CODEWITHSUNDEEP
c#
anon
anon

Reputation:

c# pass by ref not working?

Whats going on here? Why isn't the = working? It's not a value type so it should be passing by reference right?

void func()
{
    Vector2 a = new Vector2(1, 0);
    equal(a);
    // a is now (1, 0) not (0, 0)
}

void equal(Vector2 a)
{
    a = new Vector2(0, 0);
}

Upvotes: 2

Views: 3276

Answers (8)

Mario The Spoon
Mario The Spoon

Reputation: 4869

Because the reference to a is passed as value and not as reference, so you are modifying a ' local' a.

correct would be:

void equal ( ref Vector2 a)

Upvotes: 3

Motti Shaked
Motti Shaked

Reputation: 1894

A reference type can be passes by value or by reference. The default is to pass by value which is what your code sample does.

When a reference type is passed by value, you can modify the state of the object but you cannot change the reference itself so it would refer to a new object. To do that you must pass by reference.

void equal(ref Vector2 a)
{
    a = new Vector2(0, 0);
}

Here is a direct link to the part of my online .NET training which illustrates all cases with animation and code samples: http://motti.me/cf

I hope this helps!

Motti

Upvotes: 0

Damith
Damith

Reputation: 63105

Vector2 is Struct and struct type is a value type not reference. use ref keyword passing by reference

Upvotes: 1

James Hill
James Hill

Reputation: 61862

You're not passing by reference. To do that, you'll need to use the ref keyword

void func()
{
    Vector2 a = new Vector2(1, 0);
    equal(ref a);
}

void equal(ref Vector2 a)
{
    a = new Vector2(0, 0);
}

Upvotes: 2

Oded
Oded

Reputation: 499402

In order to change the reference you would need to pass by ref to ensure the reference changes.

void func()
{
    Vector2 a = new Vector2(1, 0);
    equal(ref a);
}

void equal(ref Vector2 a)
{
    a = new Vector2(0, 0);
}

I suggest reading about Value Types and Reference Types and this.

Upvotes: 1

JaredPar
JaredPar

Reputation: 755587

C# passes arguments by value by default hence it's only assigning to a within the equal method. You need to use a ref parameter here if you want pass by reference

void func()
{
    Vector2 a = new Vector2(1, 0);
    equal(ref a);
    // a is now (0, 0) as expected
}

equal(ref Vector2 a) 
{ 
  a = new Vector2(0, 0);
}

Upvotes: 6

dgvid
dgvid

Reputation: 26633

A copy of the reference to a is being passed to the equal method. The code inside equal is changing the memory location pointed to by the copy. See http://msdn.microsoft.com/en-us/library/0f66670z(v=vs.71).aspx#vclrfpassingmethodparameters_example4 for details.

Just to be clear: The Vector2 object is not being copied, the variable a is being copied when it is passed to the method.

Upvotes: 0

Marco
Marco

Reputation: 57603

You should try:

void func()
{
    Vector2 a = new Vector2(1, 0);
    equal(ref a);
}

void equal(ref Vector2 a)
{
    a = new Vector2(0, 0);

}

Upvotes: 0

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