C# out Keyword

out keyword is similiar to ref keyword, indicates that the parameter will be passed as reference. The difference is that out keyword do not need the parameter to be initialized before passed.

public void f(out int k)
{
k = 77;
}
int i;
f(out i); //i=77 now
f(ref i); //compile error, i is not defined

Since the parameter may not be initialized, it need to be initialized in the function, otherwise a compile error will occur.
public void f(out int k)
{
k = k + 5; //compile error, k is not defined
}
int i = 9;
f(out i); //error

ref do not had such issue.
public void f(ref int k)
{
k = k + 5; //ok
}
int i = 9;
f(ref i); //i=14 now



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