
Console.WriteLine(parsedValue); // -> 123
var value = 100;
var couldParse = int.TryParse("asdf", out value);   // original value gets mutated
Console.WriteLine(parsedValue); // -> 0 (the default init value)

• out enforces us to modify/set x = 11; or it wont compile
• every branch has to set the out value on Exit!

int value = 99;
Example(ref value);
Console.WriteLine(value);   // -> 11

void Example(out int x){
    x = 11;  // this mutation is enfored with the out keyword
}

in-keyword

Usecase: Passing down valuetypes (especially big structs) as function parameters will copy each time.

this causes a huge memory allocation burden.

One could use the ref keyword BUT now the function could mutate the original etc.

• we can avoid this by using in.
• this effectively makes it a ref but readonly!

int value = 99;
Example(in value);
void Example(in int x){
    // value = 123;         <- NOT ALLOWED
    Console.WriteLine(x);   // is allowed
}

When to use

• if the struct is bigger than pointer size
• if using in with a struct make sure the struct is readonly or defensive copies will degrade performance benefits.