c#marshalling binaryfiles unsafe-pointers

Reading values from binary file whose types are known at runtime

I'm trying to read a series of values from a binary file, but I won't know what the value types are until runtime.

Simplified example

I have a binary file that is 10 bytes long. The bytes represent, in order, an int, a float, and a short. I don't know this at compile-time, but I do know this at runtime, with an array like this:

        Type[] types = new Type[3];
        types[0] = typeof(int);
        types[1] = typeof(float);
        types[2] = typeof(short);

Question

So now that I have this list, is there a way I can use this information to quickly read in values from a file? The only way I can think of is using a large if block, but it looks really ugly:

        for (int i = 0; i < types.Length; i++)
        {
            if (types[i] == typeof(int))
            {
                int val = binaryfile.ReadInt32();
                //... etc ...
            }
            else if (types[i] == typeof(float))
            {
                float val = binaryfile.ReadSingle();
                //... etc ...
            }
            else if //... etc...
        }

But this is ugly and cumbersome. I'm wondering if I can use the Type information in the types array to somehow "automate" this.

What I've tried

One idea I thought about was reading in the raw bytes into an array, then performing the conversion on the byte array. So let's say my array looks like this:

        byte[] buf = new byte[10] {
            0x40, 0xE2, 0x01, 0x00,
            0x79, 0xE9, 0xF6, 0x42,
            0x39, 0x30 };

This contains the int, float, and short values 123456, 123.456, and 12345, respectively. Now I can do the following:

        fixed (byte* bp = &buf[0])
        {
            int* ip = (int*)bp;
            Console.WriteLine("int ptr: {0}", *ip);
        }

This appears to work well, but there are two problems:

I don't know how to marshal *ip back to the managed domain.

I still can't use my type list, as follows:

    fixed (byte* bp = &buf[0])
    {
        (types[0])* ip = ((types[0])*)bp;      // both errors here
        Console.WriteLine("int ptr: {0}", *ip);
    }

This produces two compile-time errors on the line indicated:

Error   1   Invalid expression term ')'
Error   2   ) expected

That's all I've thought of to try so far.

I hope someone can help. I feel like I'm missing something simple that would make my life a lot easier.

Update

I've tried Peter Duniho's suggestion and it seems to work quite well, although there is a small performance hit when compared to a large if block.

Here are some results from a ~100 MB file (all times are in ms):

Peter's method:

if block:

Nothing too significant, although since I plan to work with much, much larger files (in the GB range) those few hundred milliseconds add up, so I'm going to stick with the ugly if block until I find something as fast.

Solution

I'm not 100% sure I understand which part of this problem you're actually trying to solve. But based on what I think you're asking, this is how I'd do it:

class Program
{
    static readonly Dictionary<Type, Func<byte[], int, Tuple<object, int>>> _converters =
        new Dictionary<Type, Func<byte[], int, Tuple<object, int>>>
        {
            { typeof(int), (rgb, ib) =>
                Tuple.Create((object)BitConverter.ToInt32(rgb, ib), sizeof(int)) },
            { typeof(float), (rgb, ib) =>
                Tuple.Create((object)BitConverter.ToSingle(rgb, ib), sizeof(float)) },
            { typeof(short), (rgb, ib) =>
                Tuple.Create((object)BitConverter.ToInt16(rgb, ib), sizeof(short)) },
        };

    static void Main(string[] args)
    {
        Type[] typeMap = { typeof(int), typeof(float), typeof(short) };
        byte[] inputBuffer =
            { 0x40, 0xE2, 0x01, 0x00, 0x79, 0xE9, 0xF6, 0x42, 0x39, 0x30 };
        int ib = 0, objectIndex = 0;

        while (ib < inputBuffer.Length)
        {
            Tuple<object, int> current =
                _converters[typeMap[objectIndex++]](inputBuffer, ib);
            Console.WriteLine("Value: " + current.Item1);
            ib += current.Item2;
        }
    }
}