Win api in C#. Get Hi and low word from IntPtr

Question

I am trying to process a WM_MOUSEMOVE message in C#.

What is the proper way to get an X and Y coordinate from lParam which is a type of IntPtr?

Answer 1

Correct for both 32 and 64-bit:

Point GetPoint(IntPtr _xy)
{
    uint xy = unchecked(IntPtr.Size == 8 ? (uint)_xy.ToInt64() : (uint)_xy.ToInt32());
    int x = unchecked((short)xy);
    int y = unchecked((short)(xy >> 16));
    return new Point(x, y);
}

- or -

int GetIntUnchecked(IntPtr value)
{
    return IntPtr.Size == 8 ? unchecked((int)value.ToInt64()) : value.ToInt32();
}
int Low16(IntPtr value)
{
    return unchecked((short)GetIntUnchecked(value));
}
int High16(IntPtr value)
{
    return unchecked((short)(((uint)GetIntUnchecked(value)) >> 16));
}

These also work:

int Low16(IntPtr value)
{
    return unchecked((short)(uint)value);   // classic unchecked cast to uint
}
int High16(IntPtr value)
{
    return unchecked((short)((uint)value >> 16));
}

- or -

int Low16(IntPtr value)
{
    return unchecked((short)(long)value);   // presumption about internals
}                                           //  is what framework lib uses
int High16(IntPtr value)
{
    return unchecked((short)((long)value >> 16));
}

Going the other way

public static IntPtr GetLParam(Point point)
{
    return (IntPtr)((point.Y << 16) | (point.X & 0xffff));
}                                           // mask ~= unchecked((int)(short)x)

- or -

public static IntPtr MakeLParam(int low, int high)
{
    return (IntPtr)((high << 16) | (low & 0xffff));  
}                                           // (IntPtr)x is same as 'new IntPtr(x)'

The accepted answer is good translation of the C definition. If were dealing with just the raw 'void*' directly, then would be mostly ok. However when using 'IntPtr' in a .Net 64-bit execution environment, 'unchecked' will not stop conversion overflow exceptions from being thrown from inside IntPtr. The unchecked block does not affect conversions that happen inside IntPtr funcitons and operators. Currently the accepted answer states that use of 'unchecked' is not necesary. However the use of 'unchecked' is absolutely necessary, as would always be the case in casting to negative values from a larger type.

On 64-bit, from the accepted answer:

var xy = new IntPtr(0x0FFFFFFFFFFFFFFF);
int x = unchecked((short)xy);                // <-- throws
int y = unchecked((short)((uint)xy >> 16));  // gets lucky, 'uint' implicit 'long'
    y = unchecked((short)((int)xy >> 16));   // <-- throws

    xy = new IntPtr(0x00000000FFFF0000);     // 0, -1
    x = unchecked((short)xy);                // <-- throws
    y = unchecked((short)((uint)xy >> 16));  // still lucky  
    y = (short)((uint)xy >> 16);             // <-- throws (short), no longer lucky  

On 64-bit, using extrapolated version of DmitryG's:

var ptr = new IntPtr(0x0FFFFFFFFFFFFFFF);
var xy = IntPtr.Size == 8 ? (int)ptr.ToInt64() : ptr.ToInt32(); // <-- throws (int)
int x = unchecked((short)xy);                // fine, if gets this far
int y = unchecked((short)((uint)xy >> 16));  // fine, if gets this far
    y = unchecked((short)(xy >> 16));        // also fine, if gets this far

    ptr = new IntPtr(0x00000000FFFF0000);    // 0, -1
    xy = IntPtr.Size == 8 ? (int)ptr.ToInt64() : ptr.ToInt32(); // <-- throws (int)

On performance

return IntPtr.Size == 8 ? unchecked((int)value.ToInt64()) : value.ToInt32();

The IntPtr.Size property returns a constant as compile time literal that is capable if being inlined across assemblies. Thus is possible for the JIT to have nearly all of this optimized out. Could also do:

return unchecked((int)value.ToInt64());

- or -

return unchecked((int)(long)value);

- or -

return unchecked((uint)value);           // traditional

and all 3 of these will always call the equivalient of IntPtr.ToInt64(). ToInt64(), and 'operator long', are also capable of being inlined, but less likely to be. Is much more code in 32-bit version than the Size constant. I would submit that the solution at the top is maybe more symantically correct. Its also important to be aware of sign-extension artifacts, which would fill all 64-bits reguardless on something like (long)int_val, though i've pretty much glossed over that here, however may additionally affect inlining on 32-bit.

Useage

if (Low16(wParam) == NativeMethods.WM_CREATE)) { }

var x = Low16(lParam);

var point = GetPoint(lParam);

---

A 'safe' IntPtr mockup shown below for future traverlers.

Run this without setting the WIN32 define on 32-bit to get a solid simulation of the 64-bit IntPtr behavour.

public struct IntPtrMock
{
    #if WIN32
        int m_value;
    #else
        long m_value;
    #endif

    int IntPtr_ToInt32() {
        #if WIN32
            return (int)m_value;
        #else
            long l = m_value;
            return checked((int)l);
        #endif
    }

    public static explicit operator int(IntPtrMock value) { //(short) resolves here
        #if WIN32 
            return (int)value.m_value;
        #else
            long l = value.m_value;
            return checked((int)l); // throws here if any high 32 bits 
        #endif                      //  check forces sign stay signed
    }

    public static explicit operator long(IntPtrMock value) { //(uint) resolves here
        #if WIN32
            return (long)(int)value.m_value; 
        #else
            return (long)value.m_value;
        #endif 
    }

    public int ToInt32() {
        #if WIN32 
            return (int)value.m_value;
        #else
            long l = m_value;
            return checked((int)l); // throws here if any high 32 bits 
        #endif                            //  check forces sign stay signed
    }

    public long ToInt64() {
        #if WIN32
            return (long)(int)m_value; 
        #else
            return (long)m_value;
        #endif
    }

    public IntPtrMock(long value) { 
        #if WIN32
            m_value = checked((int)value);
        #else
            m_value = value; 
        #endif
    }

}

public static IntPtr MAKELPARAM(int low, int high)
{
    return (IntPtr)((high << 16) | (low & 0xffff));
}

public Main()
{
    var xy = new IntPtrMock(0x0FFFFFFFFFFFFFFF); // simulate 64-bit, overflow smaller

    int x = unchecked((short)xy);                // <-- throws
    int y = unchecked((short)((uint)xy >> 16));  // got lucky, 'uint' implicit 'long'
        y = unchecked((short)((int)xy >> 16));   // <-- throws

    int xy2 = IntPtr.Size == 8 ? (int)xy.ToInt64() : xy.ToInt32();   // <-- throws
    int xy3 = unchecked(IntPtr.Size == 8 ? (int)xy.ToInt64() : xy.ToInt32()); //ok

    // proper 32-bit lParam, overflow signed
    var xy4 = new IntPtrMock(0x00000000FFFFFFFF);       // x = -1, y = -1
    int x2 = unchecked((short)xy4);                                  // <-- throws
    int xy5 = IntPtr.Size == 8 ? (int)xy4.ToInt64() : xy4.ToInt32(); // <-- throws

    var xy6 = new IntPtrMock(0x00000000FFFF0000);       // x = 0, y = -1
    int x3 = unchecked((short)xy6);                                  // <-- throws
    int xy7 = IntPtr.Size == 8 ? (int)xy6.ToInt64() : xy6.ToInt32(); // <-- throws

    var xy8 = MAKELPARAM(-1, -1);                       // WinForms macro 
    int x4 = unchecked((short)xy8);                                  // <-- throws
    int xy9 = IntPtr.Size == 8 ? (int)xy8.ToInt64() : xy8.ToInt32(); // <-- throws
}