C# Decimal to VB6 Currency

Question

I have one C# DLL and one Visual Basic 6 DLL.

In C# there is a field x with data type Decimal . In VB6 there is a field y with data type Currency .

What would be the best way to pass x to y and back?

Currently I convert the fields to Double , but I am not sure if there are rounding implications.

Update 1:

Based on the helpful advice this is what my code looks now:

    public void FromVbToNet(long vb6curr)
    {
        decimal value = vb6curr / 10000;
    }

Problem is, when I try to call this from VB6 via interop, I get a compile error:

"Function or interface marked as restricted, or the function uses an Automation type not supported in Visual Basic"

So how do I declare vb6curr? String , Object , Dynamic ?

Update 2:

In case anyone needs this for reference, after further reading I came up with the following solution:

[return: MarshalAs(UnmanagedType.Currency)]
public decimal GetDecimalFromNetDll()
{
    decimal value = ... // Read from database
    return value;
}

public void SetDecimalInNetDll([MarshalAs(UnmanagedType.Currency)] decimal value)
{
    // Save to database
}

I call these from my unmanaged code in VB6 with a Currency parameter and everything seems to work so far.

Answer 1

A VB6 Currency data type is stored as a 64 bit integer, implicitly scaled by 10,000 . Armed with that knowledge it is straightforward to convert between that type and .net Decimal .

On the VB6 side you pass the data as Currency . On the C# side you pass it as long . Then on the C# side you scale by 10,000 to convert between your decimal value and the long value.

For example when you have a VB6 Currency value held in a C# long you convert to decimal like this:

long vb6curr = ...;
decimal value = vb6curr / 10000;

In the other direction it would be:

decimal value = ...;
long vb6curr = Convert.ToInt64(value*10000);

Answer 2

After some reading I came up with this solution (see also under Update 2 ).

I had to marshal the Decimal type in .Net to the Currency type in the unmanaged VB6 code and vice versa.

[return: MarshalAs(UnmanagedType.Currency)]
public decimal GetDecimalFromNetDll()
{
    decimal value = ... // Read from database
    return value;
} 

public void SetDecimalInNetDll([MarshalAs(UnmanagedType.Currency)] decimal value)
{
    // Save to database
}

For detailed information see: http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshalasattribute%28v=vs.110%29.aspx