How to get a value out of a Span<T> with Linq expression trees?

I would like to use Linq expression trees to call the indexer of a Span<T>. The code looks like:

Span<T>

var spanGetter = typeof(Span<>)
.MakeGenericType(typeof(float)).GetMethod("get_Item");

var myFloatSpan = Expression.Parameter(typeof(Span<float>), "s");

var myValue = Expression.Call(
myFloatSpan,
spanGetter,
Expression.Constant(42));

var myAdd = Expression.Add(
myValue,
Expression.Constant(13f));


Yet, this code fails because myValue is of type Single& (aka ref struct) instead of type Single (aka struct).

myValue

Single&

ref struct

Single

struct

How to evaluate a Span<T> from an expression tree?

Span<T>

Expression<Func<float>> expr = () => new Span<float>()[42];

Span<T>

Expression<Func<Span<float>, float>>

1 Answer
1

I have a solution, but it's far from being ideal, as you'll see. We re-use C# syntactic sugar engine.

class Program

static void Main(string args)

var spanGetter = typeof(Program).GetMethod("GetItem").MakeGenericMethod(typeof(float));

var myFloatSpan = Expression.Parameter(typeof(Span<float>), "s");

var myValue = Expression.Call(
null,
spanGetter,
myFloatSpan,
Expression.Constant(42));

var myAdd = Expression.Add(
myValue,
Expression.Constant(13f));

var expr = Expression.Lambda<MyFunc>(myAdd, myFloatSpan).Compile();

var span = new Span<float>(new float[43]);
span[42] = 12.3456f;
Console.WriteLine(expr(span)); // -> 25.3456


// hopefully, this shouldn't be too bad in terms of performance...
// C# knows how to do compile this, while Linq Expressions doesn't
public static T GetItem<T>(Span<T> span, int index) => span[index];

// we need that because we can't use a Span<T> directly with Func<T>
// we could make it generic also I guess
public delegate float MyFunc(Span<float> span);



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