Accessing properties on structs causes a NullReferenceException

[tlitookilakin]

Attempting to access a struct property from a Fluid template throws a NullReferenceException.

[tznind]

using Fluid;

public class M1
{
    public string S { get; set; }
}

public struct M2
{
    public string S { get; set; }
}

class Program
{
    static void Main()
    {
        var parser = new FluidParser();
        
        var m1 = new M1
        {
            S = "hello1"
        };


        var m2 = new M2
        {
            S = "hello2"
        };

        var source1 = "Answer: {{ S }} ok";
        
        parser.TryParse(source1, out var template1);
        var context1 = new TemplateContext(m1);
        var result1 = template1.Render(context1);
        Console.WriteLine(result1);


        parser.TryParse(source1, out var template2);
        var context2 = new TemplateContext(m2);
        var result2 = template2.Render(context2);
        Console.WriteLine(result2);

    }
}

Seems to work:

Answer: hello1 ok
Answer: hello2 ok

D:\Repos\fluid-test\bin\Debug\net9.0\fluid-test.exe (process 1584) exited with code 0 (0x0).
To automatically close the console when debugging stops, enable Tools->Options->Debugging->Automatically close the cons
le when debugging stops.
Press any key to close this window . . .

[sebastienros]

@tlitookilakin this is the unit test checking public properties on struct:

fluid/Fluid.Tests/MemberAccessStrategyTests.cs

Lines 258 to 271 in e23b549

	public void ShouldResolveStructs()
	{
	var options = new TemplateOptions();
	options.MemberAccessStrategy.Register<Shape>();
	options.MemberAccessStrategy.Register<Point>();
	var circle = new Shape
	{
	Coordinates = new Point(1, 2)
	};
	var template = _parser.Parse("{{Coordinates.X}} {{Coordinates.Y}}");
	Assert.Equal("1 2", template.Render(new TemplateContext(circle, options, false)));
	}

What version of Fluid are you using? This might have been implemented not that long ago.

[tlitookilakin]

I'm using 2.25.0.

I've refactored the project to just use a class instead, but previously I was using a record struct with a readonly get-only property that is not backed directly by a field. Something like this:

private record struct AssetGroup(
	string Target, AssetProperty? Prop, IList<AssetEdit> Edits, 
	AssetLoad? Load, IList<AssetInclude> Includes, string VarName
){
	public readonly HasAnyModifiers
		=> Prop?.Local is not null || Edits.Count > 0 || Load is not null || Includes.Count > 0;
}

This caused the error. Adding/removing readonly made no difference, but changing it from a record struct to a class made the error go away.

[sebastienros]

For struct properties Fluid emits IL to read the backing field ...
Here is the code that does it: https://github.com/sebastienros/fluid/blob/e23b549f665973340bc25b414a9cbdff0d0375b5/Fluid/Accessors/PropertyInfoAccessor.cs#L53C33-L53C42

Maybe there is a way to invoke the getter if it's not like this.