IntPtr and avoiding unsafe code

Question

I have an external library that takes an IntPtr. Is there any safe way to do this...

int BytesWritten = 0;
Output.WriteBytes(buffer, new IntPtr(&BytesWritten));

...without having to use 'unsafe' code? I'm not that familiar with IntPtrs, but I'd like to do something like this:

fixed int BytesWritten = 0;
Output.WriteBytes(buffer, IntPtr.GetSafeIntPtr(ref BytesWritten));

...in such a way that I don't need to compile with /unsafe.

I can't change the WriteBytes function, it's an external function.

It seems like there should be some sort of cast between 'ref int' and IntPtr, but I have not had luck finding it.

Answer 1

Yes, there is. You can use P/Invoke for your code. It will create the pointer for you automagically. Something like this:

[DllImport("yourlib", SetLastError=true)]
static extern bool WriteBytes(
    [MarshalAs(UnmanagedType.LPArray)]
    byte [] buffer,
    ref int BytesWritten);

(I added the array as a bonus). More info on P/Invoke can be found, with gazillion examples, at pinvoke.net.

Each parameter above can take out, in and ref. Out and ref parameters are translated as pointers, where an ref-parameter is two-way.

Answer 2

I assume that Output.WriteBytes is a [DllImport] method. Can you post the declaration?

You should be able to avoid the pointer by declaring the last parameter as out int instead of IntPtr -- let the P/Invoke marshaller do the rest.

Answer 3

Here is a class that will provide you with a safe IntPtr implementation. It derives from the SafeHandleZeroOrMinusOneIsInvalid class, provided by the .NET framework.

/// <summary>
/// IntPtr wrapper which can be used as result of
/// Marshal.AllocHGlobal operation.
/// Call Marshal.FreeHGlobal when disposed or finalized.
/// </summary>
class HGlobalSafeHandle : SafeHandleZeroOrMinusOneIsInvalid
{
    /// <summary>
    /// Creates new instance with given IntPtr value
    /// </summary>
    public HGlobalSafeHandle(IntPtr ptr) : base(ptr, true)
    {
    }

    /// <summary>
    /// Creates new instance with zero IntPtr
    /// </summary>
    public HGlobalSafeHandle() : base(IntPtr.Zero, true)
    {
    }

    /// <summary>
    /// Creates new instance which allocates unmanaged memory of given size 

  /// Can throw OutOfMemoryException
    /// </summary>
    public HGlobalSafeHandle(int size) :
        base(Marshal.AllocHGlobal(size), true)
    {
    }


    /// <summary>
    /// Allows to assign IntPtr to HGlobalSafeHandle
    /// </summary>
    public static implicit operator HGlobalSafeHandle(IntPtr ptr)
    {
        return new HGlobalSafeHandle(ptr);
    }

    /// <summary>
    /// Allows to use HGlobalSafeHandle as IntPtr
    /// </summary>
    public static implicit operator IntPtr(HGlobalSafeHandle h)
    {
        return h.handle;
    }

    /// <summary>
    /// Called when object is disposed or finalized.
    /// </summary>
    override protected bool ReleaseHandle()
    {
        Marshal.FreeHGlobal(handle);
        return true;
    }

    /// <summary>
    /// Defines invalid (null) handle value.
    /// </summary>
    public override bool IsInvalid
    {
        get
        {
            return (handle == IntPtr.Zero);
        }
    }
}