C# and VB.Net out parameters

Question

I've got a project in c# which is making use of another project written in vb.net. I am currently able to modify both.

I've got a method in the VB project like:

    Public Sub MethodName(ByVal param1 As String, ByRef param2 As String)
        param2 = param1 + 1
    End Sub

I am not able to call this method using the out keyword from C#:

    public void CallOtherMethod()
    {
        string param1 ="test";
        string param2;

        provider.AddTransaction(param1, out param2);
    }

Shouldn't the ByRef keyword in VB.Net have the capabilities of both "ref" and "out"?

Should I just go with ref?

Answer 1

To the runtime ref and out are fairly interchangable, as they are both just passing a reference. However, out is prefixed with an additional attribute in the IL:

public void y(ref int a)
public void z(out int a)

turns into

.method public hidebysig instance void  y(int32& a)
.method public hidebysig instance void  z([out] int32& a)

which enables the C# compiler to distinguish the two and add the special semantics that out has, namely that an out parameter does not need to have an assigned value before entering the method and must be assigned a value before exiting the method.

In contrast, ByRef in VB only provides ref, but not the additional semantics of out. There it no equivalent of outin VB.