Pointers of generic type?

Instead of supplying a pointer to the value, supply a setter:

class CommandChangeValue<T> : Command
{
    T _previous;
    T _new;
    Action<T> _set;

    public CommandChangeValue(T value, Action<T> setValue, T newValue)
    {
        _previous = value;
        _new = newValue;
        _set = setValue;
        setValue(_new);
    }

    public void Undo() { _set(_previous); }
    public void Redo() { _set(_new); }
}


// ...
double v = 42;
var c = new CommandChangeValue(v, d => v = d, 99);

Just for the record you can get a pointer to a generic type or any other type using these methods....

    /// <summary>
    /// Provides the current address of the given element
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="t"></param>
    /// <returns></returns>
    [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
    public static System.IntPtr AddressOf<T>(T t)
        //refember ReferenceTypes are references to the CLRHeader
        //where TOriginal : struct
    {
        System.TypedReference reference = __makeref(t);

        return *(System.IntPtr*)(&reference);
    }

    [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
    static System.IntPtr AddressOfRef<T>(ref T t)
    //refember ReferenceTypes are references to the CLRHeader
    //where TOriginal : struct
    {
        System.TypedReference reference = __makeref(t);

        System.TypedReference* pRef = &reference;

        return (System.IntPtr)pRef; //(&pRef)
    }

I have used them along with a few others to implement a form of slicing used with Arrays.

C# 7.3 solved that issue with new generic constraint - unmanaged.

Basically it allows to do something like that:

void Hash<T>(T value) where T : unmanaged
{
    // Okay
    fixed (T* p = &value) 
    { 
        ...
    }
}

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